Halogen activation and radical cycling initiated by imidazole-2-carboxaldehyde photochemistry

Pablo Corral Arroyo,Pablo Corral Arroyo,Raffael Aellig,Peter A Alpert,Rainer Volkamer,Rainer Volkamer,Markus Ammann

doi:10.5194/acp-19-10817-2019

Abstract

Abstract. Atmospheric aerosol particles can contain light-absorbing organic compounds, also referred to as brown carbon (BrC). The ocean surface and sea spray aerosol particles can also contain light-absorbing organic species referred to as chromophoric dissolved organic matter (CDOM). Many BrC and CDOM species can contain carbonyls, dicarbonyls or aromatic carbonyls such as imidazole-2-carboxaldehyde (IC), which may act as photosensitizers because they form triplet excited states upon UV–VIS light absorption. These triplet excited states are strong oxidants and may initiate catalytic radical reaction cycles within and at the surface of atmospheric aerosol particles, thereby increasing the production of condensed-phase reactive oxygen species (ROS). Triplet states or ROS can also react with halides, generating halogen radicals and molecular halogen compounds. In particular, molecular halogens can be released into the gas phase, which is one halogen activation pathway. In this work, we studied the influence of bromide and iodide on the photosensitized production and release of hydroperoxy radicals (HO2) upon UV irradiation of films in a coated wall flow tube (CWFT) containing IC in a matrix of citric acid (CA) irradiated with UV light. In addition, we measured the iodine release upon irradiation of IC ∕ CA films in the CWFT. We developed a kinetic model coupling photosensitized CA oxidation with condensed-phase halogen chemistry to support data analysis and assessment of atmospheric implications in terms of HO2 production and halogen release in sea spray particles. As indicated by the experimental results and confirmed by the model, significant recycling of halogen species occurred via scavenging reactions with HO2. These prevented the full and immediate release of the molecular halogen (bromine and iodine) produced. Recycling was stronger at low relative humidity, attributed to diffusion limitations. Our findings also show that the HO2 production from BrC or CDOM photosensitized reactions can increase due to the presence of halides, leading to high HO2 turnover, in spite of low release due to the scavenging reactions. We estimated the iodine production within sea salt aerosol particles due to iodide oxidation by ozone (O3) at 5.0×10-6 M s−1 assuming O3 was in Henry's law equilibrium with the particle. However, using an O3 diffusion coefficient of 1×10-12 cm2 s−1, iodine activation in an aged, organic-rich sea spray is estimated to be 5.5×10-8 M s−1. The estimated iodine production from BrC photochemistry based on the results reported here amounts to 4.1×10-7 M s−1 and indicates that BrC photochemistry can exceed O3 reactive uptake in controlling the rates of iodine activation from sea spray particles under dry or cold conditions where diffusion is slow within particles.

Highlights

Volatile halogen-containing species such as CH3X, CH2XY, HOX, XY and X2− and molecular halogens (X2) are known as activated halogen species (AHS)
The HO2 radical release exhibits first an increase starting from the baseline in the absence of halides and a peak at about w8H×iOth21O0p1r21o(dcbmulcu−tei2osmnoilfnird−om1li.nteThheinerFebaiagcs.tei1oli)nn,eowfHitthOhe2thkereePlteyHal srqeabdieisicnadgluPperHoto-q duced from the oxidation of citric acid (CA) by the triplet P∗(t)
We investigated the influence of halides on the photochemistry of imidazole-2-carboxaldehyde and its oxidative capacity

Summary

Introduction

Volatile halogen-containing species such as CH3X, CH2XY, HOX, XY and X2 (where X and Y can be Cl, Br and I) are known as activated halogen species (AHS). Halogen activation is driven by the oxidation of halides by ozone (Carpenter et al, 2013; Schmidt et al, 2016) and radicals (e.g., OH or NO3) (Sander and Crutzen, 1996), N2O5 (Behnke et al, 1997) or photochemical oxidation (Wang and Pratt, 2017; Wren et al, 2013) These volatile compounds can be emitted to the atmosphere in the form of biogenic halogencontaining organic species (Org–X) (Hepach et al, 2016; Vogt et al, 1999) or by volcanos, among other processes (Simpson et al, 2015). The production and cycling of AHS and RHS at the ocean surface or in sea spray particles are key processes to understand their release into the gas phase and the contributions to their emission fluxes (Pechtl et al, 2007; Carpenter et al, 2013; Herrmann et al, 2003)

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