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Secondary organic aerosol (SOA) yields from the photo-oxidation of α-pinene were investigated in smog chamber (SC) experiments at low(23–29 %) and high (60–69 %) relative humidity (RH), variousNOx ∕ VOC ratios (0.04–3.8) and with different aerosol seedchemical compositions (acidic to neutralized sulfate-containing orhydrophobic organic). A combination of a scanning mobility particle sizer andan Aerodyne high-resolution time-of-flight aerosol mass spectrometer was usedto determine SOA mass concentration and chemical composition. We used a MonteCarlo approach to parameterize smog chamber SOA yields as a function of thecondensed phase absorptive mass, which includes the sum of OA and thecorresponding bound liquid water content. High RH increased SOA yields by upto 6 times (1.5–6.4) compared to low RH. The yields at lowNOx ∕ VOC ratios were in general higher compared to yields at highNOx ∕ VOC ratios. This NOx dependence follows the same trend asseen in previous studies for α-pinene SOA.A novel approach of data evaluation using volatility distributions derivedfrom experimental data served as the basis for thermodynamic phasepartitioning calculations of model mixtures in this study. These calculationspredict liquid–liquid phase separation into organic-rich and electrolytephases. At low NOx conditions, equilibrium partitioning between the gasand liquid phases can explain most of the increase in SOA yields observed athigh RH, when in addition to the α-pinene photo-oxidation productsdescribed in the literature, fragmentation products are added to the modelmixtures. This increase is driven by both the increase in the absorptive massand the solution non-ideality described by the compounds' activitycoefficients. In contrast, at high NOx, equilibrium partitioning alonecould not explain the strong increase in the yields with RH. This suggeststhat other processes, e.g. reactive uptake of semi-volatile species into theliquid phase, may occur and be enhanced at higher RH, especially forcompounds formed under high NOx conditions, e.g. carbonyls

atmospheric chemistry and physics

Assessing the influence of NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; concentrations and relative humidity on secondary organic aerosol yields from &amp;lt;i&amp;gt;α&amp;lt;/i&amp;gt;-pinene photo-oxidation through smog chamber experiments and modelling calculations