Christian Weller

Modeling the Impact of Iron–Carboxylate Photochemistry on Radical Budget and Carboxylate Degradation in Cloud Droplets and Particles

To quantify the effects of an advanced iron photochemistry scheme, the chemical aqueous-phase radical mechanism (CAPRAM 3.0i) has been updated with several new Fe(III)-carboxylate complex photolysis reactions. Newly introduced ligands are malonate, succinate, tartrate, tartronate, pyruvate, and glyoxalate. Model simulations show that more than 50% of the total Fe(III) is coordinated by oxalate and up to 20% of total Fe(III) is bound in the newly implemented 1:1 complexes with tartronate, malonate, and pyruvate. Up to 20% of the total Fe(III) is found in hydroxo and sulfato complexes. The fraction of [Fe(oxalate)2](-) and [Fe(pyruvate)](2+) is significantly higher during nighttime than during daytime, which points toward a strong influence of photochemistry on these species. Fe(III) complex photolysis is an important additional sink for tartronate, pyruvate, and oxalate, with a complex photolysis contribution to overall degradation of 46, 40, and 99%, respectively, compared to all possible sink reactions with atmospheric aqueous-phase radicals, such as (•)OH, NO3(•), and SO4(•) (-). Simulated aerosol particles have a much lower liquid water content than cloud droplets, thus leading to high concentrations of species and, consequently, an enhancement of the photolysis sink reactions in the aerosol particles. The simulations showed that Fe(III) photochemistry should not be neglected when considering the fate of carboxylic acids, which constitute a major part of aqueous secondary organic aerosol (aqSOA) in tropospheric cloud droplets and aqueous particles. Failure to consider this loss pathway has the potential to result in a significant overestimate of aqSOA production.

Emerging Areas in Atmospheric Photochemistry

Sunlight is a major driving force of atmospheric processes. A detailed knowledge of atmospheric photochemistry is therefore required in order to understand atmospheric chemistry and climate. Considerable progress has been made in this field in recent decades. This contribution will highlight a set of new and emerging ideas (and will therefore not provide a complete review of the field) mainly dealing with long wavelength photochemistry both in the gas phase and on a wide range of environmental surfaces. Besides this, some interesting bulk photochemistry processes are discussed. Altogether these processes have the potential to introduce new chemical pathways into tropospheric chemistry and may impact atmospheric radical formation.

Temperature and Ionic Strength Dependence of NO3-radical Reactions with Substituted Phenols in Aqueous Solution

Abstract NO3 oxidation reactions contribute to the chemical conversion processes of organic compounds in the tropospheric multiphase system. Substituted phenols are known pollutants in cloud droplets, rain, fog and (deliquescent) particles. This study presents rate constants obtained from kinetic investigations of NO3 reactions with different substituted phenolic compounds in the aqueous solution. Effects of the temperature as well as the ionic strength on the reaction rates were measured using a laser flash photolysis – laser long path absorption (LP-LLPA) set-up. The following rate constants at T = 298 K in M-1s-1 were obtained for reactions of the NO3 radical with 4-nitrophenol (k = (8.8±4.6) · 108), 2-nitrophenol (k = (8.3±1.4) · 108), 4-hydroxybenzoic acid (k = (1.5±0.4) · 109), 4-methylphenol (k = (1.7±0.3) · 109), 4-methoxyphenol (k = (2.5±0.4) · 109), 4-aminophenol (k = (2.0±0.3) · 109), 2,6-dichlorophenol (k = (1.3±0.2) · 109), 2,6-dihydroxyphenol (k = (1.7±0.2) · 109), 2,6-dimethylphenol (k = (1.8±0.3) · 109), 2,6-dimethoxyphenol (k = (1.6±0.4) · 109), 2,6-dinitrophenol (k = (2.8±0.9) · 108), 4-hydroxy-3,5-dimethoxybenzaldehyde (k = (1.7±0.3) · 109), 4-hydroxy-3,5-dimethoxybenzoic acid (k = (1.4±0.6) · 109), 4-hydroxy-3-methoxybenzaldehyde (k = (1.1±0.2) · 109), 4-hydroxy-3-methoxybenzoic acid (k = (1.0±0.3) · 109), 3-hydroxy-4-methoxybenzoic acid (k = (1.3±0.4) · 109). Averaged activation energies of: para-substituted phenols are EAߙpara = 11±6 kJ mol-1; 2,6-substituted phenols EAߙ2,6 = 15±4 , poly-substituted phenols EAߙpoly = 16±4 kJ mol-1. From reactivity correlations it is concluded that 4-nitrophenol, 2,6-dinitrophenol and 4-hydroxybenzoic acid react via direct electron transfer. The majority of investigated compounds react in a mixed regime with contribution of both electron transfer and H-atom abstraction. Surprisingly, changes in ionic strength do not affect the reactions dominated by electron transfer but have an influence on the rate constants of reactions with a mixed mechanism.