Isoprene Suppression of New Particle Formation: Potential Mechanisms and Atmospheric Implications (Invited Presentation)

Secondary aerosols formed from anthropogenic pollutants and natural emissions have substantial impacts on human health, air quality and the Earth's climate. New particle formation (NPF) contributes up to 70% of the global production of CCN, but the effects of biogenic VOCs and their oxidation products on NPF processes are poorly understood. Observations have shown that isoprene, the most abundant biogenic species, suppresses NPF in forests. But the previously proposed chemical mechanism underlining this suppression process contradicts atmospheric observations. Here, we provide new insights on isoprene suppression of the biogenic NPF, based on comprehensive observations of key chemical precursors in a rural forest in the Southeastern U.S. and quantum chemical calculations. Our findings imply that in an isoprene-dominant forest, volatile oxidation products formed from isoprene compete with low-volatility oxidation products from monoterpenes in clustering with sulfuric acid, to suppress the growth of clusters. Current climate models treat NPF processes by considering only sulfuric acid and total low-volatility organic compounds, regardless of forest biogenic emission patterns over the globe. We conclude that NPF suppression in isoprene-emitting forests should be included in models to correctly predict the climate forcing by aerosols and clouds.