We present a new method for simulating heterogeneous (surface and multiphase) cloud chemistry in atmospheric models that do not spatially resolve clouds. The method accounts for cloud entrainment within the chemical rate expression, making it more accurate and stable than other approaches. Using this “entrainment‐limited uptake,” we evaluate the role of clouds in the tropospheric NOx cycle. Past literature suggests that on large scales, losses of N2O5 and NO3 in clouds are much less important than losses on aerosols. We find, however, that cloud reactions provide 25% of tropospheric NOx loss in high latitudes and 5% of global loss. Homogeneous, gas phase hydrolysis of N2O5 is likely 2% or less of global NOx loss. Both clouds and aerosols have similar impacts on global tropospheric O3 and OH levels, around 2% each. Accounting for cloud uptake reduces the sensitivity of atmospheric chemistry to aerosol surface area and uptake coefficient since clouds and aerosols compete for the same NO3 and N2O5.

Abstract. 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.

We evaluate the effects of rapidly changing Arctic sea ice conditions on sea salt aerosols (SSA) produced by oceanic wave‐breaking and the sublimation of wind‐lofted salty blowing snow on sea ice. We use the GEOS‐Chem chemical transport model to assess the influence of changing extent of the open ocean, multi‐year sea ice (MYI), first‐year sea ice (FYI), and snow depths on SSA emissions for 1980–2017. We combine snow depths from the Lagrangian snow‐evolution model (SnowModel‐LG) together with an empirically‐derived snow salinity function of snow depth to derive spatially and temporally varying snow surface salinity over Arctic FYI. We find that pan‐Arctic SSA surface mass concentrations have increased by 6%–12% decade−1 during the cold season (November–April) and by 7%–11% decade−1 during the warm season (May–October). The cold season trend is due to increasing blowing snow SSA originating from FYI: as MYI is replaced by FYI with thinning snow depths, snow surface salinity increases by more than 11% decade−1. During the warm season, rapid sea ice loss and thus increasing open ocean SSA are the cause of modeled SSA trends. Observations of SSA mass concentrations at Alert, Canada display positive trends during the cold season (10%–12% decade−1), consistent with our pan‐Arctic simulations. During fall, Alert observations show a negative trend (−18% decade−1), due to locally decreasing wind speeds and thus lower open ocean emissions. These significant changes in SSA concentrations could potentially affect past and future bromine explosions and Arctic climate feedbacks.

No abstract

No abstract

Abstract. 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.