Contrasting seasonal responses of sulfate aerosols to declining SO₂ emissions in the Eastern U.S.: Implications for the efficacy of SO₂ emission controls

Abstract

Stringent controls have reduced U.S. SO2 emissions by over 60% since the late 1990s. These controls have been more effective at reducing surface []SO42− in summer (June, July, and August) than in winter (December, January, and February (DJF)), a seasonal contrast that is not robustly captured by Climate Model Intercomparison Project 5 global models. We use the Geophysical Fluid Dynamics Laboratory AM3 chemistry‐climate model to show that oxidant limitation during winter causes []SO42− (DJF) to be sensitive to primary SO42− emissions, in‐cloud titration of H2O2, and in‐cloud oxidation by O3. The observed contrast in the seasonal response of []SO42− to decreasing SO2 emissions is best explained by the O3 reaction, whose rate coefficient has increased over the past decades as a result of increasing NH3 emissions and decreasing SO2 emissions, both of which lower cloud water acidity. The fraction of SO2 oxidized to SO42− is projected to keep increasing in future decades, delaying improvements in wintertime air quality.

Exploring the relationship between surface PM<sub>2.5</sub> and meteorology in Northern India

“…In particular, the efficiency of removal of tracers by convective precipitation is significantly increased compared to the unrealistic weak removal in AM3 (Fang et al, 2011), while the wet removal by frozen precipitation produced by the Bergeron process is strongly reduced (Liu et al, 2011;Paulot et al, 2017). As a result, the spatial distribution of aerosol climatology and seasonal cycle simulated by AM4 has been significantly improved relative to AM3.…”

“…The base chemical mechanism used is from AM3, as described by Naik et al (2013), with gas-phase and heterogeneous chemistry updates from Mao et al (2013a, b), and updates to the treatment of sulfate and nitrate chemistry and revised treatment of wet deposition (Paulot et al, , 2017). Here we do not take into account the uptake of SO 2 and HNO 3 on dust.…”

Agricultural ammonia emissions in China: reconciling bottom-up and top-down estimates

“…These secondary inorganic aerosols account for 40-57 % of the fine PM concentrations in eastern China (Yang et al, 2011;Huang et al, 2014). Recent studies also highlighted the possible role of NH 3 in neutralizing aerosol pH that can strongly enhance formation of sulfate through heterogeneous oxidation of SO 2 Cheng et al, 2016;Paulot et al, 2016). All this evidence leads to increasing concerns that the effectiveness of SO 2 and NO x emission controls on fine PM pollution over China may be undermined by unregulated NH 3 emissions Fu et al, 2017).…”

Impact of agricultural emission reductions on fine-particulate matter and public health

“…Regions addressed in this study, i.e., North America (blue), Europe (green), South Asia (purple) and East Asia (red). Paulot et al, 2016;Wang et al, 2011), mostly with a regional rather than a global view.…”