Abstract
Abstract. In this study, the NOx 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 NOx studies we found increases of SOA yields with increasing [NOx] at low-NOx conditions ([NOx]0 < 30 ppb, [BVOC]0 ∕ [NOx]0 > 10 ppbC ppb−1). Furthermore, increasing [NOx] at high-NOx conditions ([NOx]0 > 30 ppb, [BVOC]0 ∕ [NOx]0 ∼ 10 to ∼ 2.6 ppbC ppb−1) suppressed the SOA yield. The increase of SOA yield at low-NOx conditions was attributed to an increase of OH concentration, most probably by OH recycling in NO + HO2 → NO2 + OH reaction. Separate measurements without NOx 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 [NOx]. Measuring the NOx dependence of SOA yields at lower [NO] ∕ [NO2] 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 NOx dependencies vary for different NOx compositions. A substantial fraction of the NOx-induced decrease of SOA yields at high-NOx conditions was caused by NOx-induced suppression of new particle formation (NPF), which subsequently limits the particle surface where low volatiles condense. This was shown by probing the NOx dependence of SOA formation in the presence of seed particles. After eliminating the effect of NOx-induced suppression of NPF and NOx-induced changes of OH concentrations, the remaining effect of NOx 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 NOx. However, basic mechanisms of the NOx impacts were the same as that of β-pinene.
Highlights
Biogenic volatile organic compounds (BVOCs), such as monoterpenes (C10H16) are emitted in large quantities into the atmosphere (Guenther et al, 1995, 2012; Griffin et al, 1999a)
We investigated the secondary organic aerosol (SOA) formation from β-pinene photooxidation under varied NOx levels in the Jülich Plant Atmosphere Chamber (JPAC) to gain more insight into the impact of NOx on SOA yield and to better characterize mechanisms leading to effects of NOx on SOA yield
At low NOx conditions an increase in the initial NOx concentration increases the SOA yield, whereas at high-NOx concentrations the opposite SOA yield dependence on NOx was observed
Summary
Biogenic volatile organic compounds (BVOCs), such as monoterpenes (C10H16) are emitted in large quantities into the atmosphere (Guenther et al, 1995, 2012; Griffin et al, 1999a) These BVOCs are oxidized in the atmosphere by hydroxyl radicals (OH), ozone (O3) or nitrate radicals (NO3) resulting in the formation of secondary organic aerosol (SOA). The reported SOA mass yield for α-pinene photooxidation ranges from 8 to 37 % (Eddingsaas et al, 2012a) This variability is likely related to the numerous factors that influence the SOA yields, such as the inorganic and organic mass loading, particle acidity, NOx (NOx = NO + NO2) level, humidity, and temperature. Ambient SOA yields cannot be represented by a unique value for a given monoterpene as the yields are heavily dependent on the conditions under which the SOA is formed
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