A decade of emissions controls and their role on an ever-changing atmosphere
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The impact of emission controls in the decade 2001-2011 has been studied using a variety of models, that were updated to increase their predictive accuracy and to better represent the physics of atmospheric aerosols. Specifically, the TOMAS microphysics model was expanded to include radioactive charging, while the chemistry of the CMAQ model was improved using field data and results from the thermodynamic model ISORROPIA. For the modeling of radioactive particles, it was found that charging effects can have a profound impact on aerosol lifetimes for very large particles, but are negligible for the majority of atmospherically relevant aerosol sizes. With regards to emissions reductions, ozone formation over the decade shifted from a VOC limited regime to a NOx limited one, suggestive of diminishing returns for NOx reductions, while the reductions in SO2 emissions are found to reduce isoprene Secondary Organic Aerosol (SOA) significantly. Modeled aerosol pH and aerosol nitrate increased over the studied period, in contrast to what has been observed, and the presence of non-volatile cations in concentrations much higher than the measured ones is identified as the reason for this bias. These findings suggest that emission reductions had a profound impact on atmospheric aerosol and highlighted the need for rigorous model evaluation in order to alleviate existing biases and improve future regulatory decision-making.