Predicted impacts of heterogeneous chemical pathways on particulate sulfur over Fairbanks (Alaska), the Northern Hemisphere, and the Contiguous United States.

Abstract

A portion of Alaska's Fairbanks North Star Borough was designated as nonattainment for the 2006 24 h fine particulate matter 2.5 μm or less in diameter (PM_2.5) National Ambient Air Quality Standards (NAAQS) in 2009. PM_2.5 NAAQS exceedances in Fairbanks mainly occur during dark and cold winters, when temperature inversions form and trap high emissions at the surface. Sulfate ( ${\text{SO}}_4^{2-}$ ), often the second-largest contributor to PM_2.5 mass during these wintertime PM episodes, is underpredicted by atmospheric chemical transport models (CTMs). Most CTMs account for primary ${\text{SO}}_4^{2-}$ and secondary ${\text{SO}}_4^{2-}$ formed via gas-phase oxidation of sulfur dioxide ( ${\text{SO}}_2$ ) and in-cloud aqueous oxidation of dissolved S(IV). Dissolution and reaction of ${\text{SO}}_2$ in aqueous aerosols are generally not included in CTMs but can be represented as heterogeneous reactive uptake and may help better represent the high ${\text{SO}}_4^{2-}$ concentrations observed during Fairbanks winters. In addition, hydroxymethanesulfonate (HMS), a particulate sulfur species sometimes misidentified as ${\text{SO}}_4^{2-}$ , is known to form during Fairbanks winters. Heterogeneous formation of ${\text{SO}}_4^{2-}$ and HMS in aerosol liquid water (ALW) was implemented in the Community Multiscale Air Quality (CMAQ) modeling system. CMAQ simulations were performed for wintertime PM episodes in Fairbanks (2008) as well as over the Northern Hemisphere and Contiguous United States (CONUS) for 2015-2016. The added heterogeneous sulfur chemistry reduced model mean sulfate bias by ~0.6 μg m^-3 during a cold winter PM episode in Fairbanks, AK. Improvements in model performance are also seen in Beijing during wintertime haze events (reducing model mean sulfate bias by ~2.9 μgS m^-3). This additional sulfur chemistry also improves modeled summertime ${\text{SO}}_4^{2-}$ bias in the southeastern US, with implications for future modeling of biogenic organosulfates.