NOx enhances secondary organic aerosol formation from nighttime γ-terpinene ozonolysis

Abstract

As an important component of anthropogenic emissions, nitrogen oxides (NOx) are well known to interfere with daytime oxidation of biogenic volatile organic compounds (BVOCs) and secondary organic aerosol (SOA) formation. Nighttime chemistry is highly related to O3 and NOx level but, NOx effects on SOA formation from the ozonolysis of BVOCs, especially polyolefinic monoterpenes, have not been well understood. In the present study, SOA formation from pure and NOx-involved γ-terpinene ozonolysis was studied in a smog chamber under dark conditions. At atmospherically relevant particle mass loading of 10 μg m−3, the SOA yield from pure ozonolysis is estimated by a two-product model to be 8.6%. When NOx were incorporated into γ-terpinene ozonolysis, both the particle size and SOA yields increased simultaneously with elevated NOx mixing ratios. SOA yields doubled (from 0.38 to 0.77) when the system moved from NOx-free to [γ-terpinene]0/[NOx]0 = 3.5 (ppbC/ppb). The characteristic absorption of organic nitrates was detected by Fourier transform infrared (FTIR) spectroscopy and the fraction of organic nitrates increased with increasing NOx mixing ratios. Identification of the new constituents in SOA from NOx-involved γ-terpinene ozonolysis and their formation channels suggest that the formation of organic nitrates follows NO3 chemistry. NOx affects γ-terpinene ozonolysis via the enhanced generation of NO3 at high NOx and its subsequent more favored consumption of γ-terpinene than O3. The first-generation products from NO3 oxidation of γ-terpinene could be further oxidized by ozone, forming more oxidized products that contribute to SOA formation. Our investigation suggests that at night with high NOx levels, γ-terpinene may be a significant source of SOA and organic nitrates through anthropogenic-biogenic interactions.