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

Abstract. Atmospheric aerosol particles can contain light absorbing organic compounds, also referred to as brown carbon (BrC). In the context of the ocean surface and of sea spray aerosol deriving from the latter, light absorbing organic species are also referred to as chromophoric dissolved organic matter (CDOM). Many BrC or CDOM species (especially carbonyls, dicarbonyls or aromatic carbonyls such as imidazole-2-carboxaldehyde (IC)), referred to as photosensitizers, form triplet excited states upon UV-VIS light absorption. These triplet excited states are strong oxidants and may initiate catalytic radical reaction cycles within atmospheric aerosol particles and at their surface, therefore increasing the reactive oxygen species (ROS) production within atmospheric aerosol particles. Triplet states (or ROS resulting from them) can also react with halides generating halogen radicals and additionally molecular halogens compounds, which can be released into the gas phase and may thus contribute to halogen activation. In this work we study the influence of bromide and iodide on the photosensitized HO 2 production and release upon UV irradiation of films in a coated wall flow tube (CWFT) containing IC in a matrix of citric acid (CA). Additionally we measured the iodine release upon irradiation of IC/CA films in the CWFT. We use a kinetic model to interpret our results and to assess radical production and iodine release in sea-spray particles. As indicated by the experimental results and confirmed by the model, significant recycling of halogen species occurs via scavenging reactions with HO 2 , to prevent the full and immediate release of the molecular halogen (bromine and iodine) produced, while partially shutting down the HO x chemistry. The recycling efficiency is higher and affected by diffusion limitations at high viscosity. Our findings also show that halides can increase substantially the BrC or CDOM photosensitized HO 2 production (which in turn promotes radical and ROS production) by reacting with triplet statesin sea-spray particles. The iodine production within sea salt aerosol particles due to iodide oxidation by ozone is estimated at 5.9 × 10 −5  M s −1 assuming ozone equilibration in the particle. Under diffusion limitation this activation can drop several orders of magnitude in an aged, organic-rich sea-spray derived aerosol (1.1 × 10 −7  M s −1 for an ozone diffusion coefficient of 10 −12  cm 2  s −1 ). The estimated iodine production from BrC photochemistry amounts to 2.5 × 10 −7  M s −1 . This indicates that BrC photochemistry can exceed O 3 reactive uptake in controlling the rates of iodine activation from sea spray particles under dry or cold conditions where diffusion is slow within particles.