Detection of Iodine Monoxide in the Tropical Free Troposphere

Detection of iodine monoxide in the tropical free troposphere

Barbara Dixa, Sunil Baidara,b, James F. Breschc, Samuel R. Hallc, K. Sebastian Schmidtd, Siyuan Wanga,b, and Rainer Volkamera,b,1

aDepartment of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0215; bCooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309-0216; cEarth System Laboratory, National Center for Atmospheric Research, Boulder, CO 80307; and dLaboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303

Edited by Mark H. Thiemens, University of California at San Diego, La Jolla, CA, and approved December 21, 2012 (received for review July 19, 2012) Atmospheric iodine monoxide (IO) is a radical that catalytically production from biological sources (2, 14–16). Satellite maps of destroys heat trapping ozone and reacts further to form aerosols. IO over tropical oceans (17) support the notion that iodine Here, we report the detection of IO in the tropical free troposphere sources are mostly related to biological activity (2). However, the (FT). We present vertical profiles from airborne measurements quantitative use of satellite IO data to infer iodine sources over over the Pacific Ocean that show significant IO up to 9.5 km alti- oceans is currently limited by missing information about vertical tude and locate, on average, two-thirds of the total column above distributions, because the satellite sensitivity varies over the the marine boundary layer. IO was observed in both recent deep height of the air column. Furthermore, the relevance of halogen convective outflow and aged free tropospheric air, suggesting a chemistry for ozone loss rates is assessed using atmospheric widespread abundance in the FT over tropical oceans. Our vertical models that cannot be validated because of the lack of halogen profile measurements imply that most of the IO signal detected by radical observations in the tropical free troposphere (FT). First satellites over tropical oceans could originate in the FT, which has model estimates suggest that the combined effect of iodine and implications for our understanding of iodine sources. Surprisingly, bromine species could lead to 10% depletion of tropospheric the IO concentration remains elevated in a transition layer that is ozone per year (18); the largest impact of the halogen-driven decoupled from the ocean surface. This elevated concentration aloft ozone loss is expected in the middle to upper troposphere. To is difficult to reconcile with our current understanding of iodine life- date, there are no aircraft measurements of IO. In this study, we times and may indicate heterogeneous recycling of iodine from aero- report IO observations in the tropical troposphere. Based on our sols back to the gas phase. Chemical model simulations reveal that vertical profiles, we discuss implications for the understanding of the iodine-induced ozone loss occurs mostly above the marine bound- iodine sources and the relevance of the observed IO concen- ary layer (34%), in the transition layer (40%) and FT (26%) and ac- trations for tropospheric ozone loss rates. counts for up to 20% of the overall tropospheric ozone loss rate in the upper FT. Our results suggest that the halogen-driven ozone loss in Results and Discussion the FT is currently underestimated. More research is needed to quan- Measurements. We have measured vertically resolved concen- tify the widespread impact that iodine species of marine origin have trations of the atmospheric gases IO, water vapor (H2O), and on free tropospheric composition, chemistry, and climate. oxygen dimers (O4) and spectral irradiance over much of the tropospheric air column above the remote tropical Pacific atmospheric chemistry | oxidative capacity | halogens | Ocean. Gases are measured simultaneously by the University of heterogeneous chemistry | air-sea exchange Colorado Airborne Multi-AXis Differential Optical Absorption Spectroscopy (CU AMAX-DOAS) instrument (19). Spectral ir- eactive iodine impacts atmospheric chemistry in several radiance was measured by the HIAPER (High-performance Rways. Catalytic reaction cycles involving iodine atoms and Instrumented Airborne Platform for Environmental Research) Airborne Radiation Package (HARP) to obtain cloud optical iodine monoxide (IO; Ix = I + IO) destroy tropospheric ozone, which is a primary source for OH radicals (1, 2). Halogens thickness. Both instruments were mounted aboard the National contribute ∼45% of the ozone loss in the remote tropical marine Science Foundation/National Center for Atmospheric Research SI Text boundary layer (MBL) (2–4). IO further affects the oxidative (NSF/NCAR) GV aircraft (Methods and ). There are no capacity of the atmosphere through fast reactions with HO previous aircraft measurements of IO. Spectral proof of IO de- 2 tection at 0.3, 1.6, and 9.5 km is shown in Fig. 1A. These spectra radicals and the resulting changes in HOx (HOx = OH + HO2) (1, 2). Iodine also affects NO (NO = NO + NO ) by oxidizing provide unambiguous evidence for the presence of IO in the x x 2 tropical remote MBL, transition layer (TL), and FT and the NO to NO2 (1–4). Additionally, bromine atom recycling by IO changing IO abundance with altitude (Fig. 2). EARTH, ATMOSPHERIC, increases ozone destruction and mercury oxidation rates in the AND PLANETARY SCIENCES Spectra were recorded by collecting photons from scattered MBL, resulting in higher mercury deposition rates to ecosystems solar light along well-defined lines of sight (varying angles for- and increased availability to the food chain (2, 5, 6). Finally, in ward of the plane) on January 29, 2010 during a research flight coastal regions, the formation of ultrafine aerosol particles from conducted to show CU AMAX-DOAS performance from Kona, iodine oxides can be a source of cloud condensation nuclei that ’ HI to the south of the Hawaiian archipelago (i.e., between 2° and can modify Earth s albedo and thus, the radiative budget of the 19° N and 145° to 160° W). The flight targeted air masses inside atmosphere (2, 7). Oceans are the main source of iodine to the atmosphere. Most current knowledge of iodine sources and chemistry is based on – Author contributions: R.V. designed research; B.D. and R.V. performed research; S.B., J.F.B., measurements in the MBL (3, 8 13). IO observations at coastal S.R.H., and K.S.S. contributed data/analytic tools; B.D., J.F.B., and R.V. analyzed data; S.W. MBL sites primarily link iodine sources to macroalgae (8–10). and R.V. performed chemical modeling; and B.D. and R.V. wrote the paper. More recent studies have measured IO at open ocean sites (3, The authors declare no conflict of interest. 11–13), suggesting that there might be reactive iodine chemistry This article is a PNAS Direct Submission. over much of the open ocean. Emissions of reactive iodine 1To whom correspondence should be addressed. E-mail: [email protected]. species over the remote ocean remain poorly understood (11, 14) This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. but are currently thought to be associated with primary 1073/pnas.1212386110/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1212386110 PNAS | February 5, 2013 | vol. 110 | no. 6 | 2035–2040 Downloaded by guest on September 25, 2021 Fig. 1. Spectral proof of the detection of IO along the flight track. (A) The measured IO signals at 0.3 (MBL), 1.6 (TL), and 9.5 km (FT) altitude are overlaid on the noise level of the instrument and show the unique (fingerprint) absorption of IO as it varies with altitude. Spectra were recorded between 00:49 Coordinated Universal Time (UTC) and 01:02 UTC at 158.6° W and 6.9°–8.0° N. SCDs and rms noise values are SCD(9.5 km) = 0.7 ± 0.14 × 1013 molecules/cm2 − − (molec/cm2), RMS(9.5 km) = 1.1 × 10 4; SCD(1.6 km) = 1.4 ± 0.16 × 1013 molec/cm2, RMS(1.6 km) = 1.3 × 10 4; and SCD(0.3 km) = 2.0 ± 0.16 × 1013 molec/cm2, − RMS(0.3 km) = 1.2 × 10 4.Thefit uncertainty is indicated by the SCD error. (B)Theflight track is overlaid on a GOES-11 IR satellite image (Geostationary Operational Environmental Satellites, www.ncdc.noaa.gov/gibbs) from January 30, 2010 at 00:00 UTC; it shows a high rising cloud cover (dark blue and green) that is indicative of deep convection. Locations where IO was detected in the FT are shown; altitudes below 1.8 km are shaded orange. During the beginning and end of the flight, no high-sensitivity spectra were recorded.

the MBL as well as in the FT. Air in the FT was probed in find IO mixing ratios of 0.5 ± 0.06 ppt inside the MBL that various distances downwind of tropical deep convective activity decrease by about a factor of five in the FT and remain largely near the Equator. The flight track is shown overlaid on an IR detectable over the entire probed air column. Our vertical pro- satellite cloud image in Fig. 1B together with significant (above files establish that most of the IO vertical column is located detection limit) measurements of IO slant column densities above the MBL. When the concentration of IO is integrated over (SCDs; i.e., integrated concentrations of all photon paths along altitude, then the partial vertical column densities (VCDs) above the line of sight) in the FT (here, above 1.8 km). Lower altitudes 800 m account for about two-thirds of the overall tropospheric have been excluded to emphasize the fact that IO was found in column amounts (65.3%; average of both profiles) (Table 1). significant concentrations over large spatial scales in the FT. The water mixing ratio retrieved by CU AMAX-DOAS for the Vertical trace gas concentration profiles of IO and H2O were ascent profile ranges from 2.4 ± 0.3% to 0.25 ± 0.2% and agrees derived from one aircraft descent and ascent, because maximum well with 2.3 ± 0.4% to 0.26 ± 0.02% measured by a frost point vertical information is gained from measurements during aircraft hygrometer on the plane. This good agreement confirms the altitude changes. Concentration profiles were retrieved between accuracy of our retrievals in complex radiation fields below cirrus 0.3 and 10 km at two different locations indicated as descent and clouds (Figs. S1 and S2). ascent in Fig. 1B. Fig. 2 A and B shows the vertical profiles The retrieved IO profiles in both locations show surprising expressed as volume mixing ratio [IO in parts per trillion (ppt) by similarities over the full tropospheric column, whereas water va- − volume: 1 ppt = 10 12 ≅ 2.5 × 107 molec/cm3 for T = 298 K and por measurements are significantly different above 2.5 km. The P = 1,013 mbar; our conversion is based on aircraft temperature elevated water vapor indicates influences of convective outflow in and pressure measurements]. The vertical distributions of IO the descent profile. By contrast, the ascent profile is markedly and H2O exhibit different but strong variations with altitude. We drier. Fig. 2C shows potential temperature (based on aircraft

Fig. 2. Vertical proﬁles of IO and H2O (in volume mixing ratio; A and B) and potential temperature and aerosol extinction (C). Proﬁles were retrieved for two different locations labeled descent and ascent (Fig. 1B). Open symbols denote data points below detection limit. The corresponding descent and ascent IO VCDs are 2.91 ± 0.78 × 1012 and 2.49 ± 0.72 × 1012 molec/cm2. Error bars indicate the retrieval uncertainty.

2036 | www.pnas.org/cgi/doi/10.1073/pnas.1212386110 Dix et al. Downloaded by guest on September 25, 2021 Table 1. Comparison of averaged retrieved IO VCDs and 8° N (ascent) had not been in contact with the MBL for at least modeled satellite SCDs (×1012 molec/cm2) 60 h (Fig. S5 I–L). Based on the cumulative observational and VCD Cloud cover (%) SCD/VCD bias modeling evidence, we conclude that air inside the MBL is in direct contact with the ocean surface only at the descent location, Total whereas TL air is essentially decoupled from the MBL in both 2.71 0 4.25/1.5 profiling locations (Fig. S3). We define MBL here as the layer 2.71 20 4.34/2.5 extending from the surface to about 800 m, the TL refers to air 2.71 40 4.38/3.8 between 800 m and about 1.8 km, and FT air is above 1.8 km. MBL Implications for potential IO precursors and lifetimes in these 0.94 0 0.99 (23.3%) distinctly different compartments of the atmosphere are dis- 0.94 20 0.60 (13.8%) cussed below. 0.94 40 0.39 (8.9%) TL Widespread IO in the FT. Roughly one-third of the IO vertical 0.76 0 1.17 (27.5%) column is located in the FT. This fraction is estimated conser- 0.76 20 1.29 (29.7%) vatively, and could be up to 30% higher because of a tempera- 0.76 40 1.36 (31.1%) ture dependence of the IO absorption cross-section (24). Our FT observations are consistent with recent first measurements of IO 1.01 0 2.09 (49.2%) in the lower subtropical FT by means of mountain top MAX- 1.01 20 2.45 (56.5%) DOAS (Canary Islands at 2.4 km altitude) that estimate 0.2–0.4 1.01 40 2.63 (60.0%) ppt IO between 1 and 10 km with limited vertical resolution (25). Previous measurements of the atmospheric column abundance The VCD bias expresses the overestimation of the VCD as a factor when of IO in midlatitudes over the continental United States (Kitt the measured SCD signal is converted into a VCD based on an MBL-only vertical profile assumption. Peak Observatory in Arizona at 2 km altitude) show 0.12 ppt of IO in the stratosphere (26). These ground-based direct sun column observations constrain the sum of tropospheric and strato- temperature and pressure measurements) and aerosol extinction spheric IO column amounts, but they do not provide altitude derived from AMAX-DOAS O4 measurements (SI Text and information. The average FT vertical column of our observations Tables S1 and S2). The top of the MBL is marked by an in- is 1 × 1012 molec/cm2. A similar tropospheric VCD over land version of about a 3 K change in potential temperature, in- would have created a signal of the same order of magnitude as dicated by arrows in Fig. 2C, for both the descent and ascent the reference noise reported for the Arizona measurements (26). profiles. During ascent, the potential temperature is not constant Therefore, free tropospheric IO could be present over the conti- within the MBL but decreases continuously to the ocean surface, nental United States and would be consistent with our measure- suggesting a stratified MBL. The TL is decoupled from the MBL ments. In the context of these studies, we conclude that IO is likely and capped by a stronger temperature inversion of around 9 K at a component of lower free tropospheric air on global scales. an altitude of about 1.8 km. The high altitude of the capping Balloon-based measurements in the tropics and mid and high inversion is indicative of a decoupled TL (20, 21). Wood and latitudes reported IO upper limits of 0.1 ppt in the upper FT and Bretherton (20) find that, after the potential temperature in- lower stratosphere (27, 28). Our measurements establish that version height exceeds 1 km, a TL is often decoupled from the this mixing ratio is present over most of the free tropospheric near-surface air below, particularly in far offshore locations such air column and extended spatial scales. as our location within the trade wind belts. Consistent with this finding, our aerosol extinction profiles are relatively constant Relevance for Satellite Retrievals. The marine atmosphere over the below 800 m and decrease rapidly to background levels through open ocean is still one of the most poorly probed atmospheric the TLs in both profiling locations. Water vapor also shows environments on our planet. Satellite measurements are, there- a clear decrease near the top of the MBL and throughout the fore, a particularly useful source of data over remote oceans. To TL. The gradient in water vapor is particularly strong for the convert satellite-retrieved IO SCDs into VCDs, knowledge about ascent profile, where fewer scattered clouds were present than vertical profiles is required. Current atmospheric models cannot during descent (Fig. S3). These observations suggest that MBL, provide accurate profile information because of gaps in our un- TL, and FT are separate dynamical and chemical regimes, which derstanding of the source mechanisms of IO over oceans (11, is further supported by meteorological modeling. 14). We have simulated the satellite view of our measured vertical profiles by using a radiative transfer model. Satellite SCDs Meteorological Modeling. Air mass back trajectories were calcu- assuming a nadir viewing direction from space were calculated

lated with the weather research and forecast model (WRF) (22) for different TL cloud fractions (SI Text and Table S3). The EARTH, ATMOSPHERIC, (Methods). WRF was used to model MBL heights (23), which for comparison of IO FT VCDs (average of both profile locations) AND PLANETARY SCIENCES our study area, are found to range between 500 and 1,000 m. Fig. and corresponding satellite SCDs (Table 1) reveals that FT S4A shows that boundary layer air at both profiling locations VCDs account for roughly one-third of the total columns but originates consistently from easterly low-level trade winds, with contribute about 50% to the measured satellite SCD signal be- no apparent free tropospheric influences for about 50 h (Fig. cause of increased satellite sensitivity to the FT vs. the MBL. S4B). Flow in the TL is also from the east (Fig. S4C). Time-re- This contribution increases with cloud cover, reflecting the effect solved back trajectories initiated at 1.5 km altitude show that of increased sensitivity of satellites to partial columns located there has been almost no dynamic transport from altitudes below above a region of high albedo (i.e., above clouds) and the partial 800 m for the past 10–14 h (Fig. S4 D and E), indicating that air shielding of gases in the boundary layer by clouds. At a moderate in the TL has not been in contact with the ocean surface for cloud cover of 40%, only about 9% of the IO satellite signal about one-half of 1 d. Air masses probed above 2 km are of originates from within the MBL. a different origin for the ascent and descent profiling sites. The conversion of measured satellite SCDs into VCDs is often Starting at 3.5 km, a bifurcation of trajectories is observed near based on the assumption that trace gases are only located in the 8° N (Fig. S5 B–F). Free tropospheric air masses probed to the MBL (2, 11, 13, 17). Based on our profiles, the satellite VCDs south of 8° N (descent) were influenced by convective outflow at could be lower by a factor of 1.5–3, depending on cloud fraction multiple altitudes, whereas free tropospheric air masses north of (expressed as VCD bias in Table 1). This uncertainty reveals the

Dix et al. PNAS | February 5, 2013 | vol. 110 | no. 6 | 2037 Downloaded by guest on September 25, 2021 importance of independent vertical profile information to con- precursors with a longer lifetime and dynamical impacts on their vert SCDs into VCDs. However, if there is, indeed, a widespread distribution. presence of IO in the FT as our analysis suggests, current sat- Interestingly, in the ascent profile, IO decreases by only 24% ellite differential SCD (dSCD; i.e., retrieved SCD with respect to (±24%) from the MBL to the TL. For comparison, the TL is a reference spectrum) measurements may also indicate lower depleted by 60% (±10%) in aerosol extinction and contains 79% limits of the IO abundance, as an unknown amount of IO present (±14%) less water vapor than the MBL; an even higher chemical over the region used to record a satellite reference spectrum (17) depletion is expected for the short-lived iodine precursors (11, would reduce the retrieved satellite dSCDs. Notably, the simu- 14). If the decrease in aerosol extinction and water can be taken lated satellite SCDs from Table 1 are comparable with current as an indicator for the dilution of long-lived iodocarbons, the IO satellite dSCD detection limits over oceans, which are on the source flux from known precursors in the TL is likely much lower 12 2 order of 4–8 × 10 molec/cm (corresponding to rms noise levels (factor of 3–15) than in the MBL. Furthermore, the Ix lifetime − of 1–2 × 10 4 for single acquisitions) (17). This effect could, thus, with respect to irreversible uptake of IO and HOI to aerosol be of similar magnitude but opposite sign than the VCD bias surfaces is on the order of 1 h (for a typical aerosol surface area − caused by assumptions on vertical distributions (Table 1). Im- of 10 mm2m 3 in the TL). In contrast, a 24% decrease in IO over proved signal to noise from averaging existing satellite data in 12 h (see above) suggests a much longer effective IO lifetime space or time is a very promising method to further investigate (∼44 h). These Ix lifetimes might be conservatively estimated. the possible global presence of IO. Advances in satellite IO We conclude that, to sustain the elevated IO concentrations in measurements also mean an increasing need for an independent the TL, an efficient IO regeneration mechanism must be oper- assessment of the measured dSCDs and vertical trace gas dis- ating. We hypothesize that iodine recycling from aerosols back to tributions, which is now possible from aircraft. the gas phase sustains IO concentrations in the TL. Aqueous surfaces containing iodide are known to release I2 and IO on Implications for Iodine Sources. An increasing body of evidence reaction with ozone (32, 35); additionally, the multiphase re- from laboratory experiments (29–32), field observations (11, 12, action of HOI with dissolved halides could contribute to recy- 14), and modeling studies (11, 14) suggests that very short-lived cling of iodine back to the gas phase, analogous to bromine polyhalogenated iodocarbons, such as diiodomethane (CH2I2; chemistry (36). Iodine is also an abundant component of FT – photolytic life time of 2 10 min), bromoiodomethane (CH2IBr; aerosols (37). Based on the similarity of our IO vertical profiles – – 1 2.5 h), or chloroiodomethane (CH2ICl; 2.4 8 h), as well as in the FT, we speculate that the recycling from aerosols back to fi molecular iodine (I2; 15 s) contribute signi cantly to the iodine the gas phase could further extend the IO effective lifetime in the source flux in the MBL and are needed to sustain elevated IO FT. There are currently no simultaneous measurements of IO abundances in the remote MBL (2, 16, 33). Our airborne ob- and iodocarbons in the FT. servations of ∼0.5–0.6 ppt IO in the central Pacific MBL are slightly lower but generally consistent with ∼1.5 ppt IO at Cape Relevance for Atmospheric Chemistry. Tropospheric ozone is a Verde Islands in the tropical Atlantic ocean (3, 11), ∼0.9 ppt greenhouse gas and the primary source for OH radicals, which from ship observations over the Eastern Pacific (12, 13), and up are an important sink for methane in the tropical atmosphere to a few ppt over upwelling areas a few hundred kilometers from (38). Atmospheric models estimate that halogen-mediated ozone the Peruvian coast (12). However, in our vertical profiles, the loss could deplete the tropospheric ozone column by 10% per larger share of the IO VCD is located above the MBL. In par- year (18). These estimates are as of yet unconstrained by halogen ticular, elevated IO in the decoupled TL and aged FT air cannot radical observations in the FT and predict ∼0.02 ppt IO in the be explained by polyhalogenated iodocarbons and I2, which react upper troposphere over the tropical Atlantic (18), which is inside the MBL. Only iodocarbons like methyl iodide (CH3I; a factor of five less IO than the upper limit reported for this lifetime of 5–6 d) or ethyl iodide (C2H5I; 4 d) and possibly, region (28) and the FT–IO mixing ratio that we find over the aerosols are precursors that are long-lived enough to carry iodine Central Pacific ocean. The possibility of iodine recycling from into the remote upper tropical FT by means of tropical deep aerosols could further extend the effective iodine lifetime and convective transport pathways. Among these compounds, CH3I add an additional ozone loss pathway that is not yet considered is the most abundant (2, 15). in atmospheric models. We investigate the relevance of halogen- Distributions of chlorophyll-a (Chl-a) in the surface ocean are mediated ozone loss as a function of altitude using our IO ver- currently being used to scale very short-lived marine iodocarbon tical profiles and other flight observations to constrain a photo- emission (16, 18). Positive correlations between satellite IO and chemical model (SI Text). Fig. 3A shows that the ozone loss rate is Aqua/Moderate Resolution Imaging Spectroradiometer (MODIS) a strong function of altitude, whereas the iodine contribution to satellite-derived Chl-a (SI Text and Figs. S6 and S7), indeed, the overall ozone loss rate (Fig. 3A) is a strong function of ozone seem to provide some evidence for the relevance of biological concentration (Fig. 3B). Fig. 3A assumes that the average ozone sources over the Eastern Pacific, but they are in contrast to recent vertical profile as measured during the Pacific Exploratory Mis- ship-based measurements that show the lowest Ix in areas of sion (PEM) Tropics field campaign (same area as our flight track high Chl-a (i.e., show a negative correlation between Chl-a and during March of 1999) is representative for our case study and Ix in the MBL) (13). Our profiles suggest that scattered light further assumes that 0.5 ppt bromine oxide (BrO) is present over satellite IO signals over tropical oceans primarily indicate FT-IO. the entire tropospheric air column (6, 18, 39–41) (SI Text). Under This finding implies that satellites could provide information that the rather low ozone concentrations observed during PEM tropics is essentially decoupled from the ocean surface (Table 1). IO in (Table S4), iodine chemistry determines 23%, 26%, and 11% of the FT could, thus, help explain why IO columns measured from the overall ozone loss rate in the MBL, TL, and FT, respectively. satellites and ships scale differently with Chl-a observations. The Sensitivity studies (Fig. 3B and Fig. S8 A and B) show that, for spatial disconnect between atmospheric IO and the Chl-a con- ozone concentrations below 40–50 parts per billion (ppb), the centration, however, poses questions about which iodine emis- fraction of iodine-induced ozone loss generally is around 10% sions can legitimately be scaled using Chl-a distributions. Notably, (7–15%) over most of the tropospheric air column. Higher ozone some of the highest CH3I concentrations observed are found is the primary reason why this fraction decreases by up to two over the Eastern Pacific ocean (34), where deep convection also orders of magnitude in the stratosphere (Fig. 3B), consistent with provides a transport pathway into the FT. The apparent cor- previous estimates (26). At constant O3, our simulations show an relation between satellite IO and Chl-a could possibly be be- increase in the OH radical concentrations caused by the IO + cause of the coupled effect of biological sources producing IO HO2 reaction that is most prominent in the MBL (7.8%) and

2038 | www.pnas.org/cgi/doi/10.1073/pnas.1212386110 Dix et al. Downloaded by guest on September 25, 2021 chemistry. The chemical state of the TL remains poorly understood and warrants increased attention in future field studies. Simul- taneous measurements of IO and its precursors are now possible from research aircraft, and they hold great potential to shed light on the relative importance of organic iodine precursors and iodine recycling pathways from aerosols as sources for IO in the FT and advance our understanding of iodine chemistry in the global atmosphere. Methods We measured solar scattered light spectra with the CU AMAX-DOAS instrument (19) on board the NSF/NCAR GV research aircraft (HIAPER) during a9-hflight on January 29, 2010. The descent and ascent profiles were flown from 00:47 to 01:03 Coordinated Universal Time (UTC) and 01:40 to 02:25 UTC (January 30, 2010) at solar zenith angles of 39.0°–43.2° and 51.8°–64.2°, 6.6° N/158.7° W to 8.1° N/158.5° W, and 10.6° N/158.0° W to 14.2° N/157.2° W (a distance of about 400 km apart). The profile retrieval uses a two-step process: (i) DOAS analysis of spectra to retrieve trace gas SCDs and (ii)profile retrieval from inverse radiative transfer modeling (SI Text). Based on measurement errors and vertical information content of the SCDs, absolute detection limits in the FT for our profile retrieval are 0.06 ppt for IO and 0.05%

for H2O. Partial column amounts were integrated over altitude for the MBL (0–800 m), TL (800–1,800 m), and FT (above 1.8 km). The HARP is a comprehensive atmospheric radiation suite used to measure spectrally resolved in situ actinic flux and irradiance. Up-welling and down- welling irradiance was measured from 300 to 2,400 nm at 1 Hz. Cloud optical thickness was derived from either transmitted or reflected irradiance at 500 nm. The radiative transfer model McArtim (43) was used for the interpretation of our profile retrievals and calculation of the satellite view of our profiles. The profile retrievals used McArtim as the forward model combined with an in house–developed inversion algorithm to account for the light path dependency of the measured SCDs. Atmospheric constraints were provided

by avionics temperature and pressure proﬁles, O4 columns measured by CU AMAX-DOAS, and cloud optical depth inferred from HARP (SI Text). Satellite SCDs were simulated using McArtim-calculated weighting functions Fig. 3. Ozone loss simulations. (A) The total ozone loss rate and percent and representative settings for current instruments measuring solar scattered light [e.g., Global Ozone Monitoring Experiment (GOME), Scanning Imaging contributions constrained by IO observations and simulated bromine, HOx, Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY), Ozone photolysis, and NOy chemistry during the ascent proﬁle (base case simulation; conditions in SI Text). (B) Sensitivity of the percentage contribution Monitoring Instrument (OMI), and GOME-2] (SI Text).

of iodine-induced ozone loss to the ozone mixing ratio (base case, O3-varied). Two separate model runs were conducted with WRF (22): (i) a 60-h The shaded orange line indicates base case ozone levels (A). forecast with 30-km horizontal grid spacing and (ii) a 24-h forecast with increased horizontal resolution (12 km) to better resolve local deep convection. The model was initialized at 12:00 UTC on January 27, 2010 for the leads to increases of 4.6% and <0.5% in the TL and FT, respec- 60-h run and 00:00 UTC on January 29 for the 24-h run. The domain was tively (SI Text and Table S5). The fraction of iodine-induced centered at 9° N, 156° W and covered 20° in each direction for the longer run ozone loss is largely insensitive to photolysis frequencies and and 8° for the shorter period. Initial conditions were based on National Centers for Environmental Predictions–Global Forecast System (NCEP-GFS) NO2 (Fig. S8 C and F). A analyses. Trajectories were computed from the WRF output for a grid of In our base case simulation (Fig. 3 ), 0.5 ppt BrO and mea- ﬂ = + points along the ight track. sured IO correspond to 0.61 ppt Brx (Brx Br BrO) and 0.23 The ozone loss rate calculations were performed using a photochemical ppt Ix (average mixing ratio below 10 km), which are responsible box model in which modeled species reach steady state over multiple days. for 19% and 14% of the column average ozone loss rate below The model conceptually follows the work by Crawford et al. (44), and the 10 km, respectively. Per Xx molecule (X = Br, I), iodine is about chemical mechanism includes both gas-phase and heterogeneous reactions ﬁ

two times as ef cient at destroying ozone than bromine. In ab- of iodine and bromine species based on works by Ordóñez et al. (16), EARTH, ATMOSPHERIC, sence of any BrO, the fraction of iodine-induced ozone loss Sommariva et al. (45), and Parrella et al. (39). Details and sensitivity studies AND PLANETARY SCIENCES would be slightly larger (Fig. S8D). Arguably, uncertain BrO are in SI Text. concentrations in the FT currently limit our ability to quantify the impact of halogen-mediated tropospheric ozone loss (Fig. ACKNOWLEDGMENTS. We thank the National Center for Atmospheric Re- search/Earth Observing Laboratory for support during the aircraft integra- S8), and therefore, simultaneous observations of BrO and IO tion and operation, particularly Brigitte Baeuerle and Pavel Romashkin; from research aircraft are desirable. the whole Volkamer group for support during instrument preparation; Interestingly, most of the iodine-induced partial column ozone H. Oetjen for helpful discussions; D. Thomson of Original Code Consulting for loss occurs in the TL (40%), with MBL and FT contributing 34% developing software; and Eleanor Waxman for proofreading the manuscript. T. Deutschmann provided the McArtim radiative transfer code. S.B. is a recip- and 26%. The TL is a unique chemical environment character- ient of a ESRL/CIRES graduate fellowship. S.W. is a recipient of a Fulbright ized by moderately warm temperatures, the presence of water, fellowship. R.V. acknowledges ﬁnancial support from National Science and aqueous (low viscosity) aerosols and clouds. Entrainment of Foundation Faculty Early Career Development (CAREER) Award ATM-0847793 ozone from the FT often leads to higher ozone concentrations and National Science Foundation Grant NSF-AGS-1104104. California Air Resources Board Contract 09-317; Department of Energy Award DE-SC0006080; here than in the MBL (42), and the lack of contact with the and Electric Power Research Institute (EPRI) contracts EP-P27450/C13049 and ocean surface can extend the lifetime of reactive species like EP-P32238/C14974 supported the development of software/data analysis IO, thus increasing its relevance for HOx,ozone,andmercury tools used in this study.

Dix et al. PNAS | February 5, 2013 | vol. 110 | no. 6 | 2039 Downloaded by guest on September 25, 2021 1. von Glasow R, Crutzen PJ (2003) Tropospheric halogen chemistry. The Atmosphere 24. Schönhardt A, et al. (2012) Simultaneous satellite observations of IO and BrO over Vol. 4 Treatise on Geochemistry, eds Keeling RF, Holland HD, Turekian KK (Elsevier- Antarctica. Atmos Chem Phys 12(14):6565–6580. Pergamon, Oxford), pp 21–64. 25. Puentedura O, et al. (2012) Iodine monoxide in the north subtropical free tropo- 2. Saiz-Lopez A, et al. (2012) Atmospheric chemistry of iodine. Chem Rev 112(3): sphere. Atmos Chem Phys 12(11):4909–4921. 1773–1804. 26. Wennberg PO, Brault JW, Hanisco TF, Salawitch RJ, Mount GH (1997) The atmospheric 3. Read KA, et al. (2008) Extensive halogen-mediated ozone destruction over the column abundance of IO: Implications for stratospheric ozone. J Geophys Res Atmos tropical Atlantic Ocean. Nature 453(7199):1232–1235. 102(D7):8887–8898. 4. Keene WC, et al. (2009) Latitudinal variation in the multiphase chemical processing of 27. Pundt I, Pommereau JP, Phillips C, Lateltin E (1998) Upper limit of iodine oxide in the inorganic halogens and related species over the eastern North and South Atlantic lower stratosphere. J Atmos Chem 30(1):173–185. Oceans. Atmos Chem Phys 9(19):7361–7385. 28. Butz A, et al. (2009) Constraints on inorganic gaseous iodine in the tropical upper 5. Goodsite ME, Plane JMC, Skov H (2004) A theoretical study of the oxidation of Hg0 to troposphere and stratosphere inferred from balloon-borne solar occultation ob- – HgBr2 in the troposphere. Environ Sci Technol 38(6):1772–1776. servations. Atmos Chem Phys 9(18):7229 7242. 6. Coburn S, Dix B, Sinreich R, Volkamer R (2011) The CU ground MAX-DOAS instrument: 29. Martino M, et al. (2009) A new source of volatile organoiodine compounds in surface Characterization of RMS noise limitations and first measurements near Pensacola, seawater. Geophys Res Lett 36:L01609. FL of BrO, IO, and CHOCHO. Atmos Meas Tech 4(11):2421–2439. 30. Reeser DI, et al. (2009) Photoenhanced reaction of ozone with chlorophyll at the – 7. O’Dowd CD, et al. (2002) Marine aerosol formation from biogenic iodine emissions. seawater surface. J Phys Chem C 113(6):2071 2077. Nature 417(6889):632–636. 31. Sakamoto Y, Yabushita A, Kawasaki M, Enami S (2009) Direct emission of I2 molecule 8. Alicke B, Hebestreit K, Stutz J, Platt U (1999) Iodine oxide in the marine boundary and IO radical from the heterogeneous reactions of gaseous ozone with aqueous – layer. Nature 397(6720):572–573. potassium iodide solution. J Phys Chem A 113(27):7707 7713. 9. Saiz-Lopez A, Plane JMC (2004) Novel iodine chemistry in the marine boundary layer. 32. Hayase S, et al. (2010) Heterogeneous reaction of gaseous ozone with aqueous iodide in the presence of aqueous organic species. J Phys Chem A 114(19):6016–6021. Geophys Res Lett 31(4):L04112. 33. Yokouchi Y, Saito T, Ooki A, Mukai H (2011) Diurnal and seasonal variations of io- 10. Wada R, Beames JM, Orr-Ewing AJ (2007) Measurement of IO radical concentrations docarbons (CH2Cll, CH2I2, CH3I, and C2H5I) in the marine atmosphere. J Geophys Res in the marine boundary layer using a cavity ring-down spectrometer. J Atmos Chem Atmos 116:D06301. 58(1):69–87. 34. Yokouchi Y, et al. (2008) Global distribution and seasonal concentration change of 11. Mahajan AS, et al. (2010) Measurement and modelling of tropospheric reactive hal- methyl iodide in the atmosphere. J Geophys Res Atmos 113(D18):D18311. ogen species over the tropical Atlantic Ocean. Atmos Chem Phys 10(10):4611–4624. 35. Garland JA, Curtis H (1981) Emission of iodine from the sea-surface in the presence of 12. Volkamer R, Coburn S, Dix B, Sinreich R (2010) The Eastern Pacific Ocean is a source for ozone. J Geophys Res Ocean Atmos 86(NC4):3183–3186. short lived trace gases: Glyoxal and iodine oxide. Clivar Exchanges 53(2):30–33. 36. Sander SP, et al. (2006) Chemical Kinetics and Photochemical Data for Use in Atmo- 13. Mahajan AS, et al. (2012) Latitudinal distribution of reactive iodine in the Eastern spheric Studies—Evaluation Number 15. Technical Report, JPL Publication (Jet Pro- Pacific Ocean and its link to open ocean sources. Atmos Chem Phys 12(23):11609–11617. pulsion Laboratory, Pasadena, CA), Vol 06-2. 14. Jones CE, et al. (2010) Quantifying the contribution of marine organic gases to at- 37. Murphy DM, Thomson DS, Mahoney MJ (1998) In situ measurements of organics, mospheric iodine. Geophys Res Lett 37:L18804. meteoritic material, mercury, and other elements in aerosols at 5 to 19 kilometers. 15. Carpenter LJ, et al. (1999) Short-lived alkyl iodides and bromides at Mace Head, Ire- Science 282(5394):1664–1669. land: Links to biogenic sources and halogen oxide production. J Geophys Res Atmos 38. Change C (2007) 2007: Working Group I: The physical science basis. Contribution of – 104(D1):1679 1689. Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on 16. Ordóñez C, et al. (2012) Bromine and iodine chemistry in a global chemistry-climate Climate Change, 2007, eds Solomon S, et al. (Cambridge Univ Press, Cambridge, model: Description and evaluation of very short-lived oceanic sources. Atmos Chem United Kingdom). – Phys 12(3):1423 1447. 39. Parrella JP, et al. (2012) Tropospheric bromine chemistry: Implications for present and 17. Schönhardt A, et al. (2008) Observations of iodine monoxide columns from satellite. pre-industrial ozone and mercury. Atmos Chem Phys 12(15):6723–6740. – Atmos Chem Phys 8(3):637 653. 40. Theys N, et al. (2007) Retrieval of stratospheric and tropospheric BrO columns from fi 18. Saiz-Lopez A, et al. (2012) Estimating the climate signi cance of halogen-driven multi-axis DOAS measurements at Reunion Island (21°S, 56°E). Atmos Chem Phys 7 – ozone loss in the tropical marine troposphere. Atmos Chem Phys 12(9):3939 3949. (18):4733–4749. 19. Volkamer R, Coburn S, Dix B, Sinreich R (2009) MAX-DOAS observations from ground, 41. Theys N, et al. (2011) Global observations of tropospheric BrO columns using GOME-2 ship, and research aircraft: Maximizing signal-to-noise to measure “weak” absorbers. satellite data. Atmos Chem Phys 11(4):1791–1811. Proc SPIE 7462:746203. 42. Stoller P, et al. (1999) Measurements of atmospheric layers from the NASA DC-8 and 20. Wood R, Bretherton CS (2004) Boundary layer depth, entrainment, and decoupling P-3B aircraft during PEM-Tropics A. J Geophys Res Atmos 104(D5):5745–5764. in the cloud-capped subtropical and tropical marine boundary layer. J Clim 17(18): 43. Deutschmann T, et al. (2011) The Monte Carlo atmospheric radiative transfer model 3576–3588. McArtim: Introduction and validation of Jacobians and 3D features. J Quant Spectrosc 21. Jones CR, Bretherton CS, Leon D (2011) Coupled vs. decoupled boundary layers in Radiat Transf 112(6):1119–1137. VOCALS-Rex. Atmos Chem Phys 11(14):7143–7153. 44. Crawford J, et al. (1999) Assessment of upper tropospheric HOx sources over the 22. Dee DP, et al. (2011) The ERA-Interim reanalysis: Configuration and performance of tropical Pacific based on NASA GTE/PEM data: Net effect on HOx and other photo- the data assimilation system. Quart J Royal Met Soc 137(656):553–597. chemical parameters. J Geophys Res Atmos 104(D13):16255–16273. 23. Hong S-Y, Yign N, Dudhia J (2006) A new vertical diffusion package with an explicit 45. Sommariva R, Bloss WJ, von Glasow R (2012) Uncertainties in gas-phase atmospheric treatment of entrainment processes. Mon Weather Rev 134(9):2318–2341. iodine chemistry. Atmos Environ 57:219–232.

2040 | www.pnas.org/cgi/doi/10.1073/pnas.1212386110 Dix et al. Downloaded by guest on September 25, 2021