Chemical analysis of nitro‐aromatic compounds of secondary organic aerosol formed from photooxidation of p‐xylene with NOx

Abstract

Nitro‐aromatic compounds (NACs) measured in ambient aerosol are important contributors to brown carbon. Aromatic compounds are expected to be vital precursors of NACs in the presence of NOx in urban atmospheres. NACs of secondary organic aerosol (SOA) formed from photooxidation of p‐xylene with a variety of NO2 in smog chamber were investigated in this study. NACs were on‐line measured by aerosol laser time‐of‐flight mass spectrometer and verified by off‐line of ultraviolet absorption and electrospray ionization mass spectra. The results show that NO2 can promote the formation of p‐xylene SOA. Different from the carbonyls detected in p‐xylene SOA without NO2, nitrophenols, nitrocatechols, and other NACs via the nitration of phenolic products of p‐xylene are detected as the major components of SOA with NO2. The increase of electrophilic nitro groups in SOA strengthens absorption ability. The mass absorption coefficient averaged over 200–600 nm (<MAC>) of SOA gradually increased with the increasing of the concentration of NO2. The <MAC> of SOA with 1,000 ppb NO2 was comparable to that of biomass burning organic aerosols. These provided experimental basis for studying the formation mechanism and optics of anthropogenic NACs in the background of high concentration of NOx in Chinese urban atmosphere.

Effects of Relative Humidity and Photoaging on the Formation, Composition, and Aging of Ethylbenzene SOA: Insights from Chamber Experiments on Chlorine Radical-Initiated Oxidation of Ethylbenzene

“…Explicit investigation of these effects is needed, as many previous experiments have only examined a limited set of atmospherically relevant conditions. RH and aerosol water content will impact the partitioning behavior of small oxidized organics and enable hydrolysis reactions of species like organonitrate (ON) molecules. − Carbonyl, peroxide, organonitrate (ON), and nitroaromatic functional groups present in the products of alkylaromatic oxidation have been observed to undergo photolysis on atmospherically relevant time scales, resulting in SOA loss and changes in composition. ,− Functionalized aromatics including phenols, nitrophenols, and benzaldehydes can absorb light within the actinic spectrum, which may initiate further chemical reactions within SOA. − More generally, actinic exposure has been observed to chemically age SOA, leading to molecular degradation and structural alterations. ,, Photophysical processes have also been shown to be affected by aerosol water content, accelerating or otherwise altering reactive and partitioning processes. ,,, The effects of water vapor on SOA formation specifically during alkylaromatic oxidation have been summarized in a recent review article, showing that RH can have variable effects (positive, negative, or no effect) on SOA formation, primarily by influencing the initial SOA condensation process and different condensed-phase reaction processes. ,− Studies examining the effects of photoaging on alkylaromatic SOA are more limited but show that SOA mass is lost and that efficient chromophores initially present in SOA, including nitrophenols, undergo loss and photobleaching during aging. , …”