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.

&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; In this study, the NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; dependence of secondary organic aerosol (SOA) formation from &amp;#946;-pinene photooxidation was comprehensively investigated in the J&amp;#252;lich Plant Atmosphere Chamber. Consistent with the results of previous NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; studies we found increases of SOA yields at low NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; conditions ([NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;]&lt;sub&gt;0&lt;/sub&gt; &lt; 30 ppb, [BVOC]&lt;sub&gt;0&lt;/sub&gt;/[NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;]&lt;sub&gt;0&lt;/sub&gt; &gt; 10 ppbC ppb&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;). Furthermore, increasing [NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;] at high NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; conditions ([NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;]&lt;sub&gt;0&lt;/sub&gt; &gt; 30 ppb, [BVOC]&lt;sub&gt;0&lt;/sub&gt;/[NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;]&lt;sub&gt;0&lt;/sub&gt; ~ 10 to ~ 2.6 ppbC ppb&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;) suppressed the SOA yield. The increase of SOA yield at low NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; conditions was attributed to increase of OH concentration, most probably by OH recycling in NO + HO&lt;sub&gt;2&lt;/sub&gt; &amp;#8594; NO&lt;sub&gt;2&lt;/sub&gt; + OH reaction. Separate measurements without NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; 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&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;]. Measuring the NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; dependence of SOA yields at lower [NO]/[NO&lt;sub&gt;2&lt;/sub&gt;] 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&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; dependencies vary for different NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; compositions. A substantial fraction of the NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;-induced decrease of SOA yields at high NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; conditions was caused by NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;-induced suppression of new particle formation (NPF). This was shown by probing the NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; dependence of SOA formation in the presence of seed particles. After eliminating the effect of NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;-induced suppression of NPF and NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; induced changes of OH concentrations, the overall effect of NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; on the SOA yield from &amp;#946;-pinene photooxidation was moderate. Comparing with &amp;#946;-pinene experiments, the SOA formation from &amp;#945;-pinene photooxidation was only suppressed by increasing NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;. However, basic mechanisms of the NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; impacts were the same as that of &amp;#946;-pinene.&lt;/p&gt;

&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; The formation of organic nitrates (ON) in the gas phase and their impact on mass formation of Secondary Organic Aerosol (SOA) was investigated in a laboratory study for &lt;i&gt;&amp;#945;&lt;/i&gt;-pinene and &lt;i&gt;&amp;#946;&lt;/i&gt;-pinene photo-oxidation. Focus was the elucidation of those mechanisms that cause the often observed suppression of SOA mass formation by NO&lt;sub&gt;x&lt;/sub&gt;, and therein the role of highly oxygenated multifunctional molecules (HOM). We observed that with increasing NO&lt;sub&gt;x&lt;/sub&gt; (a) the portion of HOM organic nitrates (HOM-ON) increased, (b) the fraction of accretion products (HOM-ACC) decreased and (c) HOM-ACC contained on average smaller carbon numbers.&lt;/p&gt; &lt;p&gt;Specifically, we investigated HOM organic nitrates (HOM-ON), arising from the termination reactions of HOM peroxy radicals with NO&lt;sub&gt;x&lt;/sub&gt;, and HOM permutation products (HOM-PP), such as ketones, alcohols or hydroperoxides, formed by other termination reactions. Effective uptake coefficients &amp;#947;eff of HOM on particles were determined. HOM with more than 6 O-atoms efficiently condensed on particles (&lt;i&gt;&amp;#947;&lt;/i&gt;&lt;/sub&gt;&lt;sub&gt;eff&lt;/sub&gt;&amp;#8201;&gt;&amp;#8201;0.5 in average) and for HOM containing more than 8 O-atoms, every collision led to loss. There was no systematic difference in &lt;i&gt;&amp;#947;&lt;/i&gt;&lt;/sub&gt;&lt;sub&gt;eff&lt;/sub&gt; for HOM-ON and HOM-PP arising from the same HOM peroxy radicals. This similarity is attributed to the multifunctional character of the HOM: as functional groups in HOM arising from the same precursor HOM peroxy radical are identical, vapor pressures should not strongly depend on the character the final termination group. As a consequence, the suppressing effect of NO&lt;sub&gt;x&lt;/sub&gt; on SOA formation cannot be simply explained by replacement of terminal functional groups by organic nitrate groups.&lt;/p&gt; &lt;p&gt;The fraction of organic bound nitrate (OrgNO&lt;sub&gt;3&lt;/sub&gt;) stored in gas-phase HOM-ON appeared to be substantially higher than the fraction of particulate OrgNO&lt;sub&gt;3&lt;/sub&gt; observed by aerosol mass spectrometry. This result suggests losses of OrgNO&lt;sub&gt;3&lt;/sub&gt; for organic nitrates in particles, probably due to hydrolysis of OrgNO&lt;sub&gt;3&lt;/sub&gt; that releases HNO&lt;sub&gt;3&lt;/sub&gt; into the gas phase but leaves behind the organic rest in the particulate phase. However, the loss of HNO&lt;sub&gt;3&lt;/sub&gt; alone, could not explain the observed suppressing effect of NO&lt;sub&gt;x&lt;/sub&gt; on particle mass formation from &lt;i&gt;&amp;#945;&lt;/i&gt;-pinene and &lt;i&gt;&amp;#946;&lt;/i&gt;-pinene.&lt;/p&gt; &lt;p&gt;We therefore attributed most of the reduction in SOA mass yields with increasing NO&lt;sub&gt;x&lt;/sub&gt; to the significant suppression of gas-phase HOM-ACC which have high molecular mass and are potentially important for SOA mass formation at low NO&lt;sub&gt;x&lt;/sub&gt; conditions.&lt;/p&gt;

Abstract. The formation of organic nitrates (ONs) in the gas phase and their impact on mass formation of secondary organic aerosol (SOA) was investigated in a laboratory study for α-pinene and β-pinene photooxidation. Focus was the elucidation of those mechanisms that cause the often observed suppression of SOA mass formation by NOx, and therein the role of highly oxygenated multifunctional molecules (HOMs). We observed that with increasing NOx concentration (a) the portion of HOM organic nitrates (HOM-ONs) increased, (b) the fraction of accretion products (HOM-ACCs) decreased, and (c) HOM-ACCs contained on average smaller carbon numbers. Specifically, we investigated HOM organic nitrates (HOM-ONs), arising from the termination reactions of HOM peroxy radicals with NOx, and HOM permutation products (HOM-PPs), such as ketones, alcohols, or hydroperoxides, formed by other termination reactions. Effective uptake coefficients γeff of HOMs on particles were determined. HOMs with more than six O atoms efficiently condensed on particles (γeff>0.5 on average), and for HOMs containing more than eight O atoms, every collision led to loss. There was no systematic difference in γeff for HOM-ONs and HOM-PPs arising from the same HOM peroxy radicals. This similarity is attributed to the multifunctional character of the HOMs: as functional groups in HOMs arising from the same precursor HOM peroxy radical are identical, vapor pressures should not strongly depend on the character of the final termination group. As a consequence, the suppressing effect of NOx on SOA formation cannot be simply explained by replacement of terminal functional groups by organic nitrate groups. According to their γeff all HOM-ONs with more than six O atoms will contribute to organic bound nitrate (OrgNO3) in the particulate phase. However, the fraction of OrgNO3 stored in condensable HOMs with molecular masses > 230 Da appeared to be substantially higher than the fraction of particulate OrgNO3 observed by aerosol mass spectrometry. This result suggests losses of OrgNO3 for organic nitrates in particles, probably due to hydrolysis of OrgNO3 that releases HNO3 into the gas phase but leaves behind the organic rest in the particulate phase. However, the loss of HNO3 alone could not explain the observed suppressing effect of NOx on particle mass formation from α-pinene and β-pinene. Instead we can attribute most of the reduction in SOA mass yields with increasing NOx to the significant suppression of gas phase HOM-ACCs, which have high molecular mass and are potentially important for SOA mass formation at low-NOx conditions.