The chemical processing of organic aerosol particles is important for atmospheric chemistry, climate, and public health. The heterogeneous oxidation of oleic acid particles by ozone is one of the most frequently investigated model systems. The available kinetic data span a wide range of particle size and ozone concentration and are obtained with different experimental techniques including electrodynamic balance (EDB), optical tweezers, environmental chamber, and aerosol flow tube reactors using mass spectrometry and Raman spectroscopy as detection methods. Existing kinetic and mechanistic analyses, however, reveal systematic differences and inconsistencies that are a matter of ongoing debate. We developed and applied an inverse modeling approach using a kinetic multilayer model (KM-SUB) and Monte Carlo-based global optimization algorithms to 11 literature data sets and an additional new set of EDB data. We were able to reconcile most experimental data with consistent sets of multiphase chemical kinetic parameters. For a unique determination of these parameters, however, further experiments with simultaneous measurement of multiple observables at specific, insightful reaction conditions are required. We tested three different reaction mechanisms and conclude that secondary chemistry involving Criegee intermediates appears crucial to resolve the discrepancies found in earlier studies. Primary ozone chemistry occurs close to the particle surface and secondary reactions seem to dominate in the particle bulk, involving OH formation and radical chain reactions.

In the atmosphere pesticides can be adsorbed on the surface of particles, depending on their physico-chemical properties. They can react with atmospheric oxidants such as ozone but parameters influencing the degradation kinetics are not clear enough. In this study the heterogeneous ozonolysis of eight commonly used pesticides (i.e., difenoconazole, tetraconazole, cyprodinil, fipronil, oxadiazon, pendimethalin, deltamethrin, and permethrin) adsorbed on hydrophobic and hydrophilic silicas, and Arizona dust at relative humidity ranging from 0% to 80% was investigated. Under experimental conditions, only cyprodinil, deltamethrin, permethrin and pendimethalin were degraded by ozone. Second-order kinetic constants calculated for the pesticides degraded by ozone ranged from (4.7 ± 0.4) × 10 cm molecule s (pendimethalin, hydrophobic silica, 55% RH) to (2.3 ± 0.4) × 10 cm molecule s (cyprodinil, Arizona dust, 0% RH). Results obtained can contribute to a better understanding of the atmospheric fate of pesticides in the particulate phase and show the importance of taking humidity and particle type into account for the determination of pesticides atmospheric half-lives.

Pesticides can be adsorbed on the surface of atmospheric aerosol, depending on their physicochemical properties. They can be degraded by atmospheric oxidants such as OH radicals but the influence of some environmental parameters on the degradation kinetics, especially relative humidity and particle surface type, is not well understood. Heterogeneous degradation by OH radicals of eight commonly used pesticides (i.e., difenoconazole, tetraconazole, cyprodinil, fipronil, oxadiazon, pendimethalin, deltamethrin, and permethrin) adsorbed on hydrophobic and hydrophilic silicas at a relative humidity ranging from 0% to 70% was studied. Under experimental conditions, only cyprodinil, deltamethrin, permethrin, and pendimethalin were degraded by OH radical in atmospheric relevant concentration. Second-order kinetic constants calculated for the pesticides degraded by OH radicals ranged from (1.93 ± 0.61) x 10-13 cm 3 molecule-1 s-1 (permethrin, hydrophobic silica, 30% RH) to (4.08 ± 0.27) x 10-12 cm 3 molecule-1 s-1 (pendimethalin, hydrophilic silica, 0% RH). Results obtained can contribute to improve the understanding of the atmospheric fate of pesticides and other semi-volatile organic compounds in the particulate phase and they highlight the importance of taking humidity and particle type into account for the determination of pesticides atmospheric half-lives.