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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 (PM2.5) National Ambient Air Quality Standards (NAAQS) in 2009. PM2.5 NAAQS exceedances in Fairbanks mainly occur during dark and cold winters, when temperature inversions form and trap high emissions at the surface. Sulfate (SO42-), often the second-largest contributor to PM2.5 mass during these wintertime PM episodes, is underpredicted by atmospheric chemical transport models (CTMs). Most CTMs account for primary SO42- and secondary SO42- formed via gas-phase oxidation of sulfur dioxide (SO2) and in-cloud aqueous oxidation of dissolved S(IV). Dissolution and reaction of SO2 in aqueous aerosols are generally not included in CTMs but can be represented as heterogeneous reactive uptake and may help better represent the high SO42- concentrations observed during Fairbanks winters. In addition, hydroxymethanesulfonate (HMS), a particulate sulfur species sometimes misidentified as SO42-, is known to form during Fairbanks winters. Heterogeneous formation of SO42- 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 µg S m−3). This additional sulfur chemistry also improves modeled summertime SO42- bias in the southeastern US, with implications for future modeling of biogenic organosulfates. 

Abstract. Sulfate and nitrate aerosols degrade air quality, modulate radiative forcing and the hydrological cycle, and affect biogeochemical cycles, yet their global cycles are poorly understood. Here, we examined trends in 21 years of aerosol measurements made at Ragged Point, Barbados, the easternmost promontory on the island located in the eastern Caribbean Basin. Though the site has historically been used to characterize African dust transport, here we focused on changes in nitrate and non-sea-salt (nss) sulfate aerosols from 1990–2011. Nitrate aerosol concentrations averaged over the entire period were stable at 0.59 µg m−3 ± 0.04 µg m−3, except for elevated nitrate concentrations in the spring of 2010 and during the summer and fall of 2008 due to the transport of biomass burning emissions from both northern and southern Africa to our site. In contrast, from 1990 to 2000, nss-sulfate decreased 30 % at a rate of 0.023 µg m−3 yr−1, a trend which we attribute to air quality policies enacted in the United States (US) and Europe. From 2000–2011, sulfate gradually increased at a rate of 0.021 µg m−3 yr−1 to pre-1990s levels of 0.90 µg m−3. We used the Community Multiscale Air Quality (CMAQ) model simulations from the EPA's Air QUAlity TimE Series (EQUATES) to better understand the changes in nss-sulfate after 2000. The model simulations estimate that increases in anthropogenic emissions from Africa explain the increase in nss-sulfate observed in Barbados. Our results highlight the need to better constrain emissions from developing countries and to assess their impact on aerosol burdens in remote source regions. 

Abstract. This study describes a modeling framework, model evaluation, and source apportionment to understand the causes of Los Angeles (LA) air pollution. A few major updates are applied to the Community Multiscale Air Quality (CMAQ) model with a high spatial resolution (1 km × 1 km). The updates include dynamic traffic emissions based on real-time, on-road information and recent emission factors and secondary organic aerosol (SOA) schemes to represent volatile chemical products (VCPs). Meteorology is well predicted compared to ground-based observations, and the emission rates from multiple sources (i.e., on-road, volatile chemical products, area, point, biogenic, and sea spray) are quantified. Evaluation of the CMAQ model shows that ozone is well predicted despite inaccuracies in nitrogen oxide (NOx) predictions. Particle matter (PM) is underpredicted compared to concurrent measurements made with an aerosol mass spectrometer (AMS) in Pasadena. Inorganic aerosol is well predicted, while SOA is underpredicted. Modeled SOA consists of mostly organic nitrates and products from oxidation of alkane-like intermediate volatility organic compounds (IVOCs) and has missing components that behave like less-oxidized oxygenated organic aerosol (LO-OOA). Source apportionment demonstrates that the urban areas of the LA Basin and vicinity are NOx-saturated (VOC-sensitive), with the largest sensitivity of O3 to changes in VOCs in the urban core. Differing oxidative capacities in different regions impact the nonlinear chemistry leading to PM and SOA formation, which is quantified in this study.