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In this study, the NOx dependence of secondary organic aerosol (SOA)formation from photooxidation of the biogenic volatile organic compound (BVOC) β-pinene was comprehensivelyinvestigated in the Jülich Plant Atmosphere Chamber. Consistent with theresults of previous NOx studies we found increases of SOA yields withincreasing [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 SOAyield at low-NOx conditions was attributed to an increase of OHconcentration, most probably by OH recycling in NO + HO2  →  NO2 + OH reaction. Separate measurements without NOxaddition but with different OH primary production rates confirmed the OHdependence of SOA yields. After removing the effect of OH concentration onSOA mass growth by keeping the OH concentration constant, SOA yields onlydecreased with increasing [NOx]. Measuring the NOx dependence ofSOA yields at lower [NO] ∕ [NO2] ratio showed less pronouncedincrease in both OH concentration and SOA yield. This result was consistentwith our assumption of OH recycling by NO and to SOA yields being dependent onOH concentrations. Our results furthermore indicated that NOxdependencies vary for different NOx compositions. A substantial fractionof the NOx-induced decrease of SOA yields at high-NOx conditionswas 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 thepresence of seed particles. After eliminating the effect of NOx-inducedsuppression of NPF and NOx-induced changes of OH concentrations, theremaining effect of NOx on the SOA yield from β-pinenephotooxidation was moderate. Compared to β-pinene, the SOA formationfrom α-pinene photooxidation was only suppressed by increasingNOx. However, basic mechanisms of the NOx impacts were the same asthat of β-pinene

atmospheric chemistry and physics

Impact of NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; and OH on secondary organic aerosol formation from &amp;lt;i&amp;gt;β&amp;lt;/i&amp;gt;-pinene photooxidation