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Articles That Also Dehydrate the Weighted (12.4 × 105 Molecules Cm−3) and Methyl Chloro- Air That Ascends Into the Stratosphere (Lelieveld Et Al., 2007)

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Articles That Also Dehydrate the Weighted (12.4 × 105 Molecules Cm−3) and Methyl Chloro- Air That Ascends Into the Stratosphere (Lelieveld Et Al., 2007) Atmos. Chem. Phys., 16, 12477–12493, 2016 www.atmos-chem-phys.net/16/12477/2016/ doi:10.5194/acp-16-12477-2016 © Author(s) 2016. CC Attribution 3.0 License. Global tropospheric hydroxyl distribution, budget and reactivity Jos Lelieveld, Sergey Gromov, Andrea Pozzer, and Domenico Taraborrelli Max Planck Institute for Chemistry, Atmospheric Chemistry Department, P.O. Box 3060, 55020 Mainz, Germany Correspondence to: Jos Lelieveld (jos.lelieveld@mpic.de) Received: 29 February 2016 – Published in Atmos. Chem. Phys. Discuss.: 11 March 2016 Revised: 27 August 2016 – Accepted: 18 September 2016 – Published: 5 October 2016 Abstract. The self-cleaning or oxidation capacity of the at- ciency) has been recognized since the early 1970s (Levy II, mosphere is principally controlled by hydroxyl (OH) radicals 1971; Crutzen, 1973; Logan et al., 1981, Ehhalt et al., 1991). in the troposphere. Hydroxyl has primary (P ) and secondary The primary OH formation rate (P ) depends on the photodis- (S) sources, the former mainly through the photodissocia- sociation of ozone (O3/ by ultraviolet (UV) sunlight – with tion of ozone, the latter through OH recycling in radical reac- a wavelength of the photon (hv) shorter than 330 nm – in the tion chains. We used the recent Mainz Organics Mechanism presence of water vapor: (MOM) to advance volatile organic carbon (VOC) chemistry C ! 1 C in the general circulation model EMAC (ECHAM/MESSy O3 hv (λ < 330nm/ O. D/ O2; (R1) 1 Atmospheric Chemistry) and show that S is larger than pre- O. D/ C H2O ! 2OH: (R2) viously assumed. By including emissions of a large number · of primary VOC, and accounting for their complete break- (Note that the formal notation of hydroxyl is HO , indicat- down and intermediate products, MOM is mass-conserving ing one unpaired electron on the oxygen atom. For brevity we and calculates substantially higher OH reactivity from VOC omit the dot and use the notation OH, and similarly for other oxidation compared to predecessor models. Whereas pre- radicals.) Since the stratospheric ozone layer in the tropics viously P and S were found to be of similar magnitude, is relatively thin, UV radiation is less strongly attenuated the present work indicates that S may be twice as large, compared to the extratropics; additionally, because the so- mostly due to OH recycling in the free troposphere. Fur- lar zenith angle and water vapor concentrations are relatively ther, we find that nighttime OH formation may be signifi- high, zonal OH is highest at low latitudes in the lower to cant in the polluted subtropical boundary layer in summer. middle troposphere (Crutzen and Zimmermann, 1991; Spi- With a mean OH recycling probability of about 67 %, global vakovsky et al., 2000). OH is buffered and not sensitive to perturbations by natu- The OH radicals attack reduced and partly oxidized gases ral or anthropogenic emission changes. Complementary pri- such as methane (CH4/, non-methane volatile organic com- mary and secondary OH formation mechanisms in pristine pounds (VOCs) and carbon monoxide (CO), so that these and polluted environments in the continental and marine tro- gases only occur in trace amounts, e.g., posphere, connected through long-range transport of O , can 3 CO C OH ! CO2 C H; (R3) maintain stable global OH levels. H C O2.CM/ ! HO2.CM/; (R4) where M is an air molecule that removes excess energy from reaction intermediates by collisional dissipation. Be- 1 Introduction cause OH is highly reactive, it has an average tropospheric lifetime of about 1–2 s. After the initial OH reaction (Reac- The removal of most natural and anthropogenic gases from tion R3) peroxy radicals are produced (Reaction R4), which the atmosphere – important for air quality, the ozone layer can combine to form peroxides: and climate – takes place through their oxidation by hydroxyl (OH) radicals in the troposphere. The central role of tropo- spheric OH in the atmospheric oxidation capacity (or effi- HO2 C HO2 ! H2O2 C O2; (R5) Published by Copernicus Publications on behalf of the European Geosciences Union. 12478 J. Lelieveld et al.: Global tropospheric hydroxyl distribution, budget and reactivity influence on OH (their Fig. 6). Therefore, at r > 60 % the at- mospheric chemical system can be considered to be buffered. RO2 C HO2 ! ROOH C O2; (R6a) While globally r has remained approximately constant, the ! RO C O2 C OH; (R6b) mean tropospheric OH concentration and the lifetime of CH4 (τ / have also changed comparatively little, for example ! ROH C O3: (R6c) CH4 within a spread of about 15 % calculated by a 17-member RH is a VOC from which OH can abstract the hydrogen ensemble of atmospheric chemistry-transport models (Naik to form water and an alkyl radical, which reacts with O2 to et al., 2013). Despite substantial differences in OH concen- form a peroxy radical, RO2. After peroxide formation (Re- trations and τCH4 among the models, simulations of emission actions R5, R6a) the reaction chains can either propagate or scenarios according to several representative concentration terminate, the latter by deposition. Propagation of the chain pathways (RCPs) indicate that future OH changes will prob- leads to higher-generation reaction products and secondary ably also be small, i.e., well within 10 % (Voulgarakis et al., OH formation (S), which can be understood as OH recycling. 2013). We interpret the relative constancy of r, mean OH and For example, the photolysis of ROOH leads to OH produc- τCH4 as an indication that global OH is buffered against per- tion. In air that is directly influenced by pollution emissions S turbations. This is corroborated by studies based on obser- is largely controlled by nitrogen oxides (NO C NO2 D NOx/: vations of methyl chloroform, with known sources and OH reaction as the main sink, showing small interannual vari- NO C HO2 ! NO2 C OH: (R7) ability of global OH and small interhemispheric difference in OH (Krol and Lelieveld, 2003; Montzka et al., 2011; Patra This reaction, referred to as the NOx recycling mechanism et al., 2014). of OH, also leads to ozone production through photodissoci- For our previous estimates of P and S we used a ation of NO2 by ultraviolet and visible light: chemistry-transport model with the Carbon Bond Mech- anism (CBM) to represent non-hydrocarbon chemistry NO C hv (λ < 430nm/ ! NO C O.3P/; (R8) 2 (Houweling et al., 1998). This mechanism aggregates or- 3 O. P/ C O2.CM/ ! O3.CM/: (R9) ganic compounds into categories of species according to molecular groups and has been successfully used to simu- However, in strongly polluted air NO2 can locally be a late ozone concentrations with air quality models (Stockwell large OH sink, and in such environments the net effect of et al., 2012). However, such chemical schemes are not mass- NOx on OH is self-limiting through the reaction conserving, e.g., for carbon, and are optimized for conditions in which NOx dominates r, while in low-NOx environments NO2 C OH.CM/ ! HNO3.CM/: (R10) other mechanisms may be important, for example through Conversely, under low-NOx conditions, mostly in pristine the chemistry of non-methane VOCs emitted by vegetation air, secondary OH formation by other mechanisms is impor- (Lelieveld et al., 2008), as reviewed by Vereecken and Fran- tant: cisco (2012), Stone et al. (2012) and Monks et al. (2015). A limitation of the CBM and other, similar mechanisms is that O3 C HO2 ! 2O2 C OH; (R11) second- and higher-generation reaction products are lumped H2O2 C hv (λ < 550nm/ ! OH C OH: (R12) or ignored for computational efficiency, whereas they can contribute importantly to OH recycling and ozone chemistry These reactions are referred to as the Ox recycling mech- (Butler et al., 2011; Taraborrelli et al., 2012). anism of OH. In prior work we suggested that the strong Here we apply the Mainz Organics Mechanism (MOM), growth of air pollution since industrialization, especially in which accounts for recent developments in atmospheric VOC the 20th century, has drastically changed OH production and chemistry. MOM is a further development of the Mainz Iso- loss rates but that globally the balance between P and S prene Mechanism (Taraborrelli et al., 2009, 2012). In ad- changed little (Lelieveld et al., 2002). This is associated with dition to isoprene, MOM computes the chemistry of satu- a relatively constant OH recycling probability r, defined as rated and unsaturated hydrocarbons, including terpenes and r D 1−P =G, in which G is gross OH formation (G D P CS); aromatics (Cabrera-Perez et al., 2016). We use it to esti- P , S and G have unit moles yr−1. We computed that globally mate the role of radical production through reactions of oxi- r has changed little since preindustrial times, remaining at dized VOC, referred to as the OVOC recycling mechanism of about 50 %. Thus, in the past century G (the atmospheric ox- OH, being contrasted with the NOx and Ox recycling mech- idation power) kept pace with the growing OH sink related to anisms of OH. Based on this scheme, implemented in the the emissions of reduced and partly oxidized pollution gases. ECHAM/MESSy Atmospheric Chemistry general circula- Lelieveld et al. (2002) performed perturbation simulations, tion model (EMAC), we provide an update of global OH cal- applying pulse emissions of NOx and CH4, to compute the culations, sources, sinks, tropospheric distributions, OH re- impact on OH. This showed that, at an OH recycling proba- activity, and the lifetime of CH4 and CO, and we discuss im- bility of 60 % or higher, these perturbations have negligible plications for atmospheric chemistry. We contrast the bound- Atmos. Chem. Phys., 16, 12477–12493, 2016 www.atmos-chem-phys.net/16/12477/2016/ J.
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