A review of Secondary Organic Aerosol (SOA) formation from isoprene

Recent field and laboratory evidence indicates that the oxidation of isoprene, (2-methyl-1,3-butadiene, C₅H₈) forms secondary organic aerosol (SOA). Global biogenic emissions of isoprene (600 Tg yr⁻¹) are sufficiently large that the formation of SOA in even small yields results in substantial production of atmospheric particulate matter, likely having implications for air quality and climate. Here we present a review of field measurements, experimental work, and modeling studies aimed at understanding the mechanisms, yield, and atmospheric importance of isoprene-derived SOA. SOA yields depend on a number of factors, including organic aerosol loading (Mo), NOx level (RO₂ chemistry), and, because of the importance of multigenerational chemistry, the degree of oxidation. These dependences are not always included in SOA modules used in atmospheric transport models, and instead most yield parameterizations rely on a single set of chamber experiments (carried out over a limited range of conditions); this may lead to very different estimates of the atmospheric importance of isoprene SOA. New yield parameterizations, based on all available laboratory data (Mo=0-50 μg m⁻³), are presented here, so that SOA formation may be computed as a function of Mo, NOx level, and temperature. Current research needs and future research directions are identified.