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We examine the distribution of aerosol optical depth (AOD) across 27,707 northern hemisphere (NH) midlatitude cyclones for 2005–2018 using retrievals from the Moderate Resolution Spectroradiometer (MODIS) sensor on the Aqua satellite. Cyclone‐centered composites show AOD enhancements of 20%–45% relative to background conditions in the warm conveyor belt (WCB) airstream. Fine mode AOD accounts for 68% of this enhancement annually. Relative to background conditions, coarse mode AOD is enhanced by more than a factor of two near the center of the composite cyclone, co‐located with high surface wind speeds. Within the WCB, MODIS AOD maximizes in spring, with a secondary maximum in summer. Cyclone‐centered composites of AOD from the Modern Era Retrospective analysis for Research and Applications, version 2 Global Modeling Initiative (M2GMI) simulation reproduce the magnitude and seasonality of the MODIS AOD composites and enhancements. M2GMI simulations show that the AOD enhancement in the WCB is dominated by sulfate (37%) and organic aerosol (25%), with dust and sea salt each accounting for 15%. MODIS and M2GMI AOD are 60% larger in North Pacific WCBs compared to North Atlantic WCBs and show a strong relationship with anthropogenic pollution. We infer that NH midlatitude cyclones account for 355 Tg yr⁻¹of sea salt aerosol emissions annually, or 60% of the 30–80°N total. We find that deposition within WCBs is responsible for up to 35% of the total aerosol deposition over the NH ocean basins. Furthermore, the cloudy environment of WCBs leads to efficient secondary sulfate production.

 

Tropospheric reactive bromine (Br_y) influences the oxidation capacity of the atmosphere by acting as a sink for ozone and nitrogen oxides. Aerosol acidity plays a crucial role in Br_yabundances through acid‐catalyzed debromination from sea‐salt‐aerosol, the largest global source. Bromine concentrations in a Russian Arctic ice‐core, Akademii Nauk, show a 3.5‐fold increase from pre‐industrial (PI) to the 1970s (peak acidity, PA), and decreased by half to 1999 (present day, PD). Ice‐core acidity mirrors this trend, showing robust correlation with bromine, especially after 1940 (r = 0.9). Model simulations considering anthropogenic emission changes alone show that atmospheric acidity is the main driver of Br_ychanges, consistent with the observed relationship between acidity and bromine. The influence of atmospheric acidity on Br_yshould be considered in interpretation of ice‐core bromine trends.

 

Abstract Ambient fine particulate matter (PM 2.5 ) is the world’s leading environmental health risk factor. Reducing the PM 2.5 disease burden requires specific strategies that target dominant sources across multiple spatial scales. We provide a contemporary and comprehensive evaluation of sector- and fuel-specific contributions to this disease burden across 21 regions, 204 countries, and 200 sub-national areas by integrating 24 global atmospheric chemistry-transport model sensitivity simulations, high-resolution satellite-derived PM 2.5 exposure estimates, and disease-specific concentration response relationships. Globally, 1.05 (95% Confidence Interval: 0.74–1.36) million deaths were avoidable in 2017 by eliminating fossil-fuel combustion (27.3% of the total PM 2.5 burden), with coal contributing to over half. Other dominant global sources included residential (0.74 [0.52–0.95] million deaths; 19.2%), industrial (0.45 [0.32–0.58] million deaths; 11.7%), and energy (0.39 [0.28–0.51] million deaths; 10.2%) sectors. Our results show that regions with large anthropogenic contributions generally had the highest attributable deaths, suggesting substantial health benefits from replacing traditional energy sources. 

Snowpack emissions are recognized as an important source of gas‐phase reactive bromine in the Arctic and are necessary to explain ozone depletion events in spring caused by the catalytic destruction of ozone by halogen radicals. Quantifying bromine emissions from snowpack is essential for interpretation of ice‐core bromine. We present ice‐core bromine records since the pre‐industrial (1750 CE) from six Arctic locations and examine potential post‐depositional loss of snowpack bromine using a global chemical transport model. Trend analysis of the ice‐core records shows that only the high‐latitude coastal Akademii Nauk (AN) ice core from the Russian Arctic preserves significant trends since pre‐industrial times that are consistent with trends in sea ice extent and anthropogenic emissions from source regions. Model simulations suggest that recycling of reactive bromine on the snow skin layer (top 1 mm) results in 9–17% loss of deposited bromine across all six ice‐core locations. Reactive bromine production from below the snow skin layer and within the snow photic zone is potentially more important, but the magnitude of this source is uncertain. Model simulations suggest that the AN core is most likely to preserve an atmospheric signal compared to five Greenland ice cores due to its high latitude location combined with a relatively high snow accumulation rate. Understanding the sources and amount of photochemically reactive snow bromide in the snow photic zone throughout the sunlit period in the high Arctic is essential for interpreting ice‐core bromine, and warrants further lab studies and field observations at inland locations.

 

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. 

Using deposition observations from precipitation samples collected by the National Atmospheric Deposition Program at 125 sites across the United States, we show that the mean wet deposition flux of non‐sea‐salt chloride (NSS Cl⁻) has decreased by 83% throughout the eastern United States between 1998 and 2018. We find that 30% of the sites switch from having excess Cl⁻to being depleted in Cl⁻. We attribute the observed decreases in NSS Cl⁻deposition to a 95% decrease in U.S. anthropogenic HCl emissions since 1998. We propose that industry emission controls that remove HCl as a cobenefit of NO_xand SO₂have caused significant decreases in NSS Cl⁻deposition throughout the eastern United States, in addition to shifts from coal to natural gas and to coal with lower Cl⁻content. Our analysis implies that the lower tropospheric reactive inorganic chlorine burden was larger over the United States in the past than it is today.

 

Variability in sea ice is a critical climate feedback, yet the seasonal behavior of Southern Hemisphere sea ice and climate across multiple timescales remains unclear. Here, we develop a seasonally resolved Holocene sea salt record using major ion measurements of the South Pole Ice Core (SPC14). We combine the SPC14 data with the GEOS‐Chem chemical transport model to demonstrate that the primary sea salt source switches seasonally from open water (summer) to sea ice (winter), with wintertime variations disproportionately responsible for the centennial to millennial scale structure in the record. We interpret increasing SPC14 and circum‐Antarctic Holocene sea salt concentrations, particularly between 8 and 10 ka, as reflecting a period of winter sea ice expansion. Between 5 and 6 ka, an anomalous drop in South Atlantic sector sea salt indicates a temporary sea ice reduction that may be coupled with Northern Hemisphere cooling and associated ocean circulation changes.

 

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.