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

Abstract. A portion of Alaska’s Fairbanks North Star Borough was designated as nonattainment for the 2006 24-hour PM_2.5 National Ambient Air Quality Standard (NAAQS) in 2009. PM_2.5 NAAQS exceedances in Fairbanks mainly occur during the dark and cold winters, when temperature inversions form and trap high emissions at the surface. Sulfate (SO₄^2-), 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 SO₄^2-, and secondary SO₄^2- formed via gas-phase oxidation of sulfur dioxide (SO₂) and in-cloud aqueous oxidation of dissolved S(IV). Heterogeneous sulfur chemistry in aqueous aerosols, not often included in CTMs, may help better represent the high SO₄^2- concentrations observed during Fairbanks winters. In addition, hydroxymethanesulfonate (HMS), a particulate sulfur species sometimes misidentified as SO₄^2-, is known to form during Fairbanks winters. Heterogeneous formation of SO₄^2- 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 N. Hemisphere and Contiguous United States (CONUS) for 2015–2016. The added heterogeneous sulfur chemistry reduced model mean sulfate bias by ~0.6 µg/m³ 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 ~2.7 µgS/m³). This additional sulfur chemistry also improves modeled summertime SO₄^2- bias in the southeast U.S. with implications of future modeling of biogenic organosulfates.