Molecular composition of secondary organic aerosol from styrene under different NOx and humidity conditions

Styrene is highly reactive in the atmosphere, which has been identified as a hazardous air pollutant by the Clean Air Act, and as an important secondary organic aerosol (SOA) precursor. In this study, the SOA formation from styrene under different NOx and relative humidity (RH) conditions were explored in an indoor chamber. The chemical composition of SOA was characterized using a Fourier transform infrared spectroscopy (FTIR) and a modified atmospheric pressure chemical ionization mass spectrometry (APCI-MS). The results show that the SOA yields decrease from 11.4% to 7.2% at about 70 mu g m(-3) SOA as RH increases from 8.5% to 62.4% in the H2O2 system; the SOA yields decrease from 1.1% to 0.7% at about 10 mu g m(-3) SOA as RH increases from 8.4% to 60.9% in the NOx system. In the H2O2 system, C-7 and C-8 series compounds are major components of SOA at both RHs. The total signal ratio of less oxidized C-7 and C-8 compounds (nO < 4) to the total SOA at 8.8% RH is 2.2 times higher than that at 63.3% RH. The OSc values of SOA were determined to be -0.2 at 8.8% RH and 0.04 at 63.3% RH based on MS data. It is proposed that high RH can accelerate the heterogeneous uptake of H2O2 onto particles and promote the further oxidation of less oxidized compounds in the particle phase. In the NOx system, SOA formation is significantly suppressed by the competitive reaction of RO2 with NO. The FTIR and MS results show that the main composition of SOA is organonitrates. At 8.3% RH, OH, O-3 and NO3 are oxidants in the styrene-NOx irritation system. Among these oxidants, NO3 plays a minor role due to its photolysis. While at 60.9% RH, SOA formation from O-3 is significantly inhibited by the reaction of stabilized Criegee intermediates (SCIs) with H2O, and SOA from the NO3 pathway is suppressed by the heterogeneous uptake of N2O5 to humid particles.

Automated summary

This study investigates the formation of secondary organic aerosols (SOA) from styrene under different nitrogen oxides (NOx) concentrations and relative humidity (RH) conditions. The results show that increasing RH decreases the SOA yields, while the presence of NOx significantly suppresses SOA formation.