Enhancement of secondary organic aerosol formation and its oxidation state by SO2 during photooxidation of 2-methoxyphenol
2-Methoxyphenol (guaiacol) is derived from the lignin pyrolysis and taken as a potential tracer for wood smoke emissions. In this work, the effect of SO2 at atmospheric levels (0–56 ppbv) on secondary organic aerosol (SOA) formation and its oxidation state during guaiacol photooxidation was investigated in the presence of various inorganic seed particles (i.e., NaCl and (NH4)2SO4). Without SO2 and seed particles, SOA yields ranged from (9.46±1.71) % to (26.37±2.83) % and could be well expressed by a one-product model. According to the ratio of the average gas-particle partitioning timescale (τ‾g-p) over the course of the experiment to the vapor wall deposition timescale (τg−w), the determined SOA yields were underestimated by a factor of ∼2. The presence of SO2 resulted in enhancing SOA yield by 14.04 %–23.65 %. With (NH4)2SO4 and NaCl seed particles, SOA yield was enhanced by 23.07 % and 29.57 %, respectively, which further increased significantly to 29.78 %–53.43 % in the presence of SO2, suggesting that SO2 and seed particles have a synergetic contribution to SOA formation. The decreasing trend of the τ‾g-p/τg-w ratio in the presence of seed particles and SO2 suggested that more SOA-forming vapors partitioned into the particle phase, consequently increasing SOA yields. It should be noted that SO2 was found to be in favor of increasing the carbon oxidation state (OSC) of SOA, indicating that the functionalization or the partitioning of highly oxidized products into particles should be more dominant than the oligomerization. In addition, the average N∕C ratio of SOA was 0.037, which revealed that NOx participated in the photooxidation process, consequently leading to the formation of organic N-containing compounds. The experimental results demonstrate the importance of SO2 on the formation processes of SOA and organic S-containing compounds and are also helpful to further understand SOA formation from the atmospheric photooxidation of guaiacol and its subsequent impacts on air quality and climate.