Impact of NO_x and OH on secondary organic aerosol formation from β-pinene photooxidation

摘要

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 COH 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 β-pinene photooxidat