α-Pinene secondary organic aerosol yields increase at higher relative humidity and low NO<sub><i>x</i></sub> conditions

Abstract

<p><strong>Abstract.</strong> Secondary organic aerosol (SOA) yields from the photooxidation of α-pinene were investigated in smog chamber (SC) experiments at low (23–29 %) and high (60–69 %) relative humidity (RH), various NO<sub><i>x</i></sub>/VOC ratios (0.04–3.8) and with different aerosol seed chemical compositions (acidic to neutralized sulfate-containing or hydrophobic organic). A combination of a scanning mobility particle sizer and an Aerodyne high resolution time-of-flight aerosol mass spectrometer was used to determine SOA mass concentration and chemical composition. We present wall-loss-corrected yields as a function of absorptive masses combining organics and the bound liquid water content. High RH increased SOA yields by up to six times (1.5–6.4) compared to low RH. The yields at low NO<sub><i>x</i></sub>/VOC ratios were in general higher compared to yields at high NO<sub><i>x</i></sub>/VOC ratios. This NO<sub><i>x</i></sub> dependence follows the same trend as seen in previous studies for α-pinene SOA. A novel approach of data evaluation using volatility distributions derived from experimental data served as basis for thermodynamic phase partitioning calculations of model mixtures in this study. These calculations predict liquid-liquid phase separation into organic-rich and electrolyte phases. At low NO<sub><i>x</i></sub> conditions, equilibrium partitioning between the gas and liquid phases can explain most of the increase in SOA yields at high RH. This is indicated by the model results, when in addition to the α-pinene photooxidation products described in the literature, more fragmented and oxidized organic compounds are added to the model mixtures. This increase is driven by both the increase in the absorptive mass due to the additional particulate water and the solution non-ideality described by the activity coefficients. In contrast, at high NO<sub><i>x</i></sub>, equilibrium partitioning alone could not explain the strong increase in the yields with increased RH. This suggests that other processes including the reactive uptake of semi-volatile species into the liquid phase may occur and be enhanced at higher RH, especially for compounds formed under high NO<sub><i>x</i></sub> conditions such as carbonyls.</p>

Inflammatory responses to secondary organic aerosols (SOA) generated from biogenic and anthropogenic precursors

“…Specifically here, very high levels of IL-6 were observed post-exposure to pentadecane SOA formed under humid conditions. Prior studies reported opposing findings with some showing a significant effect of water on aerosol formation and chemical composition (Nguyen et al, 2011;Wong et al, 2015;Healy et al, 2009;Stirnweis et al, 2016), while others found little influence (Edney et al, 2000;Boyd et al, 2015;Cocker III et al, 2001). It is clear that humidity effects are highly hydrocarbon-dependent and further studies into the specific products formed under humid conditions are required to understand how these differences in chemical composition may translate to different cellular endpoints.…”

Chemical oxidative potential of secondary organic aerosol (SOA) generated from the photooxidation of biogenic and anthropogenic volatile organic compounds

“…CC-BY 3.0 License. finding significant changes in SOA composition and yields (Nguyen et al, 2011;Wong et al, 2015;Healy et al, 2009;Stirnweis et al, 2016) and others reporting little difference (Boyd et al, 2015;Edney et al, 2000;Cocker III et al, 2001). Humidity effects are therefore highly hydrocarbon-dependent.…”

“…Similarly, anthropogenic precursors were chosen to include a long-chain alkane (pentadecane), a single-ring aromatic (m-xylene), and a poly-aromatic (naphthalene). These classes of compounds are emitted as products of incomplete combustion (Robinson et al, 2007;Jia and Batterman, 2010;Bruns et al, 2016) and have been shown to have considerable SOA yields (e.g., Chan et al, 2009;Ng et al, 2007b;Lambe et al, 2011). In addition to precursor identity, the effects of humidity (dry vs. humid) and NOx (differing peroxy radical (RO2) fates, RO2 + HO2 vs. RO2 + NO) on OP WS-DTT were investigated, as these conditions have been shown to affect the chemical composition and mass loading of SOA formed (Chhabra et al, 2010;Chhabra et al, 2011;Eddingsaas et al, 2012;Ng et al, 2007b;Loza et al, 2014;Ng et al, 2007a;Chan et al, 2009;Boyd et al, 2015).…”

Chemical oxidative potential of secondary organic aerosol (SOA) generated from the photooxidation of biogenic and anthropogenic volatile organic compounds

“…It is noted that for experiment 7 (isoprene SOA under RO 2 + HO 2 -dominant, "humid" conditions), experimental conditions deviated due to extremely low SOA mass yields. For this experiment, an acidic seed solution (8 mM MgSO 4 and 16 mM H 2 SO 4 ) was used under dry conditions to promote SOA formation via the IEPOX (isoprene epoxydiols) uptake pathway, which has a higher SOA mass yield compared to the IEPOX + OH pathway and contributes significantly to ambient OA (Surratt et al, 2010;Lin et al, 2012).…”