Secondary organic aerosol formation from photooxidation of furan: effects of NO_<i>x</i> and humidity

• Main Authors: X. Jiang, N. T. Tsona, L. Jia, S. Liu, H. Zhang, Y. Xu, L. Du
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-11-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/19/13591/2019/acp-19-13591-2019.pdf

<p>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 <span class="inline-formula">NO_<i>x</i></span> 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 <span class="inline-formula">NO_<i>x</i></span> 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)&thinsp;<span class="inline-formula">∕</span>&thinsp;<span class="inline-formula">NO_<i>x</i></span> 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&thinsp;<span class="inline-formula">µ</span>g&thinsp;m<span class="inline-formula">⁻³</span> and from 0.04&thinsp;% to 1.01&thinsp;%, respectively. We found that there was a favourable relationship between the SOA yields and <span class="inline-formula">NO_<i>x</i></span> concentration. In particular, the increase in SOA yield with increasing <span class="inline-formula">NO_<i>x</i></span> concentration was continuously observed at high <span class="inline-formula">NO_<i>x</i></span> 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&thinsp;% to 88&thinsp;% increased the SOA yield from 1.01&thinsp;% to 5.03&thinsp;%. 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 <span class="inline-formula">NO_<i>x</i></span> and humidity on SOA formation during the furan–<span class="inline-formula">NO_<i>x</i></span>–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.</p>