Secondary organic aerosol from sesquiterpenes and other biogenic emissions in the United States
Secondary organic aerosol (SOA) is a major component in fine particulate matter, which is a major cause of health effects and climate change. Atmospheric chemistry and transport models typically underestimate SOA concentrations because of the lack of understanding in emission estimates and SOA formation. Sesquiterpenes (SQTs) have been hypothesized to be a significant fraction of the unexplained organic aerosol due to their significant emissions, high reactivities, and high aerosol yields. This study developed a new SQT emission inventory from the largest and most up-to-date emission rate database and updated a biogenic emission model, Model of Emissions of Gases and Aerosols from Nature (MEGAN), for SQT emission estimation. The monthly average SQT emissions for the continental U.S. domain are 298 tons hr-1 for July and 12 tons hr-1 for January, and they account for 12% and 7% of biogenic SOA precursor emissions for July and January, respectively. The new emission estimates are significantly lower than emission estimates developed from emission rates reported in the earlier literature. The estimated SQT and other biogenic volatile organic compound emissions were used in a modified and updated regional chemical transport model, Models-3 CMAQ, to explore the formation and contribution of SOA from SQTs in the United States. Inclusion of SOA production from SQTs increases the total SOA mass by a factor of 1.7 to 4. The amount of SOA mass produced from SQT is similar to the quantity produced from monoterpenes. Comparisons to observations show that the inclusion of SQTs improves the SOA prediction performance; however, the model still underpredicts the SOA concentrations. Sensitivity studies suggest that polymerization of SOA could occur in the atmosphere, and that SOA formation is very sensitive to the enthalpy of vaporization value used in the gas-particle partitioning module, which is not well understood. Although this study shows that SQT emissions are an important contributor to SOA production, the underestimation of SOA concentrations observed with the modified CMAQ model indicate the emission inventory and SOA process descriptions still need to be improved.