Abstract
Abstract. The effect of relative humidity (RH) on secondary organic aerosol (SOA) formation from the photooxidation of m-xylene initiated by OH radicals in the absence of seed particles was investigated in a Teflon reactor. The SOA yields were determined based on the particle mass concentrations measured with a scanning mobility particle sizer (SMPS) and reacted m-xylene concentrations measured with a gas chromatograph–mass spectrometer (GC-MS). The SOA components were analyzed using a Fourier transform infrared (FTIR) spectrometer and an ultrahigh-performance liquid chromatograph–electrospray ionization–high-resolution mass spectrometer (UPLC-ESI-HRMS). A significant decrease was observed in SOA mass concentration and yield variation with the increasing RH conditions. The SOA yields are 14.0 %–16.5 % and 0.8 %–3.2 % at low RH (14 %) and high RH (74 %–79 %), respectively, with the difference being nearly 1 order of magnitude. Some of the reduction in the apparent yield may be due to the faster wall loss of semi-volatile products of oxidation at higher RH. The chemical mechanism for explaining the RH effects on SOA formation from m-xylene–OH system is proposed based on the analysis of both FTIR and HRMS measurements, and the Master Chemical Mechanism (MCM) prediction is used as the assistant. The FTIR analysis shows that the proportion of oligomers with C-O-C groups from carbonyl compounds in SOA at high RH is higher than that at low RH, but further information cannot be provided by the FTIR results to well explain the negative RH effect on SOA formation. In the HRMS spectra, it is found that C2H2O is one of the most frequent mass differences at low and high RHs, that the compounds with a lower carbon number in the formula at low RH account for a larger proportion than those at high RH and that the compounds at high RH have higher O : C ratios than those at low RH. The HRMS results suggest that the RH may suppress oligomerization where water is involved as a by-product and may influence the further particle-phase reaction of highly oxygenated organic molecules (HOMs) formed in the gas phase. In addition, the negative RH effect on SOA formation is enlarged based on the gas-to-particle partitioning rule.
Highlights
Secondary organic aerosol (SOA) is a significant component of atmospheric fine particulate matter in the troposphere (Hallquist et al, 2009; Spracklen et al, 2011; Huang et al, 2014), causing serious concern as it has a significant influence on air quality, the oxidative capacity of the troposphere, global climate change and human health (Jacobson et al, 2000; Hansen and Sato, 2001; Kanakidou et al, 2005; Zhang et al, 2014)
The current study investigates the effect of relative humidity (RH) on SOA formation from the oxidation of m-xylene under low-NOx conditions in the absence of seed particles
The Fourier transform infrared (FTIR) analysis shows that the proportion of oligomers with C-O-C groups from carbonyl compounds in SOA at high RH is higher than that at low RH, but the negative RH effect on SOA formation cannot be well explained as the FTIR results cannot provide further information
Summary
Secondary organic aerosol (SOA) is a significant component of atmospheric fine particulate matter in the troposphere (Hallquist et al, 2009; Spracklen et al, 2011; Huang et al, 2014), causing serious concern as it has a significant influence on air quality, the oxidative capacity of the troposphere, global climate change and human health (Jacobson et al, 2000; Hansen and Sato, 2001; Kanakidou et al, 2005; Zhang et al, 2014). In a study on the chemical oxidative potential of SOA (Tuet et al, 2017) under lowNOx conditions, it was observed that the mass concentration of SOA from m-xylene irradiation under dry conditions was much larger than that under humid conditions, whereas the study did not focus on the mechanism of the RH effect on m-xylene SOA formation. These demonstrate that the RH effects on aromatic SOA yields, especially m-xylene, have not been fully understood, and RH effects are controversial under various NOx levels and seed particle conditions. The underlying mechanism of SOA formation for these different conditions will be discussed
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