Effects of NO and SO₂ on the secondary organic aerosol formation from isoprene photooxidation

Investigation of the effects of anthropogenic pollutants on the mass concentrations, particle number concentrations, and chemical compositions of secondary organic aerosol (SOA) formation is essential in understanding the photooxidation mechanism of volatile organic compounds but has been proven to be a very challenging experimental target because of their complex processes. Here, the effects of NO and SO2 on SOA formation from isoprene photooxidation were studied by a number of laboratory studies. The results indicate the accumulated O-3 upon the NO conversion triggers the main oxidation reaction, by which the oxidants (i.e., OH and SO4-) are derived. The SOA mass concentrations and particle number concentrations are enhanced by NO under low NO concentrations but suppressed by NO under high NO concentrations, which are consistent with previous studies (Kroll et al., 2005, 2006). This could be rationalized that the large amount of accumulated O-3 inhibit the multi-step isoprene oxidation and a large number of less oxidized products (m/z < 130) absorb or condense on the surface of existing particles, which decelerate the SOA nucleation and growth. The presence of SO2 accelerates the conversion of NO and the accumulation of O-3, which promotes the main oxidation reaction of isoprene. New compounds are observed at m/z = 134, 137, 150, 152, and 179 measured by threshold photoionization (positive ion mode) with a tunable vacuum ultraviolet free electron laser and are found to be mainly formed from the isoprene oxidization by O-3 and OH. The present findings provide both macroscopic and microscopic information to advance understanding of atmospheric components affected by the anthropogenic-biogenic interactions in the neighborhood of emission origins.