Atmospheric furan is a primary and secondary pollutant in the atmosphere, and its emission contributes to the formation of ultrafine particles. We investigate the effects of NO_x level and humidity on the formation of secondary organic aerosol (SOA) generated from the photooxidation of furan in the presence of NaCl seed particles. SOA mass concentration and yield were determined under different NO_x and humidity levels. A significant difference is observed both in the variation of SOA mass concentration and SOA yield with the initial experimental conditions. Varying VOC (volatile organic compound) ∕ NO_x ratios over the range 48.1 to 8.2 contributes to the effective formation of SOA in the presence of NaCl seed particles, with the SOA mass concentration and SOA yield ranging from 0.96 to 23.46 µg m⁻³ and from 0.04 % to 1.01 %, respectively. We found that there was a favourable relationship between the SOA yields and NO_x concentration. In particular, the increase in SOA yield with increasing NO_x concentration was continuously observed at high NO_x levels owing to a corresponding increase in the amount of low-volatility hydroxyl nitrates and dihydroxyl dinitrates that can partition into the particle phase. In addition, varying relative humidity (RH) from 5 % to 88 % increased the SOA yield from 1.01 % to 5.03 %. The enhanced SOA formation from humid conditions may result from the high OH concentration, rapid furan decay rate, enhanced carbonyl-rich products condensation, and the aqueous-phase reactions. Using hybrid quadrupole-orbitrap mass spectrometer equipped with electrospray ionization (HESI-Q Exactive-Orbitrap MS), three carbonyl-rich products and three kinds of organonitrates were identified in the collected SOA. Based on the HESI-Q Exactive-Orbitrap MS analysis and Fourier transform infrared spectroscopy (FTIR), the reaction mechanism of furan photooxidation was proposed. This study demonstrates the effects of NO_x and humidity on SOA formation during the furan–NO_x–NaCl photooxidation and provides new insights into the oxidation regime and SOA composition in furan photooxidation. The results also illustrate the importance of studying SOA formation over a comprehensive range of environmental conditions. Only such evaluations can induce meaningful SOA mechanisms to be implemented in air quality models.

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.