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

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.

Photolytic radical persistence due to anoxia in viscous aerosol particles

“…The release of HO 2 into the gas phase was measured in CWFT experiments through the efficient scavenging reaction with excess NO 42 , 58 , 59 . Films were generated by dispensing a known volume of aqueous Fe III Cit and citric acid solutions with M r from 0.01 to 0.11 in Duran glass tubes 1.2 cm in diameter and 50 cm long.…”

“…This drop was used to calculate the HO 2 release from the film following a previous procedure 42 . The NO conversion was routinely checked to exclude processes involving NO and HO 2 42 , 58 , such as a loss due to O 3 formation, a production due to the photolysis of NO 2 and the return of HO 2 to the film. We note, gas phase loss of HO 2 with volatile organic compounds is highly unlikely to affect NO conversion as known reaction rates are far too slow.…”

Aging of Atmospheric Brown Carbon Aerosol

“…Photo-excitation to generate triplet excited organic compounds has also been observed in the atmospheric condensed phase. 41,[71][72][73][144][145][146][147][148][149][150][151] Reaction of either the triplet excited organic compounds or singlet oxygen can result in significant oxidation of organic compounds in the aqueous phase. Photosensitized chemistry may also be important in formation of SOA mass, either through aqueous reactions 41,[71][72][73] or heterogeneous uptake of reactive gases.…”

“…The triplet excited state can react with organic compounds or can produce reactive photooxidants including singlet molecular oxygen ( 1 O 2 ). Photoexcitation to generate triplet excited organic compounds has also been observed in the atmospheric condensed phase. ,− ,− Reaction of either the triplet excited organic compounds or singlet oxygen can result in significant oxidation of organic compounds in the aqueous phase. Photosensitized chemistry may also be important in formation of SOA mass, either through aqueous reactions ,− or heterogeneous uptake of reactive gases. − , Measurements of collected ambient fog water and aqueous extracts of aerosol particles show that photosensitized reactions occur when chromophoric organic compounds, or BrC, are present. , Aerosol particles, which have much lower liquid water concentrations, were observed to have more than 30 times the concentration of triplet organic compounds compared to fog droplets .…”

Surface Propensity of Aqueous Atmospheric Bromine at the Liquid–Gas Interface

“…3−9 Bromine activation is the result of a complex and multiphase (i.e., in the gas and/or at the interface of condensed water) reaction cycles. Oxidation of bromide may occur through various routes involving photolytically generated radicals, excited chromophoric organic matter, 10 or ozone. In the latter case, acid-catalyzed oxidation of bromide leads to the formation of hypobromous acid (HOBr): 3,11…”