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Sulfate plays a key role in the formation and growth of aerosol particles and cloud droplets in the troposphere and is thus important for air quality and climate. The formation mechanisms of sulfate vary with oxidant levels and environmental conditions and can be partially revealed by its isotopic signatures. Here we measure oxygen (16O, 17O, 18O) and sulfur isotopes (32S, 34S) of the sulfate aerosol samples collected at coastal Hong Kong, downwind of the highly urbanized Pearl River Delta region. Based on ion measurements, most (95%) of the sulfate collected is non-sea-salt sulfate. The δ34S of sulfate is on average 4.0±2.0 ‰ (range 0.7 – 8.0 ‰), at an average sulfur oxidation ratio of 79±9%. The average oxygen-17 excess (Δ17O) is -0.1±0.3 ‰, suggesting an important role of OH / transition metals / reactive halogens. The δ18O of sulfate is on average 4.9±2.1 ‰. The Markov-Chain Monte Carlo model will be used to further constrain sulfate formation mechanisms.

Reactive halogens play a prominent role in the atmospheric chemistry of the Arctic during springtime. Field measurements and modeling studies suggest that halogens are emitted into the atmosphere from snowpack and reactions on wind-blown snow-sourced aerosols. The relative importance of snowpack and blowing snow sources is still debated, both at local scales and regionally throughout the Arctic. To understand the implications of these halogen sources on a pan-Arctic scale, we simulate Arctic reactive bromine chemistry in the atmospheric chemical transport model GEOS-Chem. Two mechanisms are included: (1) a blowing snow sea salt aerosol formation mechanism and (2) a snowpack mechanism assuming uniform molecular bromine production from all snow surfaces. We compare simulations including neither mechanism, each mechanism individually, and both mechanisms to examine conditions where one process may dominate or the mechanisms may interact. We compare the models using these mechanisms to observations of bromine monoxide (BrO) derived from multiple-axis differential optical absorption spectroscopy (MAX-DOAS) instruments on O-Buoy platforms on the sea ice and at a coastal site in Utqiaġvik, Alaska, during spring 2015. Model estimations of hourly and monthly average BrO are improved by assuming a constant yield of 0.1 % molecular bromine from all snowpack surfaces on ozone deposition. The blowing snow aerosol mechanism increases modeled BrO by providing more bromide-rich aerosol surface area for reactive bromine recycling. The snowpack mechanism led to increased model BrO across the Arctic Ocean with maximum production in coastal regions, whereas the blowing snow aerosol mechanism increases BrO in specific areas due to high surface wind speeds. Our uniform snowpack source has a greater impact on BrO mixing ratios than the blowing snow source. Model results best replicate several features of BrO observations during spring 2015 when using both mechanisms in conjunction, adding evidence that these mechanisms are both active during the Arctic spring. Extending our transport model throughout the entire year leads to predictions of enhanced fall BrO that are not supported by observations.

Column ozone variability has important implications for surface photochemistry and the climate. Ice-core nitrate isotopes are suspected to be influenced by column ozone variability and δ15N(NO3-) has been sought to serve as a proxy of column ozone variability. In this study, we examined the ability of ice-core nitrate isotopes to reflect column ozone variability by measuring δ15N(NO3-) and Δ17O(NO3-) in a shallow ice core drilled at the South Pole. The ice core covers the period 1944–2005, and during this period δ15N(NO3-) showed large annual variability ((59.2 ± 29.3) ‰ ), but with no apparent response to the Antarctic ozone hole. Utilizing a snow photochemical model, we estimated 6.9 ‰ additional enrichments in δ15N(NO3-) could be caused by the development of the ozone hole. Nevertheless, this enrichment is small and masked by the effects of the snow accumulation rate at the South Pole over the same period of the ozone hole. The Δ17O(NO3-) record has displayed a decreasing trend by ∼ 3.4 ‰ since 1976. This magnitude of change cannot be caused by enhanced post-depositional processing related to the ozone hole. Instead, the Δ17O(NO3-) decrease was more likely due to the proposed decreases in the O3 / HOx ratio in the extratropical Southern Hemisphere. Our results suggest ice-core δ15N(NO3-) is more sensitive to snow accumulation rate than to column ozone, but at sites with a relatively constant snow accumulation rate, information of column ozone variability embedded in δ15N(NO3-) should be retrievable.

The effect of post-depositional processing on the preservation of snow nitrate isotopes at Summit, Greenland, remains a subject of debate and is relevant to the quantitative interpretation of ice-core nitrate (isotopic) records at high snow accumulation sites. Here we present the first year-round observations of atmospheric nitrate and its isotopic compositions at Summit and compare them with published surface snow and snowpack observations. The atmospheric δ15N(NO3-) remained negative throughout the year, ranging from −3.1 ‰ to −47.9 ‰ with a mean of (−14.8 ± 7.3) ‰ (n=54), and displayed minima in spring which are distinct from the observed spring δ15N(NO3-) maxima in snowpack. The spring average atmospheric δ15N(NO3-) was (−17.9 ± 8.3) ‰ (n=21), significantly depleted compared to the snowpack spring average of (4.6 ± 2.1) ‰, while the surface snow δ15N(NO3-) of (−6.8 ± 0.5) ‰ was in between the atmosphere and the snowpack. The differences in atmospheric, surface snow and snowpack δ15N(NO3-) are best explained by the photo-driven post-depositional processing of snow nitrate, with potential contributions from fractionation during nitrate deposition. In contrast to δ15N(NO3-), the atmospheric Δ17O(NO3-) was of a similar seasonal pattern and magnitude of change to that in the snowpack, suggesting little to no changes in Δ17O(NO3-) from photolysis, consistent with previous modeling results. The atmospheric δ18O(NO3-) varied similarly to atmospheric Δ17O(NO3-), with summer low and winter high values. However, the difference between atmospheric and snow δ18O(NO3-) was larger than that of Δ17O(NO3-). We found a strong correlation between atmospheric δ18O(NO3-) and Δ17O(NO3-) that is very similar to previous measurements for surface snow at Summit, suggesting that atmospheric δ18O(NO3-) versus Δ17O(NO3-) relationships were conserved during deposition. However, we found the linear relationships between δ18O and Δ17O(NO3-) were significantly different for snowpack compared to atmospheric samples. This likely suggests the oxygen isotopes are also affected before preservation in the snow at Summit, but the degree of change for δ18O(NO3-) should be larger than that of Δ17O(NO3-). This is because photolysis is a mass-dependent process that would directly affect δ18O(NO3-) in snow but not Δ17O(NO3-) as the latter is a mass-independent signal. Although there were uncertainties associated with the complied dataset, the results suggested that post-depositional processing at Summit can induce changes in nitrate isotopes, especially δ15N(NO3-), consistent with a previous modeling study. This reinforces the importance of understanding the effects of post-depositional processing before ice-core nitrate isotope interpretation, even for sites with relatively high snow accumulation rates.

International audience; 17O-excess (Δ17O = δ17O − 0.52 × δ18O) of sulfate trapped in Antarctic ice cores has been proposed as a potential tool for assessing past oxidant chemistry, while insufficient understanding of atmospheric sulfate formation around Antarctica hampers its interpretation. To probe influences of regional specific chemistry, we compared year-round observations of Δ17O of non-sea-salt sulfate in aerosols (Δ17O(SO42−)nss) at Dome C and Dumont d'Urville, inland and coastal sites in East Antarctica, throughout the year 2011. Although Δ17O(SO42−)nss at both sites showed consistent seasonality with summer minima (∼1.0‰) and winter maxima (∼2.5‰) owing to sunlight-driven changes in the relative importance of O3 oxidation to OH and H2O2 oxidation, significant intersite differences were observed in austral spring–summer and autumn. The cooccurrence of higher Δ17O(SO42−)nss at inland (2.0‰ ± 0.1‰) than the coastal site (1.2‰ ± 0.1‰) and chemical destruction of methanesulfonate (MS–) in aerosols at inland during spring–summer (October–December), combined with the first estimated Δ17O(MS–) of ∼16‰, implies that MS– destruction produces sulfate with high Δ17O(SO42−)nss of ∼12‰. If contributing to the known postdepositional decrease of MS– in snow, this process should also cause a significant postdepositional increase in Δ17O(SO42−)nss over 1‰, that can reconcile the discrepancy between Δ17O(SO42−)nss in the atmosphere and ice. The higher Δ17O(SO42−)nss at the coastal site than inland during autumn (March–May) may be associated with oxidation process involving reactive bromine and/or sea-salt particles around the coastal region.

Column ozone variability has important implications for surface photochemistry and climate. Ice-core nitrate isotopes are suspected to be influenced by column ozone variability and δ15N(NO3-) has been sought to serve as a proxy of column ozone variability. In this study, we examined the ability of ice-core nitrate isotopes to reflect column ozone variability by measuring δ15N(NO3-) and Δ17O(NO3-) in a shallow ice core drilled at the South Pole. The ice core covers the period of 1944 to 2005, and during this period δ15N(NO3-) was of large annual variability ((59.2 ± 29.3) ‰), but with no apparent response to the Antarctic ozone hole. Utilizing a snow photochemical model, we estimated 6.9 ‰ additional enrichments in δ15N(NO3-) could be caused by the development of the ozone hole. But this enrichment is nevertheless small and masked by the effects of snow accumulation rate variability in addition to that of the slightly increased snow accumulation rate at the South Pole over the same period of the ozone hole. The Δ17O(NO3-) record displays a decreasing trend by ~ 3.4 ‰ since 1976. This magnitude of change can’t be caused by enhanced post-depositional processing owing to the ozone hole. Instead, the Δ17O(NO3-) decrease was more likely due to the proposed decreases in O3 / HOx ratio in the extratropical Southern Hemisphere. Our results suggest ice-core δ15N(NO3-) is more sensitive to snow accumulation rate than to column ozone, but at sites with relatively constant snow accumulation rate, information of column ozone variability embedded in δ15N(NO3-) should be retrievable. In comparison with the South Pole, up to 21 ‰ additional δ15N(NO3-) enrichments can be caused by the ozone hole at Dome A and the signal would be possibly detected if where snow accumulation rate has stayed relatively constant.

We use the GEOS-Chem chemical transport model to examine the influence of bromine release from blowing-snow sea salt aerosol (SSA) on springtime bromine activation and O3 depletion events (ODEs) in the Arctic lower troposphere. We evaluate our simulation against observations of tropospheric BrO vertical column densities (VCD tropo ) from the GOME-2 (second Global Ozone Monitoring Experiment) and Ozone Monitoring Instrument (OMI) spaceborne instruments for 3 years (2007–2009), as well as against surface observations of O3 . We conduct a simulation with blowing-snow SSA emissions from first-year sea ice (FYI; with a surface snow salinity of 0.1 psu) and multi-year sea ice (MYI; with a surface snow salinity of 0.05 psu), assuming a factor of 5 bromide enrichment of surface snow relative to seawater. This simulation captures the magnitude of observed March–April GOME-2 and OMI VCD tropo to within 17 %, as well as their spatiotemporal variability ( r=0.76 –0.85). Many of the large-scale bromine explosions are successfully reproduced, with the exception of events in May, which are absent or systematically underpredicted in the model. If we assume a lower salinity on MYI (0.01 psu), some of the bromine explosions events observed over MYI are not captured, suggesting that blowing snow over MYI is an important source of bromine activation. We find that the modeled atmospheric deposition onto snow-covered sea ice becomes highly enriched in bromide, increasing from enrichment factors of ∼5 in September–February to 10–60 in May, consistent with composition observations of freshly fallen snow. We propose that this progressive enrichment in deposition could enable blowing-snow-induced halogen activation to propagate into May and might explain our late-spring underestimate in VCD tropo . We estimate that the atmospheric deposition of SSA could increase snow salinity by up to 0.04 psu between February and April, which could be an important source of salinity for surface snow on MYI as well as FYI covered by deep snowpack. Inclusion of halogen release from blowing-snow SSA in our simulations decreases monthly mean Arctic surface O3 by 4–8 ppbv (15 %–30 %) in March and 8–14 ppbv (30 %–40 %) in April. We reproduce a transport event of depleted O3 Arctic air down to 40 ∘ N observed at many sub-Arctic surface sites in early April 2007. While our simulation captures 25 %–40 % of the ODEs observed at coastal Arctic surface sites, it underestimates the magnitude of many of these events and entirely misses 60 %–75 % of ODEs. This difficulty in reproducing observed surface ODEs could be related to the coarse horizontal resolution of the model, the known biases in simulating Arctic boundary layer exchange processes, the lack of detailed chlorine chemistry, and/or the fact that we did not include direct halogen activation by snowpack chemistry.

Significance Physical and chemical properties of individual atmospheric particles determine their climate impacts. Hygroscopic inorganic salt particles mixed with trace amounts of organic material are predicted to be liquid under typical tropospheric conditions in the summertime Arctic. Yet, we unexpectedly observed a significant concentration of solid particles composed of ammonium sulfate with an organic coating under conditions of high relative humidity and low temperature. These particle properties are consistent with marine biogenic-derived new particle formation and growth, with particle collision hypothesized to result in the solid phase. This particle source is predicted to have increasing relevance in the context of declining Arctic sea ice and increasing open water, with impacts on clouds, and therefore climate.

Reactive halogens play a prominent role in the atmospheric chemistry of the Arctic during springtime. Field measurements and models studies suggest that halogens are emitted to the atmosphere from snowpack and reactions on wind-blown snow. The relative importance of snowpack and blowing snow sources is still debated, both at local scales and regionally throughout the Arctic. To understand implications of these halogen sources on a pan-Arctic scale, we simulate Arctic reactive bromine chemistry in the atmospheric chemical transport model GEOS-Chem. Two mechanisms are included: 1) a blowing snow sea salt aerosol formation mechanism and 2) a snowpack mechanism assuming uniform molecular bromine production from all snow surfaces. We compare simulations including neither mechanism, each mechanism individually, and both mechanisms to examine conditions where one process may dominate or the mechanisms may interact. We compare the models using these mechanisms to observations of bromine monoxide (BrO) derived from multiple-axis differential optical absorption spectroscopy (MAX-DOAS) instruments on O-Buoy platforms on the sea ice and at a coastal site in Utqiaġvik, Alaska during spring 2015. Model estimations of hourly and monthly average BrO are improved by assuming a constant yield of 0.1 % molecular bromine from all snowpack surfaces on ozone deposition. The blowing snow mechanism increases BrO by providing more surface area for reactive bromine recycling. The snowpack mechanism led to increased BrO across the Arctic Ocean with maximum production in coastal regions, whereas the blowing snow mechanism increases BrO in specific areas due to high surface windspeeds. Our uniform snowpack source has a greater impact on BrO mixing ratios than the blowing snow source. Model results best replicate several features of BrO observations during spring 2015 when using both mechanisms in conjunction, adding evidence that these mechanisms are both active during the Arctic Spring. Extending our transport model throughout the entire year leads to predictions of enhanced fall BrO that are not supported by observations.

The effect of post–depositional processing on the preservation of snow nitrate isotopes at Summit, Greenland remains a subject of debate which hinders the interpretations of ice–core nitrate concentrations and isotope records. Here we present the first year–round observations of atmospheric aerosol nitrate and its isotopic compositions at Summit, and compare them with published surface snow and snowpack observations. The atmospheric δ15N(NO3–) remained negative throughout the year, ranging from –3.1 ‰ to –47.9 ‰ with a mean of (–14.8 ± 7.3) ‰, and displayed no apparent seasonality that is different from the distinct seasonal δ15N(NO3–) variations observed in snowpack. The spring average aerosol δ15N(NO3–) was (–17.9 ± 8.3) ‰, significantly depleted compared to snowpack spring average of (4.6 ± 2.1) ‰, with surface snow δ15N(NO3–) of (–6.8 ± 0.5) ‰ that is in between. The differences in aerosol, surface snow and snowpack δ15N(NO3–) are best explained by the photo-driven post–depositional processing of snow nitrate, with potential contributions from fractionation during nitrate deposition. In contrast to δ15N(NO3–), the atmospheric Δ17O(NO3–) was of similar seasonal pattern and magnitude of change to that in snowpack, suggesting little to no changes in Δ17O(NO3–) from photolysis, consistent with previous modeling results. The atmospheric δ18O(NO3–) varied similarly as atmospheric Δ17O(NO3–), with summer low and winter high values. However, the difference between atmospheric and snow δ18O(NO3–) was larger than that of Δ17O(NO3–), and the linear relationships between δ18O/Δ17O(NO3–) were different for atmospheric and snowpack samples. This suggests the oxygen isotopes are also affected before preservation in the snow at Summit, but the degree of change for δ18O(NO3–) is larger than that of Δ17O(NO3–) given that photolysis is a mass-dependent process.

The article of record may be found at https://doi.org/10.1575/1912/27821 The Surface Ocean – Lower Atmosphere Study (SOLAS) (http://www.solas-int.org/) is an international research initiative focused on understanding the key biogeochemical-physical interactions and feedbacks between the ocean and atmosphere that are critical elements of climate and global biogeochemical cycles. Following the release of the SOLAS Decadal Science Plan (2015-2025) (Brévière et al., 2016), the Ocean-Atmosphere Interaction Committee (OAIC) was formed as a subcommittee of the Ocean Carbon and Biogeochemistry (OCB) Scientific Steering Committee to coordinate US SOLAS efforts and activities, facilitate interactions among atmospheric and ocean scientists, and strengthen US contributions to international SOLAS. In October 2019, with support from OCB, the OAIC convened an open community workshop, Ocean-Atmosphere Interactions: Scoping directions for new research with the goal of fostering new collaborations and identifying knowledge gaps and high-priority science questions to formulate a US SOLAS Science Plan. Based on presentations and discussions at the workshop, the OAIC and workshop participants have developed this US SOLAS Science Plan. The first part of the workshop and this Science Plan were purposefully designed around the five themes of the SOLAS Decadal Science Plan (2015-2025) (Brévière et al., 2016) to provide a common set of research priorities and ensure a more cohesive US contribution to international SOLAS. This report was developed with federal support of NSF (OCE-1558412) and NASA (NNX17AB17G). This report was developed with federal support of NSF (OCE-1558412) and NASA (NNX17AB17G).

Studies of wintertime air quality in the North China Plain (NCP) show that particulate-nitrate pollution persists despite rapid reduction in NOx emissions. This intriguing NOx-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 of 17O 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 NOy chemistry and [PM2.5] in observations and model simulations. We find that the model on average has a negative bias of −0.9‰ and −3617O(NO3−) and [Ox,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 O3 concentrations. Our findings highlight a need to address uncertainties related to heterogeneous chemistry of NO2 in air-quality models. The combined assessment of observations and model results suggest that N2O5 uptake in aerosols and clouds is the dominant nitrate-production pathway in wintertime Beijing, but its rate is limited by ozone under high-NOx-high-PM2.5 conditions. Nitrate production rates may continue to increase as long as [O3] increases despite reduction in [NOx], creating a negative feedback that reduces the effectiveness of air pollution mitigation.

After the 1980s, atmospheric sulfate reduction is slower than the dramatic reductions in sulfur dioxide (SO2) emissions. However, a lack of observational evidence has hindered the identification of causal feedback mechanisms. Here, we report an increase in the oxygen isotopic composition of sulfate ([Formula: see text]) in a Greenland ice core, implying an enhanced role of acidity-dependent in-cloud oxidation by ozone (up to 17 to 27%) in sulfate production since the 1960s. A global chemical transport model reproduces the magnitude of the increase in observed [Formula: see text] with a 10 to 15% enhancement in the conversion efficiency from SO2 to sulfate in Eastern North America and Western Europe. With an expected continued decrease in atmospheric acidity, this feedback will continue in the future and partially hinder air quality improvements.

Oxygen-17 anomaly (Δ17O) has been used as a probe to constrain the relative importance of different pathways leading to sulfate formation. Here, we report the Δ17O values in atmospheric sulfate collected at a remote site in the Mt. Everest region to decipher the possible formation mechanisms of sulfate in such a pristine environment. The Δ17O in non-dust sulfate show higher values than most existing data in modern atmospheric sulfate. The seasonality of Δ17O in non-dust sulfate exhibits high values in the pre-monsoon and low values in the monsoon, opposite to the seasonality in Δ17O for both sulfate and nitrate (i.e., minima in warm season and maxima in cold season) observed from diverse geographic sites. This high Δ17O in non-dust sulfate found in this region clearly indicates the important role of the S(IV) + O3 pathway in atmospheric sulfate formation promoted by high cloud water pH conditions. In turn, this study highlights observational evidence that atmospheric acidity plays an important role in controlling sulfate formation pathways particularly for dust-rich environments.

17O-excess (Δ17O = δ17O − 0.52 × δ18O) of sulfate trapped in Antarctic ice cores has been proposed as a potential tool for assessing past oxidant chemistry, while insufficient understanding of atmospheric sulfate formation around Antarctica hampers its interpretation. To probe influences of regional specific chemistry, we compared year-round observations of Δ17O of non-sea-salt sulfate in aerosols (Δ17O(SO42−)nss) at Dome C and Dumont d’Urville, inland and coastal sites in East Antarctica, throughout the year 2011. Although Δ17O(SO42–)nss at both sites showed consistent seasonality with summer minima (~1.0 ‰) and winter maxima (~2.5 ‰) owing to sunlight-driven changes in the relative importance of O3-oxidation to OH- and H2O2-oxidation, significant inter-site differences were observed in austral spring–summer and autumn. The co-occurrence of higher Δ17O(SO42–)nss at inland (2.0 ± 0.1 ‰) than the coastal site (1.2 ± 0.1 ‰) and chemical destruction of methanesulfonate (MS–) in aerosols at inland during spring–summer (October to December), combined with the first estimated Δ17O(MS–) of ~16 ‰, implies that MS– destruction produces sulfate with high Δ17O(SO42–)nss of ~12 ‰. If contributing to the known post-depositional decrease of MS– in snow, this process should also cause a significant post-depositional increase in Δ17O(SO42–)nss over 1 ‰, that can reconcile the discrepancy between Δ17O(SO42–)nss in the atmosphere and ice.

Using the Whole Atmosphere Community Climate Model version 6, stratospheric ozone in the Last Glacial Maximum (LGM) is investigated. It is shown that, compared with preindustrial (PI) times, LGM modeled stratospheric temperatures are increased by up to 8 K, leading to faster ozone destruction rates for gas phase reactions, especially via the Chapman mechanism. On the other hand, stratospheric hydroxyl radical (OH) and nitrogen oxides (NOx) concentrations are decreased by 10?20%, which decreases catalytic ozone destruction, thereby decreasing ozone loss rates. The net effect of these two compensating mechanisms in the upper stratosphere (above 15 hPa) is a vertically integrated 1?3 Dobson unit (DU) decrease during the LGM. In the lower stratosphere (tropopause to 15 hPa), changes in the stratospheric overturning circulation and resulting transport dominate changes in ozone. Consistent with a weakening of the residual circulation in the LGM, lower stratospheric ozone is increased by 2?5 DU in the tropics and decreased by 5?10 DU in the extratropics, but the latter is partly compensated by ozone increases due to a lower tropopause. It is found that tropospheric ozone is decreased by about 5 DU in the LGM versus PI. Combined changes in stratospheric and tropospheric ozone lead to a decrease in total ozone column everywhere except over the northeast North America, equatorial Indian and West Pacific Oceans. Surface ultraviolet radiation in the LGM versus PI is increased over the Northern Hemisphere middle and high latitudes, especially over the ice caps, and over the Southern Hemisphere near 60°S. This research was supported by NSF Grant AGS‐1821437 and NASA Grant 80NSSC18K1031. S. S. was supported by NSF AGS project 1848863. R. H. W. was partially funded by the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska‐Curie Grant Agreement No. 797961. We would like to acknowledge high‐performance computing support from Cheyenne (doi:10.5065/D6RX99HX) provided by NCAR's Computational and Information Systems Laboratory, sponsored by the National Science Foundation, for the WACCM6 simulations and analyses presented in this study and for the data management, storage and preservation. The National Center for Atmospheric Research is sponsored by the United States National Science Foundation. We thank the anonymous reviewers for their constructive comments that have helped improve the manuscript.

The quasi-biennial oscillation (QBO) and sudden stratospheric warmings (SSWs) during the Last Glacial Maximum (LGM) are investigated in simulations using the Whole Atmosphere Community Climate Model version 6 (WACCM6). We find that the period of QBO, which is 27 months in the preindustrial and modern climate simulations, was 33 months in the LGM simulation using the proxy sea surface temperatures (SSTs) and 41 months using the model-based LGM SSTs. We show that the longer QBO period in the LGM is due to weaker wave forcing. The WACCM6 simulations of the LGM, preindustrial, and modern climates do not support previous modeling work that suggests that the QBO amplitude is smaller (larger) in a warmer (colder) climate. We find that SSWs in the LGM occurred later in the year, as compared to the preindustrial and modern climate, but that time of the final warming was similar. The difference in SSW frequency is inconclusive.

Sulfur compounds are an important constituent of particulate matter, with impacts on climate and public health. While most sulfur observed in particulate matter has been assumed to be sulfate, laboratory experiments reveal that hydroxymethanesulfonate (HMS), an adduct formed by aqueous phase chemical reaction of dissolved HCHO and SO2, may be easily misinterpreted in measurements as sulfate. Here we present observational and modeling evidence for a ubiquitous global presence of HMS. We find that filter samples collected in Shijiazhuang, China, and examined with ion chromatography within 9 days show as much as 7.6 μg m−3 of HMS, while samples from Singapore examined 9–18 months after collection reveal ~0.6 μg m−3 of HMS. The Shijiazhuang samples show only minor traces of HMS 4 months later, suggesting that HMS had decomposed over time during sample storage. In contrast, the Singapore samples do not clearly show a decline in HMS concentration over 2 months of monitoring. Measurements from over 150 sites, primarily derived from the IMPROVE network across the United States, suggest the ubiquitous presence of HMS in at least trace amounts as much as 60 days after collection. The degree of possible HMS decomposition in the IMPROVE observations is unknown. Using the GEOS‐Chem chemical transport model, we estimate that HMS may account for 10% of global particulate sulfur in continental surface air and over 25% in many polluted regions. Our results suggest that reducing emissions of HCHO and other volatile organic compounds may have a co‐benefit of decreasing particulate sulfur., Key Points New and reanalyzed observations suggest a ubiquitous global presence of hydroxymethanesulfonate (HMS) in particulate matterGEOS‐Chem simulations suggest HMS may comprise over 25% of particulate sulfur in many polluted regions, especially in continental winterReductions of formaldehyde and other volatile organic compounds may have the co‐benefit of reducing particulate sulfur

After the 1980s, atmospheric sulfate reduction is slower than the dramatic reductions in sulfur dioxide (SO

The formation of inorganic nitrate is the main sink for nitrogen oxides (NOx = NO + NO2). Due to the importance of NOx for the formation of tropospheric oxidants such as the hydroxyl radical (OH) and ozone, understanding the mechanisms and rates of nitrate formation is paramount for our ability to predict the atmospheric lifetimes of most reduced trace gases in the atmosphere. The oxygen isotopic composition of nitrate (Δ17O(nitrate)) is determined by the relative importance of NOx sinks, and thus can provide an observational constraint for NOx chemistry. Until recently, the ability to utilize Δ17O(nitrate) observations for this purpose was hindered by our lack of knowledge about the oxygen isotopic composition of ozone (Δ17O(O3)). Recent and spatially widespread observations of Δ17O(O3) have greatly reduced this uncertainty, and allow for an updated comparison of modeled and observed Δ17O(nitrate) and a reassessment of modeled nitrate formation pathways. Model updates based on recent laboratory studies of heterogeneous reactions renders dinitrogen pentoxide (N2O5) hydrolysis as important as NO2 + OH (both 41 %) for global inorganic nitrate production near the surface. All other nitrate production mechanisms 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 significant reduction of nitrate and nitrous acid (HONO) formed through this pathway in the model, and render NO2 hydrolysis a negligible pathway for nitrate formation globally. Although photolysis of aerosol nitrate may have implications for NOx, HONO and oxidant abundances, it does not significantly 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 ‰), giving confidence in the model's representation of the relative importance of ozone versus HOx (= OH + HO2 + RO2) in NOx cycling and nitrate formation.

We use the GEOS-Chem chemical transport model to examine the influence of bromine release from blowing snow sea salt aerosol (SSA) on springtime bromine activation and O3 depletion events (ODEs) in the Arctic lower troposphere. We evaluate our simulation against observations of tropospheric BrO vertical column densities (VCDtropo) from the GOME-2 and OMI spaceborne instruments for three years (2007–2009), as well as against surface observations of O3. We conduct a simulation with blowing snow SSA emissions from first-year sea ice (FYI, with a surface snow salinity of 0.1 psu) and multi-year sea ice (MYI, with a surface snow salinity of 0.05 psu), assuming a factor of 5 bromide enrichment of surface snow relative to seawater. This simulation captures the magnitude of observed March–April GOME-2 and OMI VCDtropo to within 17 %, as well as their spatiotemporal variability (r = 0.76-0.85). Many of the large-scale bromine explosions are successfully reproduced, with the exception of events in May, which are absent or systematically underpredicted in the model. If we assume a lower salinity on MYI (0.01 psu) some of the bromine explosions events observed over MYI are not captured, suggesting that blowing snow over MYI is an important source of bromine activation. We find that the modeled atmospheric deposition onto snow-covered sea ice becomes highly enriched in bromide, increasing from enrichment factors of ~ 5 in September–February to 10–60 in May, consistent with freshly fallen snow composition observations. We propose that this progressive enrichment in deposition could enable blowing snow-induced halogen activation to propagate into May and might explain our late-spring underestimate in VCDtropo. We estimate that atmospheric deposition of SSA could increase snow salinity by up to 0.04 psu between February and April, which could be an important source of salinity for surface snow on MYI as well as FYI covered by deep snowpack. Inclusion of halogen release from blowing snow SSA in our simulations decreases monthly mean Arctic surface O3 by 4–8 ppbv (15–30 %) in March and 8–14 ppbv (30–40 %) in April. We reproduce a transport event of depleted O3 Arctic air down to 40º N observed at many sub-Arctic surface sites in early April 2007. While our simulation captures a few ODEs observed at coastal Arctic surface sites, it underestimates the magnitude of other events and entirely misses some events. We suggest that inclusion of direct snowpack activation, which is a strong local source of Br radicals in the shallow Arctic boundary layer, could help reconcile the success of our simulation at capturing satellite retrievals of VCDtropo with its difficulty in reproducing local ODEs.

Observations from a Greenland ice core reveal that tropospheric oxidants are sensitive to climate-driven changes in reactive halogen chemistry and stratosphere-to-troposphere transport of ozone, in addition to ozone precursor emissions. The abundance of tropospheric oxidants, such as ozone (O3) and the hydroxyl radical (OH), determines the lifetimes of reduced trace gases such as methane, but the response of tropospheric oxidants to climate change is poorly constrained. This paper reports isotopic data from a Greenland ice core covering the most recent glacial–interglacial cycle that demonstrate the sensitivity of tropospheric oxidants to climate change. The authors observe an increase in the O3/HOx ratio in cold climates, the opposite of current expectations, and suggest that this increase might be driven by enhanced reactive halogen chemistry, enhanced stratosphere-to-troposphere transport of O3, or a combination of the two. These processes require further consideration when assessing global methane variations during periods of abrupt climate change. The abundance of tropospheric oxidants, such as ozone (O3) and hydroxyl (OH) and peroxy radicals (HO2 + RO2), determines the lifetimes of reduced trace gases such as methane and the production of particulate matter important for climate and human health. The response of tropospheric oxidants to climate change is poorly constrained owing to large uncertainties in the degree to which processes that influence oxidants may change with climate1 and owing to a lack of palaeo-records with which to constrain levels of atmospheric oxidants during past climate transitions2. At present, it is thought that temperature-dependent emissions of tropospheric O3 precursors and water vapour abundance determine the climate response of oxidants, resulting in lower tropospheric O3 in cold climates while HOx (= OH + HO2 + RO2) remains relatively buffered3. Here we report observations of oxygen-17 excess of nitrate (a proxy for the relative abundance of atmospheric O3 and HOx) from a Greenland ice core over the most recent glacial–interglacial cycle and for two Dansgaard–Oeschger events. We find that tropospheric oxidants are sensitive to climate change with an increase in the O3/HOx ratio in cold climates, the opposite of current expectations. We hypothesize that the observed increase in O3/HOx in cold climates is driven by enhanced stratosphere-to-troposphere transport of O3, and that reactive halogen chemistry is also enhanced in cold climates. Reactive halogens influence the oxidative capacity of the troposphere directly as oxidants themselves and indirectly4 via their influence on O3 and HOx. The strength of stratosphere-to-troposphere transport is largely controlled by the Brewer–Dobson circulation5, which may be enhanced in colder climates owing to a stronger meridional gradient of sea surface temperatures6, with implications for the response of tropospheric oxidants7 and stratospheric thermal and mass balance8. These two processes may represent important, yet relatively unexplored, climate feedback mechanisms during major climate transitions.

Marine cloud brightening (MCB) is proposed to offset global warming by emitting sea salt aerosols to the tropical marine boundary layer, which increases aerosol and cloud albedo. Sea salt aerosol is the main source of tropospheric reactive chlorine (Cly) and bromine (Bry). The effects of additional sea salt on atmospheric chemistry have not been explored. We simulate sea salt aerosol injections for MCB under two scenarios (212–569 Tg/a) in the GEOS‐Chem global chemical transport model, only considering their impacts as a halogen source. Globally, tropospheric Cly and Bry increase (20–40%), leading to decreased ozone (−3 to −6%). Consequently, OH decreases (−3 to −5%), which increases the methane lifetime (3–6%). Our results suggest that the chemistry of the additional sea salt leads to minor total radiative forcing compared to that of the sea salt aerosol itself (~2%) but may have potential implications for surface ozone pollution in tropical coastal regions., Key Points Sea salt aerosol emissions for Marine Cloud Brightening geoengineering are implemented in a global chemical transport modelThis leads to changes in global tropospheric Bry and Cly (+20 to 40%), ozone (−3 to −6%), OH (−2 to −4%), and methane lifetime (+3 to 6%)Chemistry of the added sea salt leads to minor total radiative forcing (−20 to −50 mW/m2) but may have implications for ozone pollution

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