Impact of NO x and OH on secondary organic aerosol formation from β-pinene photooxidation
In this study, the NO x dependence of secondary organic aerosol (SOA) formation from photooxidation of the biogenic volatile organic compound (BVOC) β-pinene was comprehensively investigated in the Jülich Plant Atmosphere Chamber. Consistent with the results of previous NO x studies we found increases of SOA yields with increasing [NO x] at low-NO x conditions ([NO x] 0 < 30 ppb, [BVOC] 0 ∕ [NO x] 0 > 10 ppbC ppb −1). Furthermore, increasing [NO x] at high-NO x conditions ([NO x] 0 > 30 ppb, [BVOC] 0 ∕ [NO x] 0 ∼ 10 to ∼ 2.6 ppbC ppb −1) suppressed the SOA yield. The increase of SOA yield at low-NO x conditions was attributed to an increase of OH concentration, most probably by OH recycling in NO + HO 2 → NO 2 + OH reaction. Separate measurements without NO x addition but with different OH primary production rates confirmed the OH dependence of SOA yields. After removing the effect of OH concentration on SOA mass growth by keeping the OH concentration constant, SOA yields only decreased with increasing [NO x]. Measuring the NO x dependence of SOA yields at lower [NO] ∕ [NO 2] ratio showed less pronounced increase in both OH concentration and SOA yield. This result was consistent with our assumption of OH recycling by NO and to SOA yields being dependent on OH concentrations. Our results furthermore indicated that NO x dependencies vary for different NO x compositions. A substantial fraction of the NO x-induced decrease of SOA yields at high-NO x conditions was caused by NO x-induced suppression of new particle formation (NPF), which subsequently limits the particle surface where low volatiles condense. This was shown by probing the NO x dependence of SOA formation in the presence of seed particles. After eliminating the effect of NO x-induced suppression of NPF and NO x-induced changes of OH concentrations, the remaining effect of NO x on the SOA yield from β-pinene photooxidation was moderate. Compared to β-pinene, the SOA formation from α-pinene photooxidation was only suppressed by increasing NO x. However, basic mechanisms of the NO x impacts were the same as that of β-pinene.