28 Mar 2023
 | 28 Mar 2023

Predicting photooxidant concentrations in aerosol liquid water based on laboratory extracts of ambient particles

Lan Ma, Reed Worland, Wenqing Jiang, Christopher Niedek, Chrystal Guzman, Keith J. Bein, Qi Zhang, and Cort Anastasio

Abstract. Aerosol liquid water (ALW) is a unique reaction medium, but its chemistry is poorly understood. For example, little is known of photooxidant concentrations – including hydroxyl radical (OH), singlet molecular oxygen (1O2*), and oxidizing triplet excited states of organic matter (3C*) – even though they likely drive much of ALW chemistry. Due to the very limited water content of particles, it is difficult to quantify oxidant concentrations in ALW directly. To predict these values, we measured photooxidant concentrations in illuminated aqueous particle extracts as a function of dilution and used the resulting oxidant kinetics to extrapolate to ALW conditions. We prepared dilution series from two sets of particles collected in Davis, California: one from winter (WIN) and one from summer (SUM). Both periods are influenced by biomass burning, with dissolved organic carbon (DOC) in the extracts ranging from 10 to 495 mg C L−1. In the winter sample, the OH concentration is independent of particle mass concentration, with an average value of 5.0 (± 2.2) × 10−15 M, while in summer OH increases with DOC in the range (0.4 − 7.7) × 10−15 M. In both winter and summer samples, 3C* concentrations increase rapidly with particle mass concentrations in the extracts, and then plateau under more concentrated conditions, with a range of (0.2 − 7) × 10−13 M. WIN and SUM have the same range of 1O2* concentrations, (0.2 − 8.5) × 10−12 M, but in WIN the 1O2* concentration increases linearly with DOC, while in SUM 1O2* approaches a plateau.

We next extrapolated the relationships of oxidant formation rates and sinks as a function of particle mass concentration from our dilute extracts to the much more concentrated condition of aerosol liquid water. Predicted OH concentrations in ALW (including mass transport of OH from the gas phase) are (5 − 8) × 10−15 M, similar to those in fog/cloud waters. In contrast, predicted concentrations of 3C* and 1O2* in ALW are approximately 10 to 100 times higher than in cloud/fogs, with values of (4 – 9) × 10−13 M and (1 – 5) × 10−12 M, respectively. Although OH is often considered the main sink for organic compounds in the atmospheric aqueous phase, the much higher concentrations of 3C* and 1O2* in aerosol liquid water suggest these photooxidants will be more important sinks for many organics in particle water.

Lan Ma et al.

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Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'RC: Comment on egusphere-2023-566', Anonymous Referee #1, 20 Apr 2023
  • RC2: 'Comment on egusphere-2023-566', Anonymous Referee #2, 20 Apr 2023
  • RC3: 'Comment on egusphere-2023-566', Anonymous Referee #3, 26 Apr 2023

Lan Ma et al.


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Short summary
Although photooxidants are important in airborne particles, little is known of their concentrations. By measuring oxidants in a series of particle dilutions, we predict their concentrations in aqueous aerosol (ALW). We find OH concentrations in ALW are on the order of 10−15 M, similar to their cloud/fog values, while oxidizing triplet excited states and singlet molecular oxygen have ALW values around 10−13 M and 10−12 M, respectively, roughly 10–100 times higher than in cloud/fog drops.