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Abstract. In this paper, we present a new version of the chemistry–climate model SOCOL-AERv2 supplemented by an iodine chemistry module. We perform three 20-year ensemble experiments to assess the validity of the modeled iodine and to quantify the effects of iodine on ozone. The iodine distributions obtained with SOCOL-AERv2-I agree well with AMAX-DOAS observations and with CAM-chem model simulations. For the present-day atmosphere, the model suggests that the iodine-induced chemistry leads to a 3 %–4 % reduction in the ozone column, which is greatest at high latitudes. The model indicates the strongest influence of iodine in the lower stratosphere with 30 ppbv less ozone at low latitudes and up to 100 ppbv less at high latitudes. In the troposphere, the account of the iodine chemistry reduces the tropospheric ozone concentration by 5 %–10 % depending on geographical location. In the lower troposphere, 75 % of the modeled ozone reduction originates from inorganic sources of iodine, 25 % from organic sources of iodine. At 50 hPa, the results show that the impacts of iodine from both sources are comparable. Finally, we determine the sensitivity of ozone to iodine by applying a 2-fold increase in iodine emissions, as it might be representative for iodine by the end of this century. This reduces the ozone column globally by an additional 1.5 %–2.5 %. Our results demonstrate the sensitivity of atmospheric ozone to iodine chemistry for present and future conditions, but uncertainties remain high due to the paucity of observational data of iodine species. 

Abstract. We use the GEOS-Chem chemical transport model to examine theinfluence of bromine release from blowing-snow sea salt aerosol (SSA) onspringtime bromine activation and O3 depletion events (ODEs) in theArctic lower troposphere. We evaluate our simulation against observations oftropospheric BrO vertical column densities (VCDtropo) from the GOME-2 (second Global Ozone Monitoring Experiment)and Ozone Monitoring Instrument (OMI) spaceborne instruments for 3 years (2007–2009), as well asagainst surface observations of O3. We conduct a simulation withblowing-snow SSA emissions from first-year sea ice (FYI; with a surface snowsalinity of 0.1 psu) and multi-year sea ice (MYI; with a surface snowsalinity of 0.05 psu), assuming a factor of 5 bromide enrichment of surfacesnow relative to seawater. This simulation captures the magnitude ofobserved March–April GOME-2 and OMI VCDtropo to within 17 %, as wellas their spatiotemporal variability (r=0.76–0.85). Many of the large-scalebromine explosions are successfully reproduced, with the exception of eventsin May, which are absent or systematically underpredicted in the model. Ifwe assume a lower salinity on MYI (0.01 psu), some of the bromine explosionsevents observed over MYI are not captured, suggesting that blowing snow overMYI is an important source of bromine activation. We find that the modeledatmospheric deposition onto snow-covered sea ice becomes highly enriched inbromide, increasing from enrichment factors of ∼5 inSeptember–February to 10–60 in May, consistent with composition observations of freshly fallen snow. We propose that this progressive enrichment indeposition could enable blowing-snow-induced halogen activation to propagateinto May and might explain our late-spring underestimate in VCDtropo.We estimate that the atmospheric deposition of SSA could increase snow salinityby up to 0.04 psu between February and April, which could be an importantsource of salinity for surface snow on MYI as well as FYI covered by deepsnowpack. Inclusion of halogen release from blowing-snow SSA in oursimulations decreases monthly mean Arctic surface O3 by 4–8 ppbv(15 %–30 %) in March and 8–14 ppbv (30 %–40 %) in April. We reproduce atransport event of depleted O3 Arctic air down to 40∘ Nobserved at many sub-Arctic surface sites in early April 2007. While oursimulation captures 25 %–40 % of the ODEs observed at coastal Arctic surfacesites, it underestimates the magnitude of many of these events and entirelymisses 60 %–75 % of ODEs. This difficulty in reproducing observed surfaceODEs could be related to the coarse horizontal resolution of the model, theknown biases in simulating Arctic boundary layer exchange processes, thelack of detailed chlorine chemistry, and/or the fact that we did not includedirect halogen activation by snowpack chemistry. 

Abstract. The formation of inorganic nitrate is the main sink for nitrogenoxides (NOx = NO + NO2). Due to the importance of NOx forthe formation of tropospheric oxidants such as the hydroxyl radical (OH) andozone, understanding the mechanisms and rates of nitrate formation isparamount for our ability to predict the atmospheric lifetimes of mostreduced trace gases in the atmosphere. The oxygen isotopic composition ofnitrate (Δ17O(nitrate)) is determined by the relativeimportance of NOx sinks and thus can provide an observationalconstraint for NOx chemistry. Until recently, the ability to utilizeΔ17O(nitrate) observations for this purpose was hindered by ourlack of knowledge about the oxygen isotopic composition of ozone (Δ17O(O3)). Recent and spatially widespread observations of Δ17O(O3) motivate an updated comparison of modeled andobserved Δ17O(nitrate) and a reassessment of modeled nitrateformation pathways. Model updates based on recent laboratory studies ofheterogeneous reactions render dinitrogen pentoxide (N2O5)hydrolysis as important as NO2 + OH (both 41 %) for globalinorganic nitrate production near the surface (below 1 km altitude). Allother nitrate production mechanisms individually represent less than 6 %of global nitrate production near the surface but can be dominant locally.Updated reaction rates for aerosol uptake of NO2 result in significantreduction of nitrate and nitrous acid (HONO) formed through this pathway inthe model and render NO2 hydrolysis a negligible pathway for nitrateformation globally. Although photolysis of aerosol nitrate may haveimplications for NOx, HONO, and oxidant abundances, it does notsignificantly impact the relative importance of nitrate formation pathways.Modeled Δ17O(nitrate) (28.6±4.5 ‰)compares well with the average of a global compilation of observations (27.6±5.0 ‰) when assuming Δ17O(O3) = 26 ‰, giving confidence in the model'srepresentation of the relative importance of ozone versus HOx (= OH + HO2 + RO2) in NOx cycling and nitrate formation on theglobal scale. 

Abstract. We present an updated mechanism for tropospheric halogen (Cl + Br + I) chemistry in the GEOS-Chem global atmospheric chemical transportmodel and apply it to investigate halogen radical cycling and implications for tropospheric oxidants. Improved representation of HOBr heterogeneouschemistry and its pH dependence in our simulation leads to less efficient recycling and mobilization of bromine radicals and enables the model toinclude mechanistic sea salt aerosol debromination without generating excessive BrO. The resulting global mean tropospheric BrO mixingratio is 0.19 ppt (parts per trillion), lower than previous versions of GEOS-Chem. Model BrO shows variable consistency and biases in comparison tosurface and aircraft observations in marine air, which are often near or below the detection limit. The model underestimates the daytimemeasurements of Cl2 and BrCl from the ATom aircraft campaign over the Pacific and Atlantic, which if correct would imply a very largemissing primary source of chlorine radicals. Model IO is highest in the marine boundary layer and uniform in the free troposphere, with a globalmean tropospheric mixing ratio of 0.08 ppt, and shows consistency with surface and aircraft observations. The modeled global meantropospheric concentration of Cl atoms is 630 cm−3, contributing 0.8 % of the global oxidation of methane, 14 % of ethane,8 % of propane, and 7 % of higher alkanes. Halogen chemistry decreases the global tropospheric burden of ozone by 11 %,NOx by 6 %, and OH by 4 %. Most of the ozone decrease is driven by iodine-catalyzed loss. The resulting GEOS-Chem ozonesimulation is unbiased in the Southern Hemisphere but too low in the Northern Hemisphere. 

Abstract. Bromine radicals influence global tropospheric chemistryby depleting ozone and by oxidizing elemental mercury and reduced sulfurspecies. Observations typically indicate a 50 % depletion of sea saltaerosol (SSA) bromide relative to seawater composition, implying that SSAdebromination could be the dominant global source of tropospheric bromine.However, it has been difficult to reconcile this large source with therelatively low bromine monoxide (BrO) mixing ratios observed in the marineboundary layer (MBL). Here we present a new mechanistic description of SSAdebromination in the GEOS-Chem global atmospheric chemistry model with adetailed representation of halogen (Cl, Br, and I) chemistry. We show thatobserved levels of SSA debromination can be reproduced in a mannerconsistent with observed BrO mixing ratios. Bromine radical sinks from theHOBr + S(IV) heterogeneous reactions and from ocean emission ofacetaldehyde are critical in moderating tropospheric BrO levels. Theresulting HBr is rapidly taken up by SSA and also deposited. Observations of SSA debromination at southern midlatitudes in summer suggest that modeluptake of HBr by SSA may be too fast. The model provides a successfulsimulation of free-tropospheric BrO in the tropics and midlatitudes in summer,where the bromine radical sink from the HOBr + S(IV) reactions iscompensated for by more efficient HOBr-driven recycling in clouds compared toprevious GEOS-Chem versions. Simulated BrO in the MBL is generally muchhigher in winter than in summer due to a combination of greater SSA emissionand slower conversion of bromine radicals to HBr. An outstanding issue inthe model is the overestimate of free-tropospheric BrO in extratropicalwinter–spring, possibly reflecting an overestimate of the HOBr∕HBr ratiounder these conditions where the dominant HOBr source is hydrolysis ofBrNO₃.

 

Abstract. We present a comprehensive simulation of tropospheric chlorinewithin the GEOS-Chem global 3-D model of oxidant–aerosol–halogen atmosphericchemistry. The simulation includes explicit accounting of chloridemobilization from sea salt aerosol by acid displacement of HCl and by otherheterogeneous processes. Additional small sources of tropospheric chlorine(combustion, organochlorines, transport from stratosphere) are also included.Reactive gas-phase chlorine Cl*, including Cl, ClO, Cl2, BrCl, ICl,HOCl, ClNO3, ClNO2, and minor species, is produced by theHCl+OH reaction and by heterogeneous conversion of sea salt aerosolchloride to BrCl, ClNO2, Cl2, and ICl. The modelsuccessfully simulates the observed mixing ratios of HCl in marine air(highest at northern midlatitudes) and the associated HNO3decrease from acid displacement. It captures the high ClNO2 mixingratios observed in continental surface air at night and attributes thechlorine to HCl volatilized from sea salt aerosol and transported inlandfollowing uptake by fine aerosol. The model successfully simulates thevertical profiles of HCl measured from aircraft, where enhancements in thecontinental boundary layer can again be largely explained by transport inlandof the marine source. It does not reproduce the boundary layer Cl2mixing ratios measured in the WINTER aircraft campaign (1–5 ppt in thedaytime, low at night); the model is too high at night, which could be due touncertainty in the rate of the ClNO2+Cl- reaction, but we haveno explanation for the high observed Cl2 in daytime. The globalmean tropospheric concentration of Cl atoms in the model is 620 cm−3and contributes 1.0 % of the global oxidation of methane, 20 % ofethane, 14 % of propane, and 4 % of methanol. Chlorine chemistryincreases global mean tropospheric BrO by 85 %, mainly through theHOBr+Cl- reaction, and decreases global burdens of troposphericozone by 7 % and OH by 3 % through the associated bromine radicalchemistry. ClNO2 chemistry drives increases in ozone of up to8 ppb over polluted continents in winter. 

We present a simulation of the global present-day composition of the troposphere which includes the chemistry of halogens (Cl, Br, I). Building on previous work within the GEOS-Chem model we include emissions of inorganic iodine from the oceans, anthropogenic and biogenic sources of halogenated gases, gas phase chemistry, and a parameterised approach to heterogeneous halogen chemistry. Consistent with Schmidt et al. (2016) we do not include sea-salt debromination. Observations of halogen radicals (BrO, IO) are sparse but the model has some skill in reproducing these. Modelled IO shows both high and low biases when compared to different datasets, but BrO concentrations appear to be modelled low. Comparisons to the very sparse observations dataset of reactive Cl species suggest the model represents a lower limit of the impacts of these species, likely due to underestimates in emissions and therefore burdens. Inclusion of Cl, Br, and I results in a general improvement in simulation of ozone (O₃) concentrations, except in polar regions where the model now underestimates O₃ concentrations. Halogen chemistry reduces the global tropospheric O₃ burden by 18.6 %, with the O₃ lifetime reducing from 26 to 22 days. Global mean OH concentrations of 1.28  ×  10⁶ molecules cm⁻³ are 8.2 % lower than in a simulation without halogens, leading to an increase in the CH₄ lifetime (10.8 %) due to OH oxidation from 7.47 to 8.28 years. Oxidation of CH₄ by Cl is small (∼  2 %) but Cl oxidation of other VOCs (ethane, acetone, and propane) can be significant (∼  15–27 %). Oxidation of VOCs by Br is smaller, representing 3.9 % of the loss of acetaldehyde and 0.9 % of the loss of formaldehyde. 

Studies of wintertime air quality in the North China Plain (NCP) show that particulate‐nitrate pollution persists despite rapid reduction in NO_xemissions. This intriguing NO_x‐nitrate relationship may originate from non‐linear nitrate‐formation chemistry, but it is unclear which feedback mechanisms dominate in NCP. In this study, we re‐interpret the wintertime observations of¹⁷O excess of nitrate (∆¹⁷O(NO₃⁻)) in Beijing using the GEOS‐Chem (GC) chemical transport model to estimate the importance of various nitrate‐production pathways and how their contributions change with the intensity of haze events. We also analyze the relationships between other metrics of NO_ychemistry and [PM_2.5] in observations and model simulations. We find that the model on average has a negative bias of −0.9‰ and −36% for ∆¹⁷O(NO₃⁻) and [O_x,major] (≡ [O₃] + [NO₂] + [p‐NO₃⁻]), respectively, while overestimating the nitrogen oxidation ratio ([NO₃⁻]/([NO₃⁻] + [NO₂])) by +0.12 in intense haze. The discrepancies become larger in more intense haze. We attribute the model biases to an overestimate of NO₂‐uptake on aerosols and an underestimate in wintertime O₃concentrations. Our findings highlight a need to address uncertainties related to heterogeneous chemistry of NO₂in air‐quality models. The combined assessment of observations and model results suggest that N₂O₅uptake in aerosols and clouds is the dominant nitrate‐production pathway in wintertime Beijing, but its rate is limited by ozone under high‐NO_x‐high‐PM_2.5conditions. Nitrate production rates may continue to increase as long as [O₃] increases despite reduction in [NO_x], creating a negative feedback that reduces the effectiveness of air pollution mitigation.