CHAPTERS

Tropospheric Ozone

Lead Authors: A. Volz-Thomas B.A. Ridley

Co-authors: M.O. Andreae W.L. Chameides R.G. Derwent I.E. Galbally J. Lelieveld S.A. Penkett M.O. Rodgers M. Trainer G. Vaughan X.J. Zhou

Contributors: E. Atlas C.A.M. Brenninkmeijer D.H. Ehhalt J. Fishman F. Flocke D. Jacob J.M. Prospera F. Rohrer R. Schmitt H.G.J. Smit A.M. Thompson CHAPTER 5

TROPOSPHERIC OZONE

Contents

SCIENTIFIC SUMMARY ...... 5.1

5.1 INTRODUCTION ...... 5.3

5.2 REVIEW OF FACTORS THAT INFLUENCE TROPOSPHERIC OZONE CONCENTRATIONS ...... 5.3 5.2.1 Stratosphere-Troposphere Exchange ...... 5.4 5.2.2 The Photochemical Balance of Ozone in the Troposphere ...... 5.5

5.3 INSIGHTS FROM FIELD OBSERVATIONS: PHOTOCHEMISTRY AND TRANSPORT ...... 5.8 5.3.1 Urban and Near-Urban Regions ...... 5.8 5.3.2 Biomass Burning Regions ...... 5.12 5.3.3 Remote Atmosphere and Free Troposphere ...... 5.14

5.4 FEEDBACK BETWEEN TROPOSPHERIC OZONE AND LONG-LIVED GREENHOUSE GASES ...... 5.20

REFERENCES ...... 5.21 TROPOSPHERIC OZONE PROCESSES

SCIENTIFIC SUMMARY

Although representing only lO percent of the total ozone column, tropospheric ozone is important because it can influence climate, as it is a greenhouse gas itself, and because its photolysis by UV radiation in the presence of water vapor is the primary source for hydroxyl radicals (OH). Hydroxyl radicals are responsible for the oxidative removal of many trace gases, such as methane (CH4), hydrofluorocarbons (HFCs), and hydrochlorofluorocarbons (HCFCs), that influence climate and/or are important for the stratospheric ozone layer.

Tropospheric ozone arises from two processes: downward flux from the stratosphere; and in situ photochemical production from the oxidation of hydrocarbons and carbon monoxide(CO) in the presence ofNOx(NO + N02). Ozone is removed from the troposphere by in situ chemistry and by uptake at the Earth's surface. The role of photochemistry in the local ozone balance depends strongly on the concentration of NOx. Human impact on tropospheric ozone and hydroxyl occurs through the emission of precursors, e.g., NOx, CO, and hydrocarbons. In the case of free tropospheric ozone, this is brought about by the export of both ozone and its precursors, in particular NOx, from source regions.

While substantial uncertainties remain in assessing the global budget of tropospheric ozone, recent studies have led to significant advances in understanding the local balance of ozone in some regions of the atmosphere.

• Recent measurements of the NOyf03 ratio have basically confirmedearlier estimates of the fluxof ozone from the stratosphere to be in the range of 240-820 Tg(03)/yr, which is in reasonable agreement with results from general circulation models (GCMs).

• The observed correlation between ozone and alkyl nitrates suggests a natural ozone concentration of 20-30 ppb in the upper planetary boundary layer (at about 1 km altitude), which agrees well with the estimate from the few reliable historic data (cf. Figure 1-16, Chapter 1).

• Measurements of the gross ozone production rate yielded values as high as several tens of ppb per hour in the polluted troposphere over populated regions, in good agreement with theoretical predictions. Likewise, the effi­ ciency of NOx in ozone formation in moderately polluted air masses was found to be in reasonable agreement with theory.

• Direct measurements of hydroxyl and peroxy radicals have become available. While they do not serve to estab­ lish a global climatology of OH, they do provide a test of our understanding of the fast photochemistry. To date, theoretical predictions of OH concentrations (from measured trace gas concentrations and photolysis rates) tend to be higher than the measurements by up to a factor of two.

• Measurements of peroxy radical concentrations in the remote free troposphere are in reasonable agreement with theory; however, significantmi sunderstanding exists with regard to the partitioning of odd nitrogen and the bud­ get of formaldehyde.

• Measurements have shown that export of ozone produced from anthropogenic precursors over North America is a significant source for the North Atlantic region during summer. It has also been shown that biomass burning is a significant source for ozone in the tropics during the dry season. These findings show the influence of human activities on the global ozone balance.

5.1 TROPOSPHERIC OZONE PROCESSES

• Photochemical net ozone destruction in the remote atmosphere has been identified in several experiments. It is likely to occur over large parts of the troposphere with rates of up to several ppb per day. Consequently, an increase in UV-B radiation (e.g. , from stratospheric ozone loss) is expected to decrease tropospheric ozone in the remote atmosphere but in some cases will increase production of ozone in and transport from the more polluted regions. The integrated effecton hydroxyl concentrations and climate is uncertain.

Uncertainties in the global tropospheric ozone budget, particularly in the free troposphere, are mainly associated with uncertainties in the global distribution of ozone itself and its photochemical precursors, especially CO and NOx. These distributions are strongly affected by dynamics, by the magnitude and spatial/temporal distribution of sources, particularly those for NOx to the middle and upper troposphere from the stratosphere, lightning, aircraft, and convective systems, and by the partitioning and removal of NOy constituents. The role of heterogeneous processes including multiphase chemistry in the troposphere is not well characterized, and the catalytic efficiency of NOx in catalyzing ozone formation in the free troposphere has not been confirmed by measurements.

5.2 TROPOSPHERIC OZONE PROCESSES

5.1 INTRODUCTION 5.2 REVIEW OF FACTORS THAT INFLUENCE TROPOSPHERIC OZONE CONCENTRATIONS As is outlined in more detail in Chapter l, there is some, albeit limited, evidence to suggest that ozone The presence of ozone in the troposphere is under­ concentrations in the troposphere of the Northern Hemi­ stood to arise from two basic processes: (1) tropospheric/ sphere have increased by a factor of two or more over the stratospheric exchange that causes the transport of strato­ past 100 years, with most of the increase having oc­ spheric air, rich in ozone, into the troposphere, and (2) curred since 1950. This conclusion is consistent with production of ozone from photochemical reactions oc­ ozone data gathered continuously since the 1970s at a curring within the troposphere. Similarly, removal of series of remote and in some cases high altitude stations. tropospheric ozone is accomplished through two com­ It is interesting to note that all stations north of about peting processes: (1) transport to and removal at the 20°N exhibit a positive trend in ozone over the past two Earth's surface, and (2) in situ chemical destruction. For decades that is significant to the 95% confidence level. the past two decades, research on tropospheric ozone has During the same time, a statistically significant negative largely focused on understanding the relative roles of trend of about 0.5%/yr is observed at the South Pole. these processes in controlling the abundance and distri­ For the most part, the trends appear to fall more or bution of ozone in the troposphere. The basic chemical less along a straight line that extends from -0.5%/yr at mechanisms that control the local ozone budget are now 90°S to +0.8%/yr at 70°N. Somewhat anomalous are the reasonably well understood, except for the role of heter­ large positive trends observed at the high elevation sites ogeneous processes. The situation is not as good in Southern Germany (1-2%/yr); these large trends per­ concerningour quantitative understanding of the natural haps reflect a regional influence above and beyond the sources of ozone and its precursors. Transport of ozone smaller global trend (Volz-Thomas, 1993). It should be and NOy (see Figure 5-l) from the stratosphere and pro­ noted, however, that the average positive trends observed duction ofNOx (= NO + N02) through lightning must be in the Northern Hemisphere are largely due to the rela­ known to a better degree in order to assess the role of tively rapid ozone increase that occurred in the seventies. anthropogenic influences, such as air traffic (see Chapter Over the last decade, no or little ozone increase has oc­ 11). Likewise, a better understanding is needed of the curred in the free troposphere except over Southern atmospheric transport processes that redistribute ozone Germany and Switzerland. Indeed, ozone concentra­ and its precursors between the polluted continental re­ tions at some locations in the polluted planetary gions and the remote atmosphere and between the boundary layer (PBL) over Europe have decreased over planetary boundary layer and the free troposphere. the last decade (Guicherit, 1988; Low et al., 1992). Boundary layer processes, including large-scale It is important that we understand the causes of the eddy mixing and smaller scale turbulence, control the apparent increase in tropospheric ozone concentrations rate at which sources ofNOx and hydrocarbon emissions in the Northern Hemisphere because of ozone's central can combine to begin ozone production chemistry. Be­ role in global biogeochemistry, its effectiveness as a cause of the nonlinear dependence of photochemical greenhouse gas, especially in the upper troposphere, and ozone formation on the precursor concentrations, mod­ its toxicity to living organisms. It is equally important, els that assume instantaneous mixing over large spatial however, to understand the causes for the decrease ob­ grids may significantly overestimate ozone production served at high latitudes in the Southern Hemisphere rates. Vertical transport of ozone and its precursors be­ because of the influence of ozone on the concentration of tween the boundary layer and higher altitudes (together hydroxyl radicals and, hence, the oxidizing capacity of with exchange of air with the stratosphere) has a strong the atmosphere, which controls the budgets of many influence on ozone distributions in the troposphere due long-lived greenhouse gases. This, in turn, requires a to the longer lifetimes of ozone and precursors in the free quantitative understanding of the chemical and meteoro­ troposphere. On the other hand, the upward flow in con­ logical processes that determine the budget of vective systems must be balanced by downward tropospheric ozone. mesoscale flow, which then carries ozone and odd nitro­ gen species from the free troposphere into the planetary

5.3 TROPOSPHERIC OZONE PROCESSES

boundary layer, where they are destroyed more rapidly yr to infer a transport of 0.28-0.6 Tg(N)/yr of NOy and (see Lelieveld and Crutzen, 1994). Observations of 03 240-820 Tg/yr of ozone into the troposphere. This corre­ 2 in convective systems suggest that both mechanisms are sponds to a t1ux of (2-6)xl 010 molecules cm- s-1 of in effect (Dickerson et al., 1987; Pickering et al., 1992b), ozone, which is slightly less than the earlier estimates but their relative magnitude has not been evaluated ex­ made from observations of tropopause folding events perimentally. Lastly, long-range horizontal advection (Danielsen and Mohnen, 1977) and is comparable to the influences ozone distributions by transport of both ozone fluxes derived from general circulation models (e.g., and its precursors from source areas into other regions, Gidel and Shapiro, 1980; Levy et al., 1985). including the marine environment. This type of long­ The most active regions of stratosphere-tropo­ range transport has been shown to be an important factor sphere exchange are in cyclonic regions of the upper in the generation oflarge regional-scale events of elevat­ troposphere, near jet streams, troughs, and cut-offlo ws. ed ozone (see, for example, Fishman et al., 1985; The contribution of tropopause folds to the exchange has Vukovich et al., 1985; Logan, 1989; Sillman et al., been well documented (e.g., WMO, 1986) and has been 1990). confirmed by recent work (Ancellet et al., 1991, 1994; Wakamatsu et al., 1989). Potential vorticity (PV) analy­ 5.2.1 Stratosphere-Troposphere Exchange ses on isentropic surfaces near the tropopause show long streamers of elevated PV curving anticyclonically from Following the elucidation by Haynes et al. (1991) high latitudes, corresponding to narrow streaks and a of the control exercised on the diabatic circulation in the low tropopause. These streaks are clearly revealed by stratosphere by waves propagating up from the tropo­ Meteosat water vapor images (Appenzeller and Davies, sphere (the so-called Downward Control Principle), a 1992), but their contribution to stratosphere-troposphere clearer picture of stratosphere-troposphere exchange exchange has yet to be assessed. The contribution of cut­ processes has emerged. Trace species such as ozone and offlows, formed by cyclonically-curving PV streamers NOy with sources in the middle stratosphere are fed into (Thorncroft et al., 1993), is better understood. These are the lower stratosphere by the diabatic circulation at a rate preferentially found in particular regions of the world, determined by the dissipation of planetary and gravity e.g., Europe (Price and Vaughan, 1992), and can promote wave fields in the stratosphere and mesosphere. The exchange by vigorous convective mixing as well as shear lower stratosphere (especially in midlatitudes) is subject instabilities near jet streams (Price and Vaughan, 1993; to efficient isentropic mixing, which maintains a close Lamarque and Hess, 1994). Recently, the contribution correlation between trace species (Plumb and Ko, 1992). of mesoscale convective systems to stratosphere-tropo­ The lower levels of the stratosphere also exchange air sphere exchange was shown to be of potential with the troposphere. importance (Poulida, 1993; Alaart et al. , 1994). Estimates of fluxes across the tropopause remain There have been no studies of trends in strato­ uncertain. For example, the net downward flux of air sphere-troposphere exchange, so the contribution of the estimated by Holton (1990) and Rosenlof and Holton stratospheric source to the observed trend in tropospheric (1993), which was based on the concepts described ozone remains an open question. A better understanding is above, is a factor of 2-3 larger than the lower limit of the also required of transport between the lower stratosphere upward flux derived by Follows (1992) from the growth and the troposphere, and of links between the opposing of CFC-11 in the troposphere. Deriving an analogous ozone trends in these two regions of the atmosphere. estimate for the trace species is even more difficult be­ Decreasing ozone concentrations in the lower strato­ cause their distributions must be accurately known. sphere would, at first approximation, imply a decreasing However, the very close correlation between nitrous ox­ t1ux into the troposphere. However, this effect could be ide (N20), NO , and ozone in the lower stratosphere y offset by changes in the meridional circulation in the (Fahey et al., 1990; Murphy et al., 1993) offers the pos­ stratosphere. Following the Downward Control Princi­ sibility of deriving the flux of trace gases from the NzO ple and assuming the primary source of ozone in the budget. Murphy and Fahey (1994) used an annual de­ stratosphere to have remained constant, changes in struction rate of N20 in the stratosphere of 8-17 Tg(N)/ downward flux would have to be forced by changes in

5.4 TROPOSPHERIC OZONE PROCESSES

gravity wave dissipation. Therefore, changes in climate olefinic and aromatic hydrocarbons (often but not exclu­ could well have led to changes in the ozone flux from the sively of anthropogenic origin) are usually the dominant stratosphere. As noted by WMO (1992), however, there fuel, while in more rural environments reactive biogenic have not been enough studies of trends in stratospheric VOC such as isoprene often dominate (Trainer et al., temperatures and transport to deduce trends in the ozone 1987; Chameides et al. , 1988). flux into the troposphere. In contrast to hydrocarbons and CO, NOx is con­ served in the process of ozone production and thus acts 5.2.2 The Photochemical Balance of Ozone in as a catalyst in ozone formation. The conversion of NO the Troposphere to N02 by peroxy radicals (H02 and R02) is the crucial step, since the rapid photolysis ofN02 yields the oxygen The production of ozone in the troposphere is ac­ atom required to produce ozone (RS-7). Indeed, the in complished through a complex series of reactions situ rate of formation of ozone is given by referred to as the "photochemical smog mechanism." The basics of this mechanism were originally identified by Haagen-Smit (1952) as being responsible for the rise of air pollution in Los Angeles in the 1950s. As is out­ As catalysis continues until NOx is permanently lined in Figure 5-l, this well-known mechanism (see removed by physical processes (deposition) or is trans­ NRC, 1991) involves the photo-oxidation of volatile or­ formed to otherNOy compounds that act as temporary or ganic compounds (VOC) and carbon monoxide (CO) in almost permanent reservoirs, the catalytic production ef­ the presence of NOx (=NO +N02). Typical of this ficiencyofNOx can, at firstapproximation, be definedas mechanism are reactions (RS-1) through (RS-7): the ratio of the rate at whichNO molecules are converted to N02 by reaction with peroxy radicals to the rate of (RS-1) RH+OH � R+H20 transformation or removal of NOx. The lifetime ofNOx (RS-2) R+02+M � R02+M varies from a few hours in the boundary layer to at least (R5-3a) R02+NO � RO+N02 several days in the upper troposphere. Thus the catalytic (RS-4) R0+02 � H02 +R'CHO production efficiency of NOx can vary considerably and (RS-5) H02 +NO � OH+N02 nonlinearly over the more than three orders of magnitude 2 X (RS-6) N02 +hv � NO+O range of concentrations (see Figure 5-9) typically found 2 X (RS-7) 0+02+M � 03+M between remote and polluted regions of the troposphere (Liu et al., 1987; Lin et al., 1988; Hov, 1989). As is seen in Figure 5-l, the conversion of NO to N02 occurs to a large extent through reaction with 03 where an in!tial reaction between a hydrocarbon (RH) itself: and a hydroxyl radical (OH) results in the production of

two 03 molecules and an aldehyde R'CHO or a ketone. (RS-10) Additional ozone molecules can then be produced from the degradation of R'CHO. In addition to the oxidation This process constitutes only a temporaryloss, be­ of hydrocarbons, ozone can be generated from CO oxi­ cause 03 (and NO) are regenerated in the photolysis of dation via (RS-8) and (RS-9) followed by (RS-5), N02 (RS-6) followed by RS-7. The cycle adjusts the NO, (RS-6), and (RS-7). photostationary state between 03, and N02, and therefore, the NO/N02 ratio (Leighton, 1961). Through (RS-8) CO+OH � C02+H this, reaction RS-10 influences the catalytic efficiency of (RS-9) H+02+M � H02+M NOx in ozone formation since it decreases the fraction of NOx that is responsible for 03 production via R5-3a and Hydrocarbons and CO provide the fuel for the pro­ R5-5 and, at the same time, increases the fraction that is duction of tropospheric ozone and are consumed in the responsible for the loss of NOx. process. In remote areas of the troposphere, CO and Because of the rapid interconversion between NO methane typically provide the fuel for ozone production and N02 during daylight, the quantity NOx =NO+ N02 (Seiler and Fishman, 1981). In urban locations, reactive was defined. Similarly, it was found to be useful to

5.5 TROPOSPHERIC OZONE PROCESSES

a b @;]

03 I H2o2 1

_ / ()H )f� �WashOut H02 Depos�ion

WashOut Deposition

Figure 5-1 a. Schematic view of the cycles of NOx and NOy and their relation to the chemical ozone balance. The quantity NOz is defined as NO - NOx and represents the sum of all oxidation products of NOx. Figure 5-1 b. Primary formation of OH from 03 photolysis and the HOx cycle in the absence of NOx. It leads to formation of hydrogen peroxide and net destruction of ozone.

define the quantity Ox = 03+ N02, in order to account where Foto is the fraction of excited oxygen atoms that for temporary losses of 03 in highly polluted environ­ react with water vapor. This expression is only approxi­ ments (Guicherit, 1988; Kley et al., 1994). It is a better mate and is more appropriate to the remote free measure of the time-integrated ozone production than troposphere, since it neglects important loss processes ozone itself (Volz-Thomas et al. , 1993a). that can occur in the continental boundary layer, such as Photochemical loss of tropospheric ozone is ac­ dry deposition and reactions with unsaturated hydrocar­ complished through photolysis followed by reaction of bons. It also neglects potential losses that have been 1 the 0( D) atom with water vapor, (R5- ll) and (R5-12). suggested to occur in cloud droplets and nighttime or Additional losses occur through reaction of the H02 rad­ wintertime losses through nitrate radical (N03) chemis­ ical formed in (R5-9) with 03 via (R5-13) and (to a try. As such, it is a lower limit for the loss rate. lesser extent) through reaction of OH with 03 (R5-14): Ultimately, the budget of ozone in a given region is governed by transport of ozone into or out of the region 1 (R5-l l) 03+hv � 0( D)+02 and the net rate of ozone formation, P(03) - L(03). Ex­ J (R5-12) Oc D) +H 20 � 2 OH cept for urban regions, where N02 is the predominant (R5-13) H02+03 � OH+2 02 sink for OH radicals and, hence, limits the formation of (R5-14) OH+03 � H02+02 R02 radicals, the rate of 03 production is most oftenlim­

The photochemical rate of ozone loss is approxi­ ited by the availability of NOx even in the boundary layer mated by over the European and North American continents. In the remote atmosphere, not only is the production rate of 03 limited by the availability of NOx, the concentration

5.6 TROPOSPHERIC OZONE PROCESSES

of NOx can be so small that L(03) exceeds P(03). These balance is negative, that is, over large areas of the South­ regions thus act as a buffer against any excess ozone im­ ern Hemisphere and the remote oceanic regions of the ported from areas having higher production rates or from Northern Hemisphere (Section 5.3). The long-term ob­ the stratosphere. servations at the South Pole (Schnell et al., 1991; A coarse estimate for the "critical" NO concentra­ Thompson, 1991) indicate that some enhanced net de­ tion at which local 03 production and loss rates are struction of tropospheric ozone may already be equivalent was given by Crutzen (1979) by simply occurring in association with the large stratospheric equating the rate of RS-13 to the rate of RS-5. The criti­ ozone losses in that region. On the other hand, a long­ cal daytime NO concentration thus derived is within a term increase in UV radiation will likely contribute to an factor of two of 10 ppt, depending on the actual ozone increase in photochemical ozone formation in the NOx­ concentration and other factors. However, this is a lower rich continental regions and possibly in large-scale limit because other loss processes have been neglected plumes downwind of these or areas of biomass burning. and the term J11 • Fo 1 D is the dominant contribution to Removal or conversion of NOx to longer-lived res­ L(03) in the remote lower-to-middle troposphere. For ervoirs clearly decreases the local catalytic efficiency of example, from experimental observations made at 3.4 03 production. During daytime, losses of NOx proceed km in the mid-North Pacific Ocean region, this term ac­ through the reaction of N02 with OH radicals (RS-15) counted for nearly 50% of the total loss rate when and the formation of peroxyacetylnitrate (PAN) and its averaged over 24 hours (Ridley et al., 1992). homologues (RS-16): Nevertheless, any non-zero concentration of NOx (RS-15) N02 + OH + M -7 HN03 + M contributes to 03 production, compensates the loss rate, (RS-16) N02 + RC(0)02 H RC(0)02N02 and increases the lifetime of 03. Since P(03) is so sensi­ tive to the NOx abundance and L(03) is, to first While nitric acid (HN03), at least in the planetary approximation, insensitive to NOx in the remote atmo­ boundary layer, provides an effective sink for NOx, the sphere, possible trends in tropospheric 03 are intimately thermally unstable compound PAN provides only a tem­ dependent upon trends in NOx concentrations. Clearly, porary reservoir for N02. Most important, the lifetime assessing the contribution of photochemical processes to of PAN becomes long enough at the colder temperatures trends in global and regional ozone relies on a good of the middle and upper troposphere that it can be trans­ knowledge of the distribution of 03, the fuels (CO, CH4, ported over long distances and serve as a carrier of NOx NMHC), and especially the distribution of NOx. Reac­ into remote regions. NOx is also removed by the forma­ tions RS-11 and RS-12 not only constitute an important tion of alkyl nitrates (RONOz) that are formed in the 03 loss rate but also initiate the oxidation cycles via OH alternative reaction path of R02 with NO (R5-3b) (At­ radicals (see Section 5.4) and therefore link stratospheric kinson et al., 1982): 03 change to tropospheric photochemistry through the sensitive dependence of J11 on the overhead column of (R5-3b) 03. Similar to PAN, alkyl nitrates could provide a source of The depletion of stratospheric ozone during the NOx to more remote regions via photolysis or through last decade has led to increased ultraviolet radiation of reaction with OH following transport (Atlas, 1988). wavelength 290-320 nm penetrating to the troposphere An important loss process for NOx that was not (see Chapter 9). Liu and Trainer (1988) studied the in­ included in earlier model studies (Liu et al., 1987) is the fluence of enhanced UV radiation on tropospheric ozone oxidation of N02 by ozone itself. The N03 radical with a simple photochemical model and found that the formed in reaction (RS-17) is extremely sensitive to pho­ net effect depended on ambient NOx levels. To first or­ tolysis but can build up at night to concentrations of der, an increase in the ultraviolet flux essentially several hundred ppt (Platt et al. , 1981; Wayne et al., accelerates the already-existing production and destruc­ 1991). Because of the thermal equilibrium (RS-18) that tion processes. For this reason, a positive trend in UV is established between N03, N02, and N20s, heteroge­ radiation will, most likely, cause a negative trend in tro­ neous losses of NzOs or N03 in addition to reactions of pospheric ozone in regions where the net photochemical N03 with some hydrocarbons provide a sink for NOx in

5.7 TROPOSPHERIC OZONE PROCESSES

addition to the reaction with OH (R5-15). For example, 5.3 INSIGHTS FROM FIELD OBSERVATIONS: (R5-19) constitutes a non-photochemical conversion of PHOTOCHEMISTRY AND TRANSPORT active NOx to long-lived aerosol nitrate (or HN03 in During the summer months, elevated and poten­ case of evaporation of the droplets). According to model tially harmful levels of ozone are commonly observed in calculations, this mechanism could provide a significant urban and rural areas of North America and Europe (Cox sink for NOx on a global scale (Dentener and Crutzen, et al. , 1975; Logan, 1985). Slow-moving high pressure 1993). Observations of the chemical lifetime of N03 systems with predominantly clear skies and elevated (Platt et al., 1984) indicate that in the boundary layer, the temperatures set the stage for the photochemical forma­ initial reaction R5-17 is often the rate-limiting step for tion and accumulation of ozone and other oxidants over the removal of NOx by these processes. wide regions during episodes that last several days

(R5-17) N02 + 03 --7 N03 + 02 (Guicherit and van Dop, 1977; Vukovich et al. , 1977). (R5-18) N02 + N03 B N20s There is substantial evidence from field measurements - (R5-19) N20s + H201q --7 2 H+ + 2 N03 and model calculations that most of this ozone is being produced photochemically from ozone precursors emit­ The occurrence of clouds changes the chemical ted within the region. The export of 03 and its precursors processing in an air mass significantly (Chameides and from the urban to regional and global scales represents Davis, Although the volume fraction of liquid 1982). the greatest potential impact on trends in global ozone by water in clouds is only of the order of o-6 or less, some 1 anthropogenic activities. gases are so soluble that they largely go into the aqueous phase. This has several consequences: (1) the soluble 5.3.1 Urban and Near-Urban Regions gases are concentrated in a relatively small volume, which can enhance reaction rates, and (2) soluble gases High ozone levels in and downwind of urban re­ are separated from insoluble ones, so that some reaction gions remain an important air quality problem rates are significantly reduced. An important example is throughout the world. While most industrialized coun­ reaction R5-5, which almost ceases within clouds be­ tries have made significant progress in lowering peak cause H02 is very soluble, whereas NO remains in the ozone concentrations over the last two decades, un­ interstitial air. Furthermore, the dissociation of dis­ healthy levels of ozone persist in and around many larger - solved H02 yields 02 , which destroys 03 in the cities. In particular, in many developing countries, the aqueous phase. The production of H02 in the droplets absence or ineffectiveness of emissions control efforts results to a large extent from the oxidation of dissolved can result in extremely high ozone concentrations. formaldehyde. A radical reaction cycle is thus initiated The limited atmospheric chemical measurements in which both formaldehyde and 03 are destroyed. from urban areas in developing countries suggest that The estimated effect of cloud chemistry is that the conditions in many of these areas essentially mimic con­ photochemical 03 production rate in the lower tropo­ ditions observed in the Organization for Economic sphere (where most clouds occur) is reduced by 30-40%, Cooperation and Development (OECD) countries during while 03 destruction reactions are enhanced by up to a the 1960s before implementation of large-scale emis­ sions control programs. For example, observations of factor 2 (Lelieveld and Crutzen, 1990). The net effect of cloud processes on the 03 burden in the troposphere is individual hydrocarbon ratios in Mexico City, Mexico, estimated to be much smaller, however, since these pro­ during 1992 (Seila et al. , 1993) were similar to those cesses compete with dry deposition (Dentener et al. , observed in Los Angeles, California, during the 1960s and are consistent with motor vehicles as the major 1993). Model simulations suggest a 10-30% lower tro­ pospheric ozone burden as compared to a cloud-free source of hydrocarbon emissions in this area. Motor ve­ hicle emissions have also been demonstrated to be the atmosphere (Johnson and Isaksen, 1993; Dentener et al. , major source of hydrocarbons in Athens, Greece; Rio 1993). de Janeiro, Brazil; and Beijing, China (see Tang et al., 1993; Xiuli et al., 1994).

5.8 TROPOSPHERIC OZONE PROCESSES

The impact of anthropogenic NOx and VOC emis­ 140 sions on regional and global ozone levels depends on the 0 rate of ozone formation, the amount of ozone that is 120 I c formed per precursor, and the rate and the pathway of 100 :0 c. transport out of the source regions. Direct and indirect 00 I !:: c ltlSCotia " 80 measurements of peroxy radical concentrations that " ¢ Bondville 0 I N Whitetop were made at several rural sites indicate concentrations 0 60 o Egbert --model of up to several hundred ppt at noontime on clear sum­ 40 I ······modelx (NMHCx4) mer days (Parrish et al., 1986; Volz et al., 1988; Mihelcic 20 et al., 1990; Mihelcic and Volz-Thomas, 1993; Cantrell 10 15 20 25 et al., 1993). When combined with concurrent NO mea­ Products of NO, oxidation [ppb] surements, these R02 radical concentrations indicate substantial in situ ozone production rates of several tens 140 of ppb/h at rural locations in the vicinity of industrialized 120 -- _, regions (Volz et al., 1988; Trainer et al., 1991; Cantrell et :0 !too al., 1993). Such measurements can be used to determine 0 z 80 the relative roles of UV radiation, NOx, and VOC for in +

0 -� • situ ozone production. The observed ozone increase is II 60 Schauinsland • · • • · · NOz/Ox-fit usually much smaller than the gross production rate de­ 0 40 --- Ox/NOz-fit rived from the R02 measurements, which indicates that -- Orthog.-regr. the losses through dry deposition or reactions with un­ 20 0 5 10 15 20 25 saturated VOCs such as terpenes and by dilution must be NO,= NO,· NO, [ppb] of similar magnitude as the production rate. This indi­ cates that the characteristic lifetime of ozone in polluted air masses is rather small, e.g., less than one day. Figure 5-2a. Ozone versus the concentration of In photochemically aged air in summer, 03 was NOx oxidation products (e.g., N02 in Figure 5-1), as measured at four sites in the eastern United States found to increase with increasing NO concentration, y and Canada during summer 1988 and the results from a background value of 30-40 ppb 03 at NOy mixing from a model calculation (based upon Trainer et at., ratios below 1 ppb to values between 70 to 100 ppb at 1993). NOy levels of 10-20 ppb (Fahey et al., 1986). As is ex­ Figure 5-2b. Same relation as 5-2a measured at pected from photochemical theory, an even better Schauinsland, Germany, during summer 1990 in air masses advected from the Rhine Valley (based correlation is observed between ozone and the products upon Volz-Thomas et at., 1993a). The quantity Ox of the NOx oxidation (Trainer et al., 1993; Volz-Thomas = 03 + N02 is used to account for titration of 03 et al. , 1993a). Figure 5-2 shows the results from mea­ under high NOx conditions (RS-1 0 in Section 5.2.2). surements made during summertime at several rural locations in the U.S. and Canada, and at Schauinsland in The data also indicate a significantly lower production Europe. The slope of the correlation provides, at first approximation, experimental information on the ozone efficiency for the air masses encountered at Schauin­ production efficiency, e.g. , the number of ozone mole­ sland in Europe. cules produced by each NOx molecule before oxidation The role of hydrocarbons and nitrogen oxides for ozone formation on the urban sub-urban scale was stud­ to more stable products such as HN03 and peroxyacetyl I nitrate (see Section 5.2.2). The increments in the indi­ ied by Hess et al., (1992a, b, c) in an outdoor smog vidual data sets in Figure 5-2 range from 4 to 10 and chamber using a synthetic gas blend that closely resem­ suggest a somewhat smaller production efficiency than bled that of automobile exhaust. The most important what has been predicted by models for NOx levels typi­ finding was that the initial rate of ozone formation de­ cally encountered in rural regions of the industrialized pended on the mix of hydrocarbons used and, of course, countries (Liu et al., 1987; Lin et al., 1988; Hov, 1989). on the availability of UV light. However, the final

5.9 TROPOSPHERIC OZONE PROCESSES

amount of 03 produced during one day depended mainly 140

on the availability of NOx. To some extent, the latter ·-- 120 -----I--· - --- _ ___.__ -�-- . ./ . depends on the hydrocarbon mix, specificallyon the ex­ v • :0 -� � a. 100 f-- • istence of NOx sinks in the chemistry through formation • � . • . v.• • . � - � 80 -- • -; - -- of organic nitrates (Carter and Atkinson, 1989). "' • a: . . •• � Insight into the chemical breakdown of hydrocar­ Cl ..../ . . -� c: 60 !· ";( �· bons and their role in ozone formation can be obtained :E . . 40 .A1 -- • y = 0. 12X+ 26 from field measurements of alkyl nitrates (RON02), 0 , . . -- since these species are formed as a by-product in reac­ 20

tion (R5-3), which is rate-limiting in ozone formation. 0 0 1 00 200 300 400 500 From an extensive series of measurements made at Alkylnitrate Mixing Ratio [ppt] Schauinsland, a mountain site in Southern Germany, in summer, a linear relation was found between ozone and Figure 5-3. Correlation of Ox = 03+ N02 concen­ alkyl nitrate concentrations, which is shown in Figure trations with those of alkyl nitrates (RON02) as 5-3(Flockeetal., 1991, 1993). The high degree of cor­ obseNed at Schauinsland, Germany, in summer relation found in air masses that originate in the Rhine under polluted conditions (based upon Flocke et Valley, and thus represent a relatively uniform mix of at., 1992). Ox and RON02 emerge from the same reaction (R5-3). hydrocarbons, clearly points out that most of the ozone observed at Schauinsland in summer (70 ppb average and peak values of 130 ppb) is formed in situ from an­ consistent with the fact that measured ratios of organic thropogenic precursors emitted within the region. By peroxy radicals to H02 are significantly larger than those extrapolation to RON02 concentrations of zero, an predicted by models that do not include this mechanism estimate of 20-30 ppb is obtained for today's non-photo­ (Mihelcic and Volz-Thomas, 1993). The conclusion is chemical background mixing ratio of ozone in the that the rate of production of R02 radicals is greater than continental boundary layer in summer (Flocke, 1992; originally assumed in these models. Flocke et al., 1993; Volz-Thomas et al., 1993b). This Carbon monoxide is an anthropogenic pollutant finding supports the conclusions drawn by Vo lz and Kley that has a relatively long photochemical lifetime (1 ( 1988) and by Staehelinet al ., (1994) from historic mea­ month in summer) and is not affected by rainout. Thus, surements (see Chapter 1) and proves the predominant it is a suitable tracer of anthropogenic pollution on long­ anthropogenic influence on ozone levels in some rural er time scales (Fishman and Seiler, 1983). Parrishet al. areas today. Since alkyl nitrates are not removed by rain­ (1993) observed a strong correlation between ozone and

out, they are better suited for such an extrapolation than CO with a consistent slope il03/ilCO = 0.3 at several either NOy or NOz ( = NOy - NOx), since the latter con­ island sites in eastern Canada (Figure 5-4). The sites tain soluble HN03 as a major constituent. were located at approximately 500-km intervals down­ The European studies also led to the conclusion wind of the northeastern urban corridor of the United that about one ozone molecule per carbon atom is States, and covered approximately one-third of the dis­ formed from the oxidation of hydrocarbons in these air tance from Boston to Ireland. By scaling the observed masses (Flocke, 1992). Furthermore, the relative abun­ slope to a CO emission inventory, they inferred a net ex­ dance of the different alkyl nitrates indicates that most of port of 5 Tg anthropogenic 03 out of the eastern U.S. in the smaller R02 radicals are not formed from the oxida­ summer. Chinet al. ( 1994) successfully simulated the tion of the respective parent hydrocarbons but by observed 03-CO relationship in a continental-scale decomposition of larger alkoxy radicals. This findingis three-dimensional (3-D) model and concluded that the in agreement with results from laboratory studies (Atkin­ correlation slope of 0.3 is a general characteristic of aged son et al., 1992) and R02 production from the polluted air in the U.S. The model allowed in particular decomposition of RO radicals is now a common feature in to correct for the effectof 03 deposition. From this cal­ detailed chemical mechanisms used in urban airshed mod­ culation, Chin et al. (1994) estimated that export of els (Carter, 1990; Atkinson 1990). The finding is also eastern North American pollution contributes 7 Tg of 03

5. 10 TROPOSPHERIC OZONE PROCESSES

slope = 0.29 = ..... slope 0.27 R2 = 0.71 . . R2 = 0.74 ·. . . : . . � .. . slope = 0.2 2 R2 = 0.46 . � ... ' ·� :�=: : ...... ' /:;:· -:::. : · ) . . . � . . . . . · -= ...... ··: . . ·

Seal Island Sable Island Cape Race

0 �-L�Li-L��_L��-L��-L-L��-L��-L�Li-L��-L� 0 100 200 0 100 0 100 200 300

CO (ppbv)

Figure 5-4. Relation between 03 and CO observed at three island sites in the North Atlantic west of Canada during summer 1992 (based upon Parrish et at., 1993). in summer. Jacob et al. (1993) used the same model to Simmonds, 1993; Derwent et al. , 1994). Derwent et al. estimate that pollution from all of North America con­ conclude from their analysis of the air masses that arrive tributes 30 Tg of 03 to the Northern Hemisphere in at Mace Head, Ireland, that the European continent is a summer, of which 15 Tg is due to direct export and 15 Tg net source of ozone in summer, but is a net sink in winter. is due to export of NOx leading to 03 production in the This estimate, however, is only valid for the planetary remote troposphere. This anthropogenic source of 03 is boundary layer and does not include the influence of about one-third of the estimated cross-tropopause trans­ NOx export on the net photochemical balance of ozone. port of 03 in the Northern Hemisphere in summer. A seasonal trend is also apparent in the correlation Considering that the U.S. accounts for about 30% of fos­ of ozone with NOy and NOz (Fahey et al. , 1986; Vo lz­ sil fuel NOx emissions in the Northern Hemisphere, it Thomas et al., 1993a). The wintertime measurements of can be concluded that anthropogenic sources make a 03, NOx, and NOy at Schauinsland indicate a decrease major contribution to tropospheric ozone on the hemi­ of ozone with increasing concentrations of the products spheric scale, of magnitude comparable to influx from of the NOx oxidation and, hence, support the importance the stratosphere. of nighttime chemistry in the oxidation of NOx at the While the summertime measurements show a expense of ozone in polluted air masses. strong positive correlation of ozone with anthropogenic Since anthropogenic NOx emissions do not have a tracers such as NOy and CO, a negative correlation was strong seasonal variation, Calvert et al. (1985) argued observed during winter. A decrease in the 03 concentra­ that the absence of a seasonal cycle in nitrate deposition tion with increasing CO concentration is observed at a rates provided evidence for the importance of N03 number of locations in North America and Europe (Poul­ chemistry in the removal of NOx. However, more recent ida et al., 1991; Parrish et al., 1993; Scheel et al., 1993; data from the National Acid Deposition Program and

5. 11 TROPOSPHERIC OZONE PROCESSES

SEASONAL DEPICTIONS OF TROPOSPHERIC OZONE DISTRIBUTION

December - February March- May

50 50

25 25 ([) u ::J +- 0 0 :;::: 0 --' -25 -25

-50 -50 -180 -120 -60 0 60 120 180 -180 -120 -60 0 60 120 180

June- August September- November

50 50

25 25 ([) u ::J +- 0 0 :;::: 0 --' -25 -25

-50 -50 -180 -120 -60 0 60 120 180 -180 -120 -60 0 60 120 180 Longitude Longitude

<20 25 35 45 55 60>

Dobson Units

Figure 5-5. Tropospheric ozone column in Dobson units (DU) as derived from satellite observations for different times of the year (based upon Fishman eta/., 1991 ). The red areas of high ozone can be associated with export from North America and Europe and from biomass burning regions in South America and Africa. other North American sites do indeed show a seasonal 5.3.2 Biomass Burning Regions cycle in bulk nitrate deposition rates, with larger values Biomass burning takes many forms; among them, in summer (Correll et al., 1987; Doddridge et al., 1992), forest and savanna fires, burning of agricultural wastes, as would be expected if OH radicals played the dominant and the use of biomass fuels as a domestic energy source role in controlling the NOx budget. Some further sup­ are the most important. Biomass fires release a mixture port for the dominance of OH in controlling the removal of gases containing the same ozone precursors emitted of NOx is provided by the observation of larger NOx lev­ from fossil fuel combustion: NOx, CO, CH4, and non­ els in the Arctic winter (Dickerson, 1985; Honrath and methane hydrocarbons (NMHC), including a large Jaffe, 1992). proportion of alkenes. Ozone production in aged bio­ mass-burning plumes has been shown by numerous investigators (Delany et al., 1985; Andreae et al. , 1988, 1992; Cros et al. , 1988; Kirchhoff et al., 1989, 1992;

5.12 TROPOSPHERIC OZONE PROCESSES

Kirchhoff and Marinho, 1994). The global emissions of --··--- 1.0 ozone precursors from biomass burning have been esti­ mated in a recent review by Andreae (1993) to be • comparable in magnitude to the emissions from fossil

fuel burning. Evidence for the importance of tropo­ 0 • u spheric ozone production from pyrogenic precursors has

the persistence of high ozone levels in the mid-tropo­ L1NO I L1CO y sphere during the burning season (Cros et al., 1992; Fishman et al., 1992). Aircraft measurements have dem­ Figure 5-6. Ratio of 03 to CO in aged biomass onstrated the origin of these ozone-enriched air masses burn�ng plumes as � function of the NOyiCO ratio. from biomass burning (Marenco et al., 1990; Andreae et The 1ncrease seen 1n the data clearly indicates the al., 1988, 1992, 1994a). Compelling evidence was also important role of NOx for ozone formation in the collected in more recent aircraft campaigns that docu­ plumes {based upon Andreae et a/., 1994b). The straight line represents a fit to the majority of the mented the distribution of ozone and its pyrogenic data. It is ?on�is�ent with an El:verage OyNOy ratio precursors in a region extending from South America of 4-5, qUite s1m1lar to the rat1os observed in sub­ across the Atlantic Ocean to southern Africa (Andreae et urban air masses over Europe. al., 1994b). Due to the dispersed nature of biomass burning and the relatively small number of fieldinvestigations on hemisphere-wide. The accurate description of these ozone production from pyrogenic precursors, it is still transport processes probably represents the largest diffi­ difficultto provide a quantitative estimate of ozone pro­ culty in current global models, as is discussed in more duction from this source. The observed ratio of ozone to detail in Chapter 7. CO enhancements in aged burning plumes varies from The secular trends of biomass burning are highly near zero in some tundra fire emissions (Wofsy et al., uncertain. Obviously, fire has been present on Earth 1992) to almost one in some aged savanna fire plumes since the evolution of land plants, and human activity (Andreae et al., 1994a). As shown in Figure 5-6, these has resulted in large fires in the savannas of Africa and differencesappear to be related to the ratio of NOx to CO South America since the advent of human beings. How­ (and consequently NMHC) in the emissions. By using ever, other types of biomass burning have clearly been an average 03/CO-ratio of 0.3 and a CO emission of 300 increasing over the last century, especially deforestation Tg C/yr from biomass burning, Andreae (1993) estimat­ fires and domestic biomass fuel use. These types of bio­ ed a global gross 03 production of ca. 400 Tg 03/yr from mass burning have especially high emission factors for h biomass burning, with an uncertainty of at least a factor ozone precursors. Andreae ( 1994) estimated that t e re­ of two. A recent model study estimated a similar gross lease of trace gases from biomass burning has increased rate of 540 Tg/yr; however, the net production of ozone by about a factor of two or three since 1850. Semi-quan­ from biomass burning was found to be only 100 Tg/yr titative measurements made during the last century, (Lelieveld and Crutzen, 1994). This large difference albeit not considered sufficientlyreliable for an indepen­ emphasizes, as already discussed for the northern mid­ dent quantitative assessment (Kley et al., 1988), would latitudes above, the crucial role of transport processes in indeed support a secular increase in tropospheric ozone distributing the ozone between the PBL, where it is de­ concentrations in the Southern Hemisphere (Sandroni et stroyed rapidly, and the free troposphere, where its a!., 1992). No trend is seen in the surface ozone records chemical lifetime is long enough for it to be dispersed obtained over the last two decades at Cape Point, South

5. 13 TROPOSPHERIC OZONE PROCESSES

Africa (Scheel et al., 1990) and American Samoa, Pacif­ there are often large changes in 03 concentration; these ic Ocean (Oltmans and Levy, 1994). changes are strongly anticorrelated with a number of - aerosol species, including N03 (Savoie et al. , 1992). 5.3.3 Remote Atmosphere and The changes in 03 are driven by changing transport pat­ Free Tro posphere terns over the North Atlantic as opposed to chemical reactions involving 03 and nitrogen species in the atmo­ While recent work has provided major advances in sphere. Analyses of isentropic trajectories clearly show our understanding of the ozone budget over continental - that high 03 and low N03 are associated with transport regions relatively close to the centers of precursor emis­ from the middle and high latitudes and from relatively sions and clearly demonstrated the anthropogenic high altitudes in the free troposphere. Conversely, high perturbation of tropospheric ozone on a regional scale, _ N03 and relatively low 03 are associated with transport the system is still far from being understood on a hemi­ from Africa. The lack of association of high 03 with spheric or global scale. Compounding issues are: (1) the ground-level sources is supported by the strong anticor­ strong, if not overriding, influenceof transport from both relation of 03 with 210-Pb; conversely, the strong the stratosphere and the continental source regions, - correlation of N03 and 210-Pb (and a weaker correla­ transport that is episodic rather than steady; (2) the large - tion with Saharan dust) indicates that N03 is derived difference in chemical time constants between the principally from continental surface sources, probably in boundary layer and the free troposphere; (3) the uncer­ Europe and North Africa. These associations suggest tainty in the production rate for NOx by lightning and its that African biomass burning could be a significant distribution; (4) the extremely low NOx and NOy constit­ - source of N03 , but appears to be a minor source for o3 uent concentrations, which represent a real measurement at Barbados. Although substantial amounts of 03 may challenge; and (5) the large volume that must be covered have been produced as a consequence of the burning, a to establish a climatology. Concerns have also been substantial fraction must have been destroyed in transit raised about the validity of N02 and NOy measurements . in the marine PBL. m t h e troposphere obtained by commonly used tech- The importance of transport processes for the glo­ niques (see Davis et al. , 1993; Crosley, 1994). bal ozone distribution is also emphasized in studies Nevertheless, the large number of fieldexperiments per­ made at the Spanish Meteorological observatory at Iza­ formed over the last years (see Carroll and Thompson, na, Tenerife. The station is located at an elevation of 2.4 1994) has led to a better understanding of the 03 budget km, above the top of the marine inversion most of the on a global scale. Even greater insight is expected to time. At Izana, ozone concentrations have a well-de­ come out of the recently completed or ongoing experi­ fined seasonal cycle, with monthly means of about 40 ments that have included direct measurements of OH ppb in winter, about 55 ppb in spring, and about 50 ppb and R02 radicals and the seasonal variations of active in July (Schmitt et al. , 1988). The concentrations in nitrogen compounds in the remote atmosphere. summer are much higher on average than those observed Plumes of "pollution" from biomass burning re­ at Mauna Loa, Hawaii, and exhibit a bimodal distribu­ gions and from the industrialized regions of North tion. Low mixing ratios of -20 ppb are advected from America, Europe, and Asia have been identified from the open ocean and the Saharan desert, and high values satellite observations (Fishman et al. , 1990). Being of up to 100 ppb generally result from relatively rapid downwind of continental source regions, measurements transport from northern latitudes (Schmitt and Hanson made in the marine boundary layer at Barbados, West 1993). It has been suggested that the persistence of hig� Indies, show that large variations in 03 concentrations ozone concentrations in the summer could be due to the can be associated with changes in long-range transport transport of ozone from Europe, based on isentropic patterns. There is a pronounced seasonal cycle for 03 at back-trajectories and the correlation of high ozone epi­ Barbados (Oltmans and Levy, 1992, 1994). During the sodes with increased concentrations of tracers of winter and spring, daily averaged values are typically in anthropogenic origin such as CH4, PAN, VOC, and CO the range of 20-35 ppb, while during the summer, values (Schmitt et al., 1988, 1993; Vo lz-Thomas et al., 1993c). are typically 10-20 ppb. During the winter-spring period The seasonal cycle of ozone at lzana is similar to that for

5. 14 TROPOSPHERIC OZONE PROCESSES

a 500 a

...... c. 0 z

b

,·· : . . � . ' ' ' ' ' 0 . ' . � . z

Figure 5-7. NO concentrations measured in the free troposphere during STRATOZ Ill and TROPOZ II (based upon Ehhalt et at., 1992; Wahner et at., 1994). The flight track was similar in both missions and extended over the North and South Atlantic and the west coast of South America.

- non-seasalt sulfate (nss-S04=) and N03 , which could this regard, the results obtained at Izana are similar to be interpreted as supporting an anthropogenic source for those obtained in the marine boundary layer at Barba­ ozone. However, on a day-to-day basis, ozone is strong­ dos. - ly anticorrelated with aerosol N03 and nss-S04= Evidence that stratospheric input is an important (Prospero et al. , 1993). This and the coherence between component of the upper tropospheric ozone budget, es­ ozone and 7-Be, which is produced from cosmic rays in pecially in spring and early summer, was presented by the upper troposphere and lower stratosphere (see Brost Beekmann et al. (1994), based on the correlation be­ et al., 1991), could imply a major contribution of ozone tween ozone mixing ratio and potential vorticity, and by from the stratosphere or effective losses of aerosol ni­ Smit et al. (1993), based on a time series of ozonesonde trate and sulfate during convective transport from the measurements in the upper and lower troposphere. The planetary boundary layer into the free troposphere. In poleward increase in upper tropospheric ozone suggests

5. 15 TROPOSPHERIC OZONE PROCESSES

AASE 1 that this component is even more important at high lati­ NOx (pptv) tudes. Evidence for stratospheric input to the Arctic troposphere was presented by Shapiro et al. ( 1987) and 12.------, Oltmans et al. ( 1989). Furthermore, airborne lidar mea­ surements made over the Arctic region in summer found " �'"7·-�··----/--·"7"' "''"""""""""""""'"'""'""""""""""" 11 40 that stratospheric intrusions dominated the ozone budget

_ 40 __ _ "' in the free troposphere (Browell et al., 1992; Gregory et 10 al., 1992). There is also a suggestion in ozonesonde data from the South Pole (Gruzdev and Sitnov, 1993) that ozone depletion in the Antarctic polar vortex extends into the upper troposphere. An example of progress in determining large-scale reactive nitrogen distributions over the complete tropo­ 7 spheric altitude regime is shown in Figure 5-7, which contrasts the seasonal distribution of NO from aircraft measurements made during the Tropospheric Ozone II 6 (TROPOZ II) mission in January 1991 (Wahner et al., 1994) and the Stratospheric Ozone III (STRATOZ III) 5 ;--,---.--.--r--r-�--�----,-� mission in June 1984 (Drummond et al. , 1988; Ehhalt et 35 40 45 50 55 60 65 70 75 80 85 aL, 1992). The mixing ratios are considerably higher in Latitude the Northern Hemisphere, particularly at high latitudes in winter, and at 20-50°N at high altitudes in summer. AASE2 Vertical gradients are strongest in June north of 20°S. NOx (pptv) 12,------� The high mixing ratios of NO at northern midlatitudes are attributed to stratospheric input, aircraft emissions,

11 and convective transport from the "polluted" boundary layer(Ehhalt et al., 1992). 10 The NO concentrations observed during TROPOZ II are much larger than what has been observed by other investigators at similar latitudes and seasons. Figure 5-8 'E g � shows NOx concentrations observed during the Arctic � 8 Airborne Stratospheric Expedition (AASE) I and II mis­ :J ·E sions (Carroll et al., 1990a; Weinheimer et al., 1994). <( 7 While these flightswere made during the same season as TROPOZ II and at overlapping latitudes, they show 6 much lower NOx (=N0+N02) concentrations than the NO concentrations alone that were observed during 5 TROPOZ II. The AASE measurements are in general 4 1--.---.--.--r--��--.--,--.-� agreement although separated by a three-year period. 35 40 45 50 55 60 65 70 75 80 85 The difference may be due to the shorter measurement Latitude period of the TROPOZ program, and an unusual synop­ tic event, compared to the longer-term AASE programs.

Figure 5-8. Summary of NOx = NO+ N02 concen­ Barring unexpected measurement uncertainty, the differ­ trations in the free troposphere measured in the ences demonstrate the difficulty in ascertaining a Northern Hemisphere during the AASE I and AASE climatology of a short-lived species like NOx over larger II missions (based upon Carroll et a/., 1990a and Weinheimer et a/., 1994). scales.

5.16 TROPOSPHERIC OZONE PROCESSES

10.000 • Ground Data I{�- <) Flight Data -J ·� rl-H . �� � rr 1.000

f-

,....., .0 i--1 c.. p 3 3 c.. � .__. 0.100 -::f---· X � 0 � .&>--- z Tl � I f- � it'-� � 3 r���ll_L I 0.010 -::: ----

lp m �

0.001

0.010 0.100 1.000 10.000 100.000 NOy [ppb]

Figure 5-9. Summary of NOx and NOy concentrations in the PBL and free troposphere (from Prather et at., 1994, based on Carroll and Thompson, 1994). The majority of the airborne measurements shows NOx concentrations that are too small to sustain net ozone production. The letters and numbers within the sym­ bols refer to the following measurement campaigns (see Appendix for acronym definitions): a=ABLE3a; A= AASE; b = ABLE3b; B = Barrow, Alaska; H = Harvard Forest; K = Kinterbush, Alabama; M = MLOPEX; n = NACNEMS; N = Niwot Ridge, Colorado; P = Point Arena, California; s = SOS/SONIA, S = Scotia, Pennsylva­ nia; T = TOR; 2 = CITE2; 3 = CITE3.

Murphy et al.(1 993) have measured vertical distri­ compared to 03 in the free troposphere, mixing, and in­ butions of NOy and 03 into the stratosphere. Although a put from the stratosphere. strong correlationbetween NOy and 03 was found in the A summary of tropospheric NOx and NOy concen­ stratosphere, they observed only weak to no correlation trations from Prather et al. ( 1994) is shown in Figure 5-9. between these constituents in the troposphere, i.e., the It is based on the compilation of Carroll and Thompson,

tropospheric NOyf03 ratio can be larger and more vari­ (1994) of measurements made by various groups in the able, a reflection of the variety of sources, sinks, and lower and middle troposphere over the U.S. and Europe.

transport processes of NOy and 03 in the troposphere. In Although very high concentrations from urban areas are contrast, Wofsy et al. (1992) and Hubler et al. (1992a, b) excluded, the concentrations of NOx span a range of reported a significant positive correlation when the data three orders of magnitude. On the average, a correlation

are averaged over a large number of observations. The between NOx and NOy is seen. However, the individual observed slope was much steeper than that derived from data sets clearly show that the shorter-lived NOx can still continental boundary layer studies (Section 5.3.2) and vary over an order of magnitude for a given N Oy concen­ approached that found in the stratosphere. The large de­ tration. From this and the differencesin NOx observations crease in the NOyi03 ratio between the continental in the upper troposphere at northern latitudes discussed surface studies and the remote free atmosphere is be­ above, it is clear that present measurements are insufficient

lieved to largely reflect the shorter lifetime of NOy to reasonably describe a meaningful climatology.

5. 17 TROPOSPHERIC OZONE PROCESSES

Aircraft programs have continued to strengthen the median mixing ratio of only 25 ppt, insufficientto over­ role of PAN as a reservoir for NOx, at least in the 3-6 km come average net photochemical destruction of 03 altitude range over continental regions (Singh et al. , (Sandholm et al., 1992). Earlier studies over the conti­ 1992, 1994), where PAN decomposition was able to ac­ nental U.S. by Carroll et al. (1990b) found that air count for much of the observed NOx, a result that masses between the boundary layer and 5-6 km, were emphasizes the role of transport of odd nitrogen reser­ nearly equipartitioned between net loss, approximate voirs. Very high PAN concentrations of up to 200 ppt balance, and net production of 03. were also observed in long-range transport events at Iza­ An extremely interesting finding that yet awaits na during spring, whereas PAN concentrations remained complete explanation is the occurrence of nearly com­ below 20 ppt at the higher temperatures of summer plete 03 depletion in the Arctic surface layer in spring (Schmitt and Hanson, 1993). Other studies have shown (Barrie et al. , 1988; Bottenheim et al. , 1990; McConnell that the importance of PAN as a NOx reservoir is not glo­ et al. , 1992; Fan and Jacob, 1992). A recent analysis of bal. Measurements made in the Northern Hemisphere the ratios of different hydrocarbons provides evidence upper troposphere mostly over the Atlantic Ocean have for bromine chemistry being responsible for the ozone generally shown smaller mixing ratios than observed in removal (Jobson et al. , 1994), although Platt and Haus­ the middle troposphere over continental regions, and mann (1994) argue that the measured BrO concentrations Southern Hemisphere mixing ratios were very small were too small to explain the complete ozone depletion throughout the troposphere (Rudolph et al. , 1987; Per­ on the short time scales implied by the observations. ras, 1994). Similarly, during studies at the Mauna Loa Net ozone loss of 0.5 ppb/day, or -1%/ day, was Observatory experiment, PAN was not a major constitu­ also found in the free troposphere near 3.4 km from ob­ ent. HN03 was the dominant reservoir (median of 43% servations at the Mauna Loa Observatory (Ridley et al., of NOy), followed by NOx (14%), particulate nitrate 1992). The concentrations of peroxy radicals and the (5%), PAN (5%), and alkyl nitrates (2%) (Atlas et al., rate of ozone formation, P(03), were derived from the 1992). photostationary state of NOx (Figure 5-11) and the loss The role of the remote marine PBL as a strong net rate, L(03), was inferred from model calculations based sink for ozone has been clearly identifiedin a large num­ on the measured concentrations of all relevant parame­ ber of investigations, a finding first reported by Liu et al. ters. It is noteworthy that both the total concentration of (1983). For example, a clear anticorrelation in the diur­ H02 and R02 determined during this study, as well as nal and seasonal variation of 03 and H202 was observed the modeled HOz/R02 ratio, are in good agreement with by Ayers et al. (1992) in marine air at Cape Grim, Tas­ recent direct measurements made by matrix isolation mania (Figure 5-10). As is seen in Figure 5-l, H02 and ESR spectroscopy at Izana, Tenerife (D. Mihelcic, radical recombination leads to formation of H202, which private communication). can thus be utilized as a tracer for photochemical activi­ The net destruction rate found in spring at Mauna ty. The results are consistent with net photochemical Loa in the free troposphere is slow enough that vertical destruction of 03 in a very low NOx atmosphere. Net exchange with the marine boundary layer can overrule in photochemical destruction of 03 in the tropical PBL of situ chemistry. Vertical soundings made from a ship up to 25%/day was also inferred from the data gathered cruise in the Pacific clearly demonstrate the importance during several ship cruises (Thompson et al., 1993; Smit of convective transport for the ozone balance of the free et al. , 1989; Smit and Kley 1993; Harris et al., 1992). troposphere. Extremely low ozone concentrations, that The photochemical bufferregions are not confined had their origin in the marine boundary layer, were to the remote maritime lower atmosphere. Aircraft found in the upper troposphere (Smit and K1ey, 1993). flights covering Alaska, northern Ontario and Quebec, These observations contrast those made or modeled over and Labrador have concluded that the surface layer, es­ continental regions, where an emphasis has been on the pecially the boreal forest, was an efficient sink for 03 role of convection of boundary layer precursors in aug­ and NOy (Gregory et al. , 1992; Jacob et al., 1992; Bak­ menting 03 production in the middle and upper win et al., 1992). In some regions of these flights, NOx troposphere (Dickerson et al., 1987; Pickering et al. , was nearly independent of altitude up to 6 km with a l992a, b; Thompson et al. , 1994). As was suggested by

5. 18 TROPOSPHERIC OZONE PROCESSES

a Peroxide and Ozone Average Diurnal Cycles at Cape Grim

15 1100

1000 > > .c 14.5 Q_ 0. .3 .3 �rv��\ w w 900 "' c A< ·;;; 0 14 0 a..� 0 800 0 c c 0 0 .., 13.5 � 700 c ·gc "w w c " 0 c u 13 0 600 u

12.5 500 12 18 24

b Peroxide and Ozone Seasonal Cycles at Cape Grim

35 1400

> 30 1200 > .c Q_ 0. .3 .3 25 1000 -15 w c x � 0 0 20 . //� 800 :;:; "- 0 0 c c 0 15 600 0 ·� -� c w 10 400 c " "w c c 0 0 u v 200 u

0 0 > c > " N 6. 0 "' Q_ u 0 w ;;; lL{; � w "' �

---o-- 03 -- H202

Figure 5-1 0a. Average diurnal cycles for peroxide and ozone in background air at Cape Grim for January 1992 (based upon Ayers et a/., 1992). Figure 5-1 Ob. Seasonal cycles of peroxide and ozone in background air at Cape Grim (based upon Ayers et at., 1992).

modeling studies (Lelieveld and Crutzen, 1994), down­ reduce tropospheric 03 in regions that are removed from ward mesoscale flow in the cloud environment can carry polluted areas. 03 to the Earth's surface, where it is destroyed more rap­ Intensive studies at Mauna Loa have suggested idly. Although these model studies yet await some possible discrepancies in our understanding of the confirmation by experimental data, it is likely that deep atmospheric oxidizing capacity. Programs completed convection tends to increase free tropospheric ozone lev­ more recently may help to determine whether these re­ els downwind of continental source areas but may sults are more universal in the remote troposphere. First, the abundance of formaldehyde (HCHO) predicted from

5. 19 TROPOSPHERIC OZONE PROCESSES

100 weakened compared to that modeled previously. How­ ever, simply decreasing the model lifetime of HN03 will 90 > ..,_ eventually cause significant discrepancies in simulating CL CL 80 the mixing ratios of NOx in the remote atmosphere, since NOx is ultimately lost through HN03 formation. Clear­ "- 70 trl �I ly, more systematic investigations of 03, NOx, and other _J 60 <( NOy species and suitable tracers for transport need to be u 50 I"' conducted in remote locations in order to better under­ 0 <( k stand the interplay between transport and chemistry in 40 v 0::: determining the "global" budget of ozone and its poten­ >- v I\ 30 tial for future increase. X ll i\ 0 II 0::: 20 1':\ w 0... I/ 5.4 FEEDBACK BETWEEN TROPOSPHERIC 10 ��� 1'1 y�.r- OZONE AND LONG-LIVED GREENHOUSE 0 � GASES 4 6 8 10 12 14 16 18 20 HAWAII STANDARD TIME The concentrations of many trace gases that con­ tribute to the greenhouse effect of the atmosphere or are involved in the budget of ozone in the stratosphere or the Figure 5-1 1. Average diurnal variation of peroxy troposphere, i.e. , CH4, CO, NMHC, NOx, methyl bro­ radical mixing ratios derived from measurements of trace gases and photolysis rates during the Mauna mide (CH3Br), HFCs, and HCFCs, are mediated through Loa Observatory Photochemistry Experiment (Rid­ oxidation by OH radicals. Reaction RS-11 followed by ley et a/., 1992). The bars give the mean and RS-12 provides the major source for OH in the unpollut­ standard deviation of the total peroxy radical mixing ed troposphere. Therefore, OH concentrations are ratio estimated from the photostationary state of strongly linked to the UV fluxbelow 320 nm (UV-B) and NOx. The solid lines are model predictions for the the concentrations of water vapor and ozone itself. In mixing ratios of (H02 + CH302) and of CH302, respectively. addition, OH is affected by other trace gases. For this reason, rising levels of CH4, CO, and NOx may lead to changes in the oxidizing capacity of the troposphere (see a model (Liu et al., 1992) was three times larger than the Thompson and Cicerone, 1986), which in turn influ­ observed median (Heikes, 1992). Since measurements ences the concentrations of gases relevant to global of a variety of hydrocarbons (Greenberg et al., 1992) warming and/or stratospheric ozone depletion. showed that CH4 oxidation was the dominant source of On short time scales, increases in UV flux, H20, HCHO, the results implied that model abundance of OH and 03 lead to increases in OH, as is clearly borne out by was too high or, more likely, that other HCHO removal the good correlation found between OH concentrations processes not included in the model were important. and the photolysis frequency of ozone (Platt et al. , Second, the observed HN03/NOx ratio was also in poor 1988). On longer time scales, however, the net effect of agreement with the photochemical model, unless the re­ enhanced UV radiation and H20 concentrations on OH moval rate for HN03 was increased equivalent to a 3-5 depends on the net photochemical balance of ozone day lifetime. More limited aircraft measurements have P(03) - L(03) in the particular region of the atmosphere, also indicated a smaller-than-expected ratio (Huebert et that is, on the NOx concentration, and advective trans­ al. , 1990). The model used by Ehhalt et al. (1992) to port of ozone from other regions. describe the aircraft observations of NO also implied a Besides being required for ozone maintenance, very short average lifetime of NOy, on the order of a few NOx increases change the partitioning between OH and days. If the HN 03 reservoir is indeed removed faster H02 to favor OH via reaction RS-5. Thus, increasing than commonly described in models, the increased NOx will lead to an increase in OH, at least for NOx con­ efficiency of 03 production in the remote atmosphere is centrations below 1 ppb (Hameed et al. , 1979; Logan et

5.20 TROPOSPHERIC OZONE PROCESSES al., 1981). At higher concentrations, reaction (R 5-l5) Among these are CH3CCl3, with an atmospheric turn­ 1 4 becomes the maj or loss process for OH (and HOx == OH over time of about 6 years, and CO, with a turnover + H02) and a further increase of NOx will tend to reduce time of a few months. A detailed discussion of these OH concentrations. At very low NOx levels, e.g., below indirect attempts is given in Chapter 7. a few tens of ppt, recycling of OH occurs via reaction More recently, other potentially important oxi­ (R 5-l3). In this sense, the dependence of OH on NOx in dants in the troposphere have been suggested in addition the remote atmosphere and on long time scales is much to OH. Among these are chlorine atoms (Pszenny et at. , stronger than implied by models that use fixed ozone 1993), which may be formed in the marine boundary concentration fields. layer from reactions of NzOs with aerosol chloride (Fin­ The exact concentration of NOx at which the influ­ laysen-Pitts et at. , 1989; Zetsch and Behnke, 1992). ence of NOx upon OH changes sign depends on the Penkett et at. ( 1993) concluded from the measured ratios concentrations of ozone (see above) and those of other of iso- to normal alkanes in the atmosphere that N03 trace gases such as CO, CH4, and NMHC. The latter radicals could play a significantrole in the atmospheric gases change the HOx partitioning in favor of HOz (R 5-l oxidation of NMHC on larger regional scales, in particu­ to R5-4 and R5-8 to R5-9) and thereby serve to reduce lar at higher latitudes. The importance of these OH. However, this negative influence is not a linear one additional oxidizing reagents is, however, still in the hy­ because of the reduction in HOx losses that proceed via pothesis stage. While it has been suggested that atomic OH reactions, i.e., R5-15. chlorine and bromine could play a role in certain regions Although attempts to measure OH were made in of the troposphere, for example during spring in the Arc­ the early seventies (see Wang et al., 1976; Perner et al., tic (Fan and Jacob, 1992; Jobson et al., 1994), 1976), direct measurements are still extremely sparse concentrations larger than 1% of the average global OH (Perner et al. , 1987; Platt et al., 1988; Felton et al., 1988; concentration seem to be inconsistent with the budgets Dorn et al., 1988; Mount and Eisele, 1992; Eisele et al., of some trace gases (J. Rudolph, private communica­ 1994), in particular in the remote atmosphere. One rea­ tion). son for this is the experimental difficulty involved given the extremely low concentrations of OH radicals due to REFERENCES their reactivity. In addition, because of the fast response Alaart, M., H. Kelder, and L.C.G. Heijboer, On the trans­ of OH to changes in the controlling boundary condi­ port of trace gases by extra-tropical cyclones, in tions, these measurements will not and cannot produce a Proc. Quadr. Ozone Symp., Charlottesville, Virginia, global field of OH concentrations. They can, however, June 4- 13, 1992, in press, 1994. lead to improvements in understanding the chemical Ancellet, G., J. Pelon, M. Beekmann, A. Papayannis, and budget when accompanied by measurements of the con­ G. Megie, Ground-based lidar studies of ozone ex­ trolling factors (Ehhalt et al., 1991) and, hence, help to changes between the stratosphere and the calibrate the photochemical models used to deriveglobal troposphere, J. Geophys. Res., 96, 22401-22421, OH fields (Ehhalt et al. , 1991; Poppe et at., 1994 ), in 1991. particular with the recent advances in measurement ca­ Ancellet, G., M. Beekmann, and A. Papayannis, Impact pability for OH. However, any attempt in modeling the of a cut-off low development on downward trans­ global OH field and, in particular, its secular trend, for port of ozone in the troposphere, J. Geophys. Res., example that induced by the increase in methane con­ 99, 345 1 -3468, 1994. centrations (see Chapter 7) or UV radiation, relies on an Andreae, M.O., The influenceof tropical biomass burn­ accurate knowledge of the distribution and trends of ing on climate and the atmospheric environment, ozone, water vapor, and a number of other parameters, in Biogeochemistryof Global Change: Radiatively but most importantly, on the distribution in space and Active Tra ce Gases, edited by R.S. Oremland, time of NOx. Chapman & Hall, New Yo rk, 113-150, 1993. Average figures on global OH concentrations have been derived from the concentrations of tracers that are removed from the atmosphere preferentially by OH.

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