Reactive Uptake of Ammonia by Biogenic and Anthropogenic Organic Aerosols

Secondary organic aerosol (SOA) produced by photooxidation of biogenic and anthropogenic volatile organic compounds (VOCs) represent a large fraction of fine atmospheric particulate matter (PM2.5). The chemical composition of SOA particles is continuously changing as a result of various chemical and physical aging processes. One of the recently discovered aging processes, observed in both laboratory and field experiments, is the reactive uptake of ammonia by carbonyl species in SOA, leading to the formation of nitrogen-containing organic compounds (NOC). These NOC have attracted a lot of attention because of their propensity to absorb visible radiation and increase the amount of solar energy trapped in the atmosphere. Another potentially important, but poorly explored, consequence of NOC formation is that these compounds are less efficient than ammonia at neutralizing acids in particles. This paper summarizes existing experimental evidence for the reactive uptake of ammonia by SOA particles and describes our recent efforts to model the effect of this complicated process on air quality at the regional and continental scale. The modeling results predict that the reactive uptake of ammonia by SOA particles can significantly reduce the concentration of gas-phase ammonia, thereby indirectly affecting particle acidity as well as the amount of ammonium sulfate and ammonium nitrate in PM2.5. Since emissions of ammonia to the atmosphere are expected to increase due to the growing agricultural needs of the human population, these findings have important implications for future air pollution control strategies.