Impact of NO<sub><i>x</i></sub> and OH on secondary organic aerosol (SOA) formation from β-pinene photooxidation

Abstract

<p><strong>Abstract.</strong> In this study, the NO<sub><i>x</i></sub> dependence of secondary organic aerosol (SOA) formation from β-pinene photooxidation was comprehensively investigated in the Jülich Plant Atmosphere Chamber. Consistent with the results of previous NO<sub><i>x</i></sub> studies we found increases of SOA yields at low NO<sub><i>x</i></sub> conditions ([NO<sub><i>x</i></sub>]<sub>0</sub> < 30 ppb, [BVOC]<sub>0</sub>/[NO<sub><i>x</i></sub>]<sub>0</sub> > 10 ppbC ppb<sup>−1</sup>). Furthermore, increasing [NO<sub><i>x</i></sub>] at high NO<sub><i>x</i></sub> conditions ([NO<sub><i>x</i></sub>]<sub>0</sub> > 30 ppb, [BVOC]<sub>0</sub>/[NO<sub><i>x</i></sub>]<sub>0</sub> ~ 10 to ~ 2.6 ppbC ppb<sup>−1</sup>) suppressed the SOA yield. The increase of SOA yield at low NO<sub><i>x</i></sub> conditions was attributed to increase of OH concentration, most probably by OH recycling in NO + HO<sub>2</sub> → NO<sub>2</sub> + OH reaction. Separate measurements without NO<sub><i>x</i></sub> 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<sub><i>x</i></sub>]. Measuring the NO<sub><i>x</i></sub> dependence of SOA yields at lower [NO]/[NO<sub>2</sub>] ratio showed less pronounced increase in both; OH concentration and SOA yield. This result was consistent to our assumption of OH recycling by NO and to SOA yields being dependent on OH concentrations. It furthermore indicated that NO<sub><i>x</i></sub> dependencies vary for different NO<sub><i>x</i></sub> compositions. A substantial fraction of the NO<sub><i>x</i></sub>-induced decrease of SOA yields at high NO<sub><i>x</i></sub> conditions was caused by NO<sub><i>x</i></sub>-induced suppression of new particle formation (NPF). This was shown by probing the NO<sub><i>x</i></sub> dependence of SOA formation in the presence of seed particles. After eliminating the effect of NO<sub><i>x</i></sub>-induced suppression of NPF and NO<sub><i>x</i></sub> induced changes of OH concentrations, the overall effect of NO<sub><i>x</i></sub> on the SOA yield from β-pinene photooxidation was moderate. Comparing with β-pinene experiments, the SOA formation from α-pinene photooxidation was only suppressed by increasing NO<sub><i>x</i></sub>. However, basic mechanisms of the NO<sub><i>x</i></sub> impacts were the same as that of β-pinene.</p>

Effects of NO<sub><i>x</i></sub> and SO<sub>2</sub> on the secondary organic aerosol formation from photooxidation of <i>α</i>-pinene and limonene

“…The loss of organic vapors on chamber walls can influence SOA yield Zhang et al, 2014;Ehn et al, 2014;Sarrafzadeh et al, 2016;McVay et al, 2016;Nah et al, 2016;Matsunaga and Ziemann, 2010;Ye et al, 2016;Loza et al, 2010). The wall loss rate of organic vapors in our chamber was estimated by following the decay of organic vapor concentrations after photooxidation was stopped in the experiments with low particle surface area (∼ 5 × 10 −8 cm 2 cm −3 ) and thus low condensational sink on particles.…”

“…The wall loss rate of organic vapors in our chamber was estimated by following the decay of organic vapor concentrations after photooxidation was stopped in the experiments with low particle surface area (∼ 5 × 10 −8 cm 2 cm −3 ) and thus low condensational sink on particles. Such method is similar to the method used in previous studies Sarrafzadeh et al, 2016;Krechmer et al, 2016;Zhang et al, 2015). A high-resolution time-of-flight chemical ionization mass spectrometer (HRToF-CIMS, Aerodyne Research Inc.) with nitrate ion source ( 15 NO − 3 ) was used to measure semi/low-volatility organic vapors.…”

“…The decrease in SOA yield with increasing NO x was proposed to be due to the formation of more volatile products like organic nitrate under high-NO x conditions (Presto et al, 2005). In contrast, a recent study found that the suppressing effect of NO x is in large part attributed to the effect of NO x on OH concentration for the SOA from β-pinene oxidation, and, after eliminating the effect of NO x on OH concentration, SOA yield only varies by 20-30 % (Sarrafzadeh et al, 2016). In addition to the effect of NO x on SOA yield, NO x has been found to suppress the new particle formation from VOC directly emitted by Mediterranean trees (mainly monoterpenes; Wildt et al, 2014) and β-pinene (Sarrafzadeh et al, 2016), thereby reducing the condensational sink present during high-NO x experiments.…”

“…The anthropogenic modulation of the SOA formation from monoterpenes can have important impacts on the regional and global biogenic SOA budget (Spracklen et al, 2011). The influence of various anthropogenic pollutants on SOA formation of monoterpenes has been investigated by a number of laboratory studies (Sarrafzadeh et al, 2016;Flores et al, 2014;Emanuelsson et al, 2013;Eddingsaas et al, 2012a;Offenberg et al, 2009;Kleindienst et al, 2006;Presto et al, 2005;Ng et al, 2007;Zhang et al, 1992;Pandis et al, 1991;Draper et al, 2015;Han et al, 2016). In particular, NO x and SO 2 have been shown to affect SOA formation from monoterpenes.…”

“…Yet, the effects of NO x and SO 2 at concentration ranges for ambient anthropogenic-biogenic interactions (sub-ppb to several tens of ppb for NO 2 and SO 2 ) have seldom been directly addressed. Moreover, many previous studies on the SOA formation from monoterpene oxidation focus on ozonolysis or do not distinguish OH oxidation and ozonolysis in photooxidation, and only a few studies on OH oxidation have been conducted (Eddingsaas et al, 2012a;Zhao et al, 2015b;McVay et al, 2016;Sarrafzadeh et al, 2016;Henry et al, 2012;Ng et al, 2007). More importantly, studies that investigated the combined effects of NO x and SO 2 are scarce, although they are often co-emitted from anthropogenic sources.…”