Aqueous Phase Photo-oxidation of Brown Carbon Nitrophenols: Reaction Kinetics, Mechanism, and Evolution of Light Absorption

Abstract

Always cite the published version, so the author(s) will receive recognition through services that track citation counts, e.g. Scopus. If you need to cite the page number of the author manuscript from TSpace because you cannot access the published version, then cite the TSpace version in addition to the published version using the permanent URI (handle) found on the record page. ABSTRACT 7Light absorbing organic aerosol particles, referred to as brown carbon, are geographically 8 widespread and can have an important climate impact through the absorption of solar radiation. 9Recent studies, both in the laboratory and the field, have shown that brown carbon aerosols can be 10 bleached of their color by direct photolysis and photo-oxidation reactions on the timescale of hours 11 to days. However, the photo-oxidation of nitrophenol molecules, which are colored compounds 12 often associated with biomass burning organic aerosol, show an enhancement in light absorption 13 before the color is lost. This study investigates the mechanism of color enhancement and the fate 14 of three nitrophenol compounds, specifically nitrocatechol, nitroguaiacol, and dinitrophenol, in 15 aqueous aerosol using online aerosol chemical ionization mass spectrometry (aerosol-CIMS). The 16 second order rate constants for the three nitrophenols with OH radicals in the aqueous phase at pH 17 2 7 (298 K), were determined to be 5 × 10 9 M -1 s -1 , 5.2 × 10 9 M -1 s -1 , and 3.7 × 10 9 M -1 s -1 for 18 nitrocatechol, nitroguaiacol, and dinitrophenol, respectively. For a representative aqueous OH 19 concentration, these rate constants correspond to an aqueous lifetime with respect to OH on the 20 order of hours. While the nitrophenol molecules react rapidly with OH, the initial products, which 21 are functionalized by additional electron-donating OH groups, likely lead to the observed 22 absorption increase in the visible range. Further photo-oxidation fragments the aromatic structure 23 to produce smaller, highly oxygenated molecules which no longer absorb strongly at visible 24 wavelengths. These products include furoic acid, glyoxylic acid, malonic acid, oxalic acid, and 25 isocyanic acid. All three nitrophenols investigated formed similar products during photo-26 oxidation, suggesting that these results could be generalized to this larger class of compounds. 27Keywords: brown carbon aerosol, biomass burning, photo-bleaching, aqueous oxidation, aerosol 28 aging 29 aldehydes, 8-10 as well as photo-oxidation of phenolic compounds. 11,12 Nitrophenols are a class of 40 light absorbing compounds often detected in significant concentrations (10s of ng m -3 ) in 41 biomass burning BrC. 13,14 These phenolic compounds are derived from pyrolysis of lignin, 14 but 42 can also be formed by photo-oxidation of anthropogenic pollutants such as xylene 15 and 43 toluene. 16 Nitrophenols are readily water soluble and have been measured in cloud, fog, and rain 44 water at concentrations of up to tens of ug L -1 . 13 45The magnitude of the climate impact of BrC d...

Molecular composition and photochemical lifetimes of brown carbon chromophores in biomass burning organic aerosol

“…First, different matrices could quench the electronic excitation in the chromophores to a different extent. Another possibility is that photodegradation of BrC chromophores could be not due to direct photolysis but rather occurring through condensed-phase photosensitized reactions (Malecha and Nizkorodov, 2017;Monge et al, 2012), in which case the rate of decomposition would depend on concentration of photosensitizers in the samples as well as viscosity of the material (Hinks et al, 2016;Kaur et al, 2019). Lastly, other absorbing species, such as black carbon, could be shielding BrC chromophores from irradiation, altering the amount of radiation absorbed by BrC chromophores.…”

“…Several studies have characterized changes in the UV-Vis spectra of nitrophenols, common in BBOA, as they are exposed to UV radiation. For example, Hinks et al (2016) irradiated 2,4dinitrophenol incorporated in limonene secondary organic aerosol material and observed the absorbance decrease in the range of 250-320 nm, while the absorbance from 400 to 450 nm increased. Similarly, Zhao et al (2015) observed a photo-enhancement at 420 nm for a 4-nitrocatechol aqueous solution, in response to direct photolysis.…”

Amplification of black carbon light absorption induced by atmospheric aging: temporal variation at seasonal and diel scales in urban Guangzhou

“…OC can be separated into two categories based on the formation nature. Primary organic carbon (POC) can be emitted from traffic emission (Huang et al, 2014), biomass burning (Simoneit, 2002), trash burning and cooking (Mohr et al, 2009). Secondary organic carbon (SOC) can be formed through the oxidation of volatile organic compounds (VOCs) or semivolatile POC (Hallquist et al, 2009).…”

Aqueous Reactions of Sulfate Radical-Anions with Nitrophenols in Atmospheric Context

“…Vione, et al [54] showed that OH radicals reacted faster than NO 3 radicals with 2-NP and 4-NP in aqueous solutions to lower the atmospheric levels of 2-NP below those of 4-NP. Hems and Abbatt [55] studied the aqueous-phase photo-oxidation of 2,4-DNP by OH radicals, identified numerous intermediate products thereof and showed the corresponding evolution of UV absorbance of the reacting solutions.…”

“…In addition, many laboratories studied the aqueous-phase reactions of other substituted phenols of atmospheric interest like guaiacol, nitro-guaiacol, vanillin, or syringol [55][56][57][58][59].…”