Secondary organic aerosol formation from OH-initiated oxidation of <i>m</i>-xylene: effects of relative humidity on yield and chemical composition

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.

Secondary organic aerosol formation from monocyclic aromatic hydrocarbons: insights from laboratory studies

“…Increasing the RH shis the reaction equilibrium to small compounds, thereby suppressing oligomer formation. The negative effect of RH on SOA formation from the m-xylene photooxidation system without seed particles was also observed by Zhang et al 32 They suggested that both the faster wall losses of organic vapors and the shis of oligomer equilibrium reactions to monomers under higher RH condition can lead to the decrease in apparent m-xylene SOA yields. In marked difference to the ndings of Hinks et al, the mass concentrations of toluene SOA generated with inorganic seed particles were generally higher under higher-RH conditions due to the occurrence of aqueous-phase reactions in certain studies.…”

“…Over the past few decades, many efforts have been made to investigate the mass yield of aromatic SOA (the ratio of the mass of SOA generated to the mass of precursors consumed) and its formation mechanism. [31][32][33][34][35][36][37] However, the reported yields and formation mechanism of aromatic SOA remain highly variable even for a specic MAH. This variability can be attributed to the differences in the complex environmental conditions including physical parameters (i.e., solar intensity, temperature, and relative humidity (RH)), 32,35 inorganic pollutant emissions (i.e., nitrogen oxides (NO x ), ammonia, and sulfur dioxide (SO 2 )), 31,33 and reactant properties (VOC and oxidant concentrations as well as VOC structure).…”

“…[31][32][33][34][35][36][37] However, the reported yields and formation mechanism of aromatic SOA remain highly variable even for a specic MAH. This variability can be attributed to the differences in the complex environmental conditions including physical parameters (i.e., solar intensity, temperature, and relative humidity (RH)), 32,35 inorganic pollutant emissions (i.e., nitrogen oxides (NO x ), ammonia, and sulfur dioxide (SO 2 )), 31,33 and reactant properties (VOC and oxidant concentrations as well as VOC structure). 36,37 Ambient SOA mass yields cannot be represented by a xed value for a given MAH.…”

“…Over the past few decades, many efforts have been made to investigate the mass yield of aromatic SOA (the ratio of the mass of SOA generated to the mass of precursors consumed) and its formation mechanism. 31–37 However, the reported yields and formation mechanism of aromatic SOA remain highly variable even for a specific MAH. This variability can be attributed to the differences in the complex environmental conditions including physical parameters ( i.e.…”

Evaluation of Secondary Organic Aerosol (SOA) Simulations for Seoul, Korea

“…Unlike inorganic aerosol (i.e., SIA) formation, which favors humid conditions (Liu et al., 2018), observed SOA concentrations at Seoul during KORUS‐AQ show a negative relationship with relative humidity (RH; Figure S3 in Supporting Information ). Observational studies found that photochemistry was suppressed under high RH conditions, and larger anthropogenic SOA mass loadings and yields were observed with lower RH due to increased formation of oligomers in the absence of seed particles (Hinks et al., 2018; Liang et al., 2019; Liu et al., 2019; Zhang et al., 2019). Total PM loading and inorganic aerosol concentrations were particularly low during this period (Figure S2 in Supporting Information ), providing a less‐favorable condition for particle growth onto existing aerosols.…”