Modeling the Tropospheric Aqueous-Phase Chemistry of Photosensitizers under Wildfire-Plume and Urban Conditions with CAPRAM-PS1.0

Abstract

A detailed CAPRAM mechanism describing the sunlight-induced photosensitization chemistry is developed and applied to a chemical process model. The mechanism contains the gas–aqueous phase partitioning and further chemical processing of nine commonly investigated photosensitizers and particulate HULIS. In addition, the chemistry of secondarily formed singlet oxygen is included in a detailed manner. Overall, the newly developed mechanism module CAPRAM-PS1.0 comprises 284 processes. Performed model simulations focus on the environmental conditions of (i) a fresh smoke plume from a wildfire and (ii) an urban polluted background. The simulation revealed that the quenching of activated photosensitizers is dominated either by the organic matrix and copper under particle conditions or by oxygen under cloud conditions. The modeled average photosensitizer and ¹O₂ concentrations range between (0.007–9.5) × 10^–12 and (0.0002–1.1) × 10^–11 mol l^–1, respectively, and agree with measurements depending on the simulations. The main modeled loss of ¹O₂ is quenching by the water matrix with a 97% yield. The residual reactions are dominated by quenching through the organic matrix. The simulations indicate that effective quenching of photosensitizers and their main product ¹O₂ into the ground state inhibits the efficient production of particulate mass. Accordingly, great care should be taken not to overstate possible effects of photosensitization chemistry in atmospheric organic-containing aerosol particles. Besides and interestingly, photosensitizer chemistry is modeled to contribute up to 25% to chlorine activation, affecting the tropospheric oxidation budget.