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.

Secondary organic aerosol (SOA) formation from a cyclohexene ∕ NO_x system with various SO₂ concentrations under UV light was investigated to study the effects of cyclic alkenes on the atmospheric environment in polluted urban areas. A clear decrease at first and then an increase in the SOA yield was found with increasing SO₂ concentrations. The lowest SOA yield was obtained when the initial SO₂ concentration was in the range of 30–40 ppb, while higher SOA yield compared to that without SO₂ could not be obtained until the initial SO₂ concentration was higher than 85 ppb. The decreasing SOA yield might be due to the fact that the promoting effect of acid-catalysed reactions on SOA formation was less important than the inhibiting effect of decreasing OH concentration at low initial SO₂ concentrations, caused by the competition reactions of OH with SO₂ and cyclohexene. The competitive reaction was an important factor for SOA yield and it should not be neglected in photooxidation reactions. The composition of organic compounds in SOA was measured using several complementary techniques including Fourier transform infrared (FTIR) spectroscopy, ion chromatography (IC), and Exactive Plus Orbitrap mass spectrometer equipped with electrospray interface (ESI). We present new evidence that organosulfates were produced from the photooxidation of cyclohexene in the presence of SO₂.