Fresh versus Photo-recalcitrant Secondary Organic Aerosol: Effects of Organic Mixtures on Aqueous Photodegradation of 4-Nitrophenol

Abstract

In the atmosphere, brown carbon (BrC) molecules can contribute to photochemistry and to the warming and evaporation of cloud droplets. The lifetimes of BrC molecules in these cloud droplets are not well constrained, especially considering the droplets can contain mixtures of organic and inorganic molecules. Here we investigated the kinetics of photodegradation for a representative brown carbon molecule, 4-nitrophenol (4NP), in aqueous solutions at low pH. Three different solutions were compared: one containing 4NP alone and the other two additionally containing either fresh or photo-recalcitrant (pre-photobleached) α-pinene secondary organic aerosol (SOA). Fresh SOA slightly accelerates the photodegradation compared to that with 4NP alone. In contrast, photo-recalcitrant SOA decelerates photodegradation, increasing the estimated atmospheric lifetime of 4NP by 4 h (from ∼11 h for BrC alone to ∼14 h when it is mixed with photo-recalcitrant SOA). Photo-recalcitrant SOA also shows minimal chemical changes after irradiation for 20 h in aqueous solution. These results suggest that, in contrast to fresh SOA, aqueous photorecalcitrant SOA does not serve as a source of OH radicals when irradiated but may instead act as an OH radical scavenger. The presence of photo-recalcitrant SOA could have important effects on atmospheric lifetimes of organics in mixed aqueous systems.

Measurement report: Molecular characteristics of cloud water in southern China and insights into aqueous-phase processes from Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

“…The oligomerization of tryptophan were also observed in synthetic cloud water (Bianco et al, 2016a). For the N-and S-containing organics, photochemistry may also cause the release of inorganic nitrate and sulfate (Braman et al, 2020;Laskin et al, 2015;Bruggemann et al, 2020). The main radicals in the atmosphere at night is NO3•, which can form from the gas-phase reaction between NO2 and O3 and enter into cloud droplets.…”

Environmental effects of stratospheric ozone depletion, UV radiation, and interactions with climate change: UNEP Environmental Effects Assessment Panel, Update 2020

“…12 ). However, a fraction of SOA appears not to be degraded by UV radiation [ 366 , 368 , 369 ], with the overall effect of the UV radiation depending on the chemical composition of the many different aerosols present in the atmosphere. No attempt has yet been made to quantify the sensitivity of the lifetimes of different SOA to changes in UV radiation as would occur with changes in stratospheric ozone.…”

New SOA Treatments Within the Energy Exascale Earth System Model (E3SM): Strong Production and Sinks Govern Atmospheric SOA Distributions and Radiative Forcing

“…Zawadowicz et al (2020) measured a higher photolysis rate constant for SOA generated from some precursors (up to 2% of the NO 2 photolysis) as compared to the Hodzic et al (2016) rate constant of 0.04%. However, previous studies also showed that a fraction of SOA does not photolyze (Braman et al, 2020;O'Brien & Kroll, 2019;Zawadowicz et al, 2020); for example, 20% of α-pinene and β-caryophyllene SOA, as well as 10% of isoprene SOA, were measured to be nonphotolabile in their environmental chamber measurements. In addition to wet removal and photolytic loss, recent studies suggest that the heterogenous reactions of gas-phase oxidants (i.e., ozone and hydroxyl radical) with SOA could be additional sinks of SOA (Hu et al, 2016;.…”