Organosulfate-driven amplification of secondary organic aerosol formation from isoprene oxidation intermediates: Coupled effects of SO2 and relative humidity.

Methacrolein (MACR), a major primary oxidation product of isoprene, plays an essential role in secondary organic aerosol (SOA) formation. Here, we investigate the effect of SO2 levels and relative humidity (RH) on the SOA formation from the MACR/OH photooxidation system. From a series of experiments performed in an indoor smog chamber, the aerosol chemical composition was analyzed using gas chromatography-mass spectrometer (GC-MS) and Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR). The experimental results indicate that the particle mass concentration, maximum particle number concentration, geometric mean diameter and the SOA yields increased with increasing initial SO2 concentration under high (61 %-65 %), medium (20 %-22 %) and low (9 %-12 %) RH conditions. The SOA mass concentration increased as RH decreased in the presence of SO2, which could be attributed to reduced wall loss of hydrophilic species and the weak dilution of aerosol acidity under low RH conditions. Furthermore, we found that the presence of SO2 can affect the changes in the formed SOA mass concentration under different RH conditions. IR spectra results revealed the formation of organosulfate and inorganic sulfate compounds in the particle phase, and the formation mechanism of the detected SOA products is proposed. The results of this study provide a deeper understanding of SOA formation under complex polluted environments.