Abstract
Photooxidation of volatile organic compounds (VOCs) produces condensable oxidized organics (COOs) to yield secondary organic aerosol (SOA), but the fundamental chemical mechanism for gas-to-particle conversion remains uncertain. Here we elucidate the production of COOs and their roles in SOA and brown carbon (BrC) formation from m-xylene oxidation by simultaneous monitoring the evolutions of gas-phase products and aerosol properties in an environmental chamber. Four COO types with the distinct functionalities of dicarbonyls, carboxylic acids, polyhydroxy aromatics/quinones, and nitrophenols are identified from early-generation oxidation, with the yields of 25 %, 37 %, 5 %, and 3 %, respectively. SOA formation occurs via several heterogeneous processes, including interfacial interaction, ionic dissociation/acid-base reaction, and oligomerization, with the yields of (20 ± 4) % and (32 ± 7) % at 10 % and 70 % relative humidity (RH), respectively. Chemical speciation shows the dominant presence of oligomers, nitrogen-containing organics, and carboxylates at RH and carboxylates at low RH. The identified BrC includes N-heterocycles/N-heterochains and nitrophenols, as evident from reduced single scattering albedo. The measured uptake coefficient (γ) for COOs is dependent on the functionality, ranging from 3.7 × 10−4 to 1.3 × 10−2. A kinetic framework is developed to predict SOA production from the concentrations and uptake coefficients for COOs. This functionality-based approach well reproduces SOA formation from m-xylene oxidation and is broadly applicable to VOC oxidation for other species. Our results reveal that photochemical oxidation of m-xylene represents a major source for SOA and BrC formation under urban environments, because of its large abundance, high reactivity with OH, and high yields for COOs.
Highlights
Photooxidation of anthropogenic and biogenic volatile organic compounds (VOCs) produces tropospheric ozone and secondary organic aerosol (SOA), with profound implications for air quality, human health, and climate (Pope et al, 2002; Li et al, 2007; IPCC, 2013; NASEM, 2016; Zhu et al, 2017; Molina, 2021; Zhang et al, 2021)
condensable oxidized organics (COOs) types consisting of dicarbonyls, carboxylic acids, polyhydroxy aromatics/quinones, and nitrophenols from early-generation (P2 and P3), with the yields of 25%, 37%, 5%, and 3%, respectively
The nitrogen-containing organics consisting of N-heterocycles, N-heterochains, and nitrophenols are light-absorbing, characterized by low single scattering albedo (SSA)
Summary
Photooxidation of anthropogenic and biogenic volatile organic compounds (VOCs) produces tropospheric ozone and secondary organic aerosol (SOA), with profound implications for air quality, human health, and climate (Pope et al, 2002; Li et al, 2007; IPCC, 2013; NASEM, 2016; Zhu et al, 2017; Molina, 2021; Zhang et al, 2021). VOC oxidation is initiated by various oxidants (e.g., OH, O3, NO3, etc.) and proceeds via multiple pathways and stages (Atkinson, 2000; Suh et al, 2001; Zhang et al, 2002; Zhao et al, 2004; Wennberg et al, 2018), yielding condensable oxidized organics (COOs) to form SOA and brown carbon (BrC) via gas-to-particle conversion (Finlayson-Pitts and Pitts, 2000; Moise et al, 2015; Seinfeld and Pandis, 2016). The enormous chemical complexity for VOC oxidation and gas-to-particle conversion represents one of the greatest challenges in atmospheric chemistry research (Ravishankara, 1997; Zhang et al, 2015; NASEM, 2016). Aromatic hydrocarbons (e.g., benzene, toluene, xylenes, and trimethylbenzene) account for 20-30% of the total VOCs and are the major anthropogenic SOA precursors in the urban atmosphere (Calvert et al, 2002; Ng et al, 2007; Song et al, 2007; Guo et al, 2014; Seinfeld and Pandis, 2016). The OH-m-xylene reactions occur via dominantly OH-addition to the aromatic ring to yield m-xylene-OH adducts and minorly H-
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