Effects of SO<sub>2</sub> on optical properties of secondary organic aerosol generated from photooxidation of toluene under different relative humidity conditions

Abstract

Abstract. Secondary organic aerosol (SOA) has great impacts on air quality, climate change and human health. The composition and physicochemical properties of SOA differ greatly because they form under different atmospheric conditions and from various precursors as well as differing oxidation. In this work, photooxidation experiments of toluene were performed under four conditions (dry, dry with SO2, wet and wet with SO2) to investigate the effect of SO2 under different relative humidities on the composition and optical properties of SOA at wavelengths of 375 and 532 nm. According to our results, the increase in humidity enhances not only light absorption but also the scattering property of the SOA. Oligomers formed through multiphase reactions might be the reason for this phenomenon. Adding SO2 slightly lowers the real part of the complex refractive index, RI(n), of toluene-derived SOA (RI(n)dry,SO2<RI(n)dry, RI(n)wet,SO2<RI(n)wet), which might be a result of the partitioning of low-oxidation-state products. The imaginary part of the complex refractive index, RI(k), is enhanced under dry conditions with SO2 compared to that of only dry conditions, which might be due to acid-catalyzed aldol condensation reactions. Wet conditions with SO2 shows the combined effect of SO2 and humidity. The extinction properties of toluene-derived SOA under wet conditions with SO2 increased by approximately 30 % compared to that of toluene-derived SOA formed under dry conditions. Our results suggest that various atmospheric conditions will affect the composition and optical proprieties of SOA, which has significant implications for evaluating the impacts of SOA on the rapid formation of regional haze, global radiative balance and climate change.

The Roles of N, S, and O in Molecular Absorption Features of Brown Carbon in PM2.5 in a Typical Semi‐Arid Megacity in Northwestern China

“…BrC can be photobleached by oxidations with hydroxyl radical (·OH), ozone (O 3 ), and oxygen (O 2 ) (Schnitzler & Abbatt, 2018; Sumlin et al., 2018), and transformed through photo‐enhancement involving nitrate radical (NO 3 ) and ammonium salt (NH 4 + ) (De Haan et al., 2017; Lee et al., 2013; Li et al., 2020). Sulfur dioxide (SO 2 ) can effectively enhance BrC absorption with the presence of N‐containing precursors under high relative humidity (RH) (Nakayama et al., 2015, 2018; Zhang et al., 2020). Through chamber simulation coupling with chromatographic separation and mass spectrometric detection (MSD), N‐containing aromatic compounds were recognized as vital BrC contributors (Desyaterik et al., 2013; Lin et al., 2017).…”

Important Oxidants and Their Impact on the Environmental Effects of Aerosols

“…The oxidants (OH radical, NO x , O 3 , NO 3 radicals) also have a great influence on the chemical reaction processes, attributing to the different chemical composition and RI values of the formed products. For example, Zhang et al 98 conducted experiments with toluene in the presence of SO 2 under different RH conditions and showed that an increase in RH could enhance both the scattering and absorbing properties of SOA. The presence of SO 2 could slightly lower the scattering property of SOA and improve the absorption property of SOA.…”

Heterogeneous reaction of toluene/NO2/O3 on α-Fe2O3 nanoparticles: the impacts of O3, light illumination, and relative humidity on the formation of N-containing organic compounds (NOC)

“…The relative humidity (RH) has been highlighted in recent research on the atmospheric environment because it changes with different regions, seasons, and times of day. [23][24][25] The RH effects on the formation processes of aromatic SOA have been investigated in previous studies. 9,25 Due to the formation of hydroxy products from aqueous reactions, a high RH can lead to a much higher SOA yield from the photooxidation of toluene.…”