Secondary organic aerosol formation from photooxidation of furan: effects of NO_x level and humidity

Abstract. Atmospheric furan is both primary and secondary pollutants in the atmosphere, and their emission contributes to the formation of ultrafine particles and ground-level ozone. 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. The particle mass concentration and size distribution were determined with a scanning mobility particle sizer (SMPS). SOA mass concentration and yield were determined under different NO_x and humidity levels. Owing to condensation and coagulation, the particle number concentration decreases with increasing particle size. A significant difference is observed both in the SOA mass concentration and SOA yield variation with the initial experiment conditions. A relatively high NO_x level, ranging from 16.8 to 97.5 ppb, contributes to effective formation of SOA in the presence of NaCl seed particles, with the mass concentration of SOA and SOA yield ranging from 0.96 μg m⁻³ to 23.46 μg m⁻³ and from 0.04 % to 1.01 %, respectively. Likewise, the SOA mass concentration and yield increase with increasing humidity, because the increasing RH increases the aerosol liquid water content, which contributes to the liquid phase reactions. Nine organic nitrate species were detected by electrospray ionization exactive orbitrap mass spectrometry (ESI-Exactive-Orbitrap MS). The -COOH，-OH，-C = O and NO₂ functional groups were assigned in the FTIR spectra and used as the indicator for the mechanism inference. The present study directly addresses NO_x effects and reinforces the implication of humidity on SOA formation during the furan-NO_x-NaCl photooxidation. Furthermore, the results 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.