Reactive uptake of ammonia by secondary organic aerosols: Implications for air quality

Abstract Reactions between ammonia (NH 3 ) and organic compounds containing carbonyl functional groups in aerosol particles can form organic products that are less basic than NH 3 and are thus unable to neutralize efficiently nitric and sulfuric acids. In this exploratory study, the University of California, Irvine - California Institute of Technology (UCI-CIT) model is used to investigate the potential air quality impacts of including of the chemical uptake of NH 3 by secondary organic aerosols (SOA) in a regional airshed model. A surface reaction of NH 3 with SOA is implemented into the model to determine the impact of this process on NH 3 and PM 2.5 concentrations in the South Coast Air Basin of California (SoCAB). Air quality simulations are conducted using uptake coefficients ranging from 10 −5 to 10 −2 to explore the sensitivity of changes in NH 3 and PM 2.5 concentrations to the magnitude of the uptake coefficient. Results indicate that the chemical uptake of NH 3 by SOA can potentially deplete gaseous NH 3 concentrations, causing indirect reductions in the amount of ammonium nitrate and ammonium sulfate in particulate matter. The magnitude of the impact on NH 3 and PM 2.5 concentrations exhibits a strong but non-linear dependence on the value of the uptake coefficient, with evidence for small but notable impacts on air quality even with the lowest assumed uptake coefficient of 10 −5 .