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. Theeffect of relative humidity (RH) on secondary organic aerosol (SOA) formationfrom the photooxidation of m-xylene initiated by OH radicals in the absenceof seed particles was investigated in a Teflon reactor. The SOA yields weredetermined based on the particle mass concentrations measured with a scanningmobility particle sizer (SMPS) and reacted m-xylene concentrationsmeasured with a gas chromatograph–mass spectrometer (GC-MS). The SOAcomponents were analyzed using a Fourier transform infrared (FTIR)spectrometer and an ultrahigh-performance liquid chromatograph–electrosprayionization–high-resolution mass spectrometer (UPLC-ESI-HRMS). A significantdecrease was observed in SOA mass concentration and yield variation with theincreasing RH conditions. The SOA yields are 14.0 %–16.5 % and0.8 %–3.2 % at low RH (14 %) and high RH (74 %–79 %),respectively, with the difference being nearly 1 order of magnitude. Some ofthe reduction in the apparent yield may be due to the faster wall loss ofsemi-volatile products of oxidation at higher RH. The chemical mechanism forexplaining the RH effects on SOA formation from m-xylene–OH system isproposed based on the analysis of both FTIR and HRMS measurements, and theMaster Chemical Mechanism (MCM) prediction is used as the assistant. The FTIRanalysis shows that the proportion of oligomers with C-O-C groups fromcarbonyl compounds in SOA at high RH is higher than that at low RH, butfurther information cannot be provided by the FTIR results to well explainthe negative RH effect on SOA formation. In the HRMS spectra, it is foundthat C2H2O is one of the most frequent mass differences at lowand high RHs, that the compounds with a lower carbon number in the formula atlow RH account for a larger proportion than those at high RH and that thecompounds at high RH have higher O : C ratios than those at low RH. TheHRMS results suggest that the RH may suppress oligomerization where water isinvolved as a by-product and may influence the further particle-phasereaction of highly oxygenated organic molecules (HOMs) formed in the gasphase. In addition, the negative RH effect on SOA formation is enlarged basedon the gas-to-particle partitioning rule.

atmospheric chemistry and physics

Secondary organic aerosol formation from OH-initiated oxidation of &amp;lt;i&amp;gt;m&amp;lt;/i&amp;gt;-xylene: effects of relative humidity on yield and chemical composition