Recent environmental chamber studies have suggested that acid-catalyzed particle-phase reactions of organic carbonyls contribute to the formation of secondary organic aerosol (SOA). We report the first measurements of uptake of methylglyoxal on liquid H2SO4 over the temperature range of 250-298 K and acidic range of 55-85 wt %. From the time-dependent uptake the effective Henry's law solubility constant (H*) was determined. Heterogeneous reactions of methylglyoxal are shown to decrease with acidity and involve negligible formation of sulfate esters. Hydration and polymerization likely explain the measured uptake of methylglyoxal on H2SO4 and the measurements do not support an acid-catalyzed uptake of methylglyoxal. The results imply that heterogeneous reactions of methylglyoxal contribute to organic aerosol formation in less acidic media and hydration and polymerization of methylglyoxal in the atmospheric aerosol-phase are dependent on the hygroscopicity, rather than the acidity of the aerosols.

Photochemical oxidation of aromatic hydrocarbons leads to tropospheric ozone and secondary organic aerosol (SOA) formation, with profound implications for air quality, human health, and climate. Toluene is the most abundant aromatic compound under urban environments, but its detailed chemical oxidation mechanism remains uncertain. From combined laboratory experiments and quantum chemical calculations, we show a toluene oxidation mechanism that is different from the one adopted in current atmospheric models. Our experimental work indicates a larger-than-expected branching ratio for cresols, but a negligible formation of ring-opening products (e.g., methylglyoxal). Quantum chemical calculations also demonstrate that cresols are much more stable than their corresponding peroxy radicals, and, for the most favorable OH (ortho) addition, the pathway of H extraction by O2 to form the cresol proceeds with a smaller barrier than O2 addition to form the peroxy radical. Our results reveal that phenolic (rather than peroxy radical) formation represents the dominant pathway for toluene oxidation, highlighting the necessity to reassess its role in ozone and SOA formation in the atmosphere.

Recent experimental studies using the environmental chamber have suggested that acid‐catalyzed particle‐phase reactions of organic carbonyls lead to multifold increases in secondary organic aerosol (SOA) mass, but the kinetics and mechanism of the heterogeneous chemistry of carbonyls with sulfuric acid remain largely uncertain. We report the first measurements of heterogeneous uptake of octanal and 2, 4‐hexadienal on liquid H2SO4 in the acid range of 60 to 85 wt % and between 250 and 298 K. Octanal was physically absorbed by sulfuric acid without undergoing irreversible reaction. From the time‐dependent uptake the effective Henry's law solubility constant (H*) was determined. Irreversible reactive uptake was observed for 2, 4‐hexadienal, and the uptake coefficient decreased with decreasing acid concentrations. The uptake of octanal and 2, 4‐hexadienal on liquid H2SO4 is explained by aldol condensation, dependent on acidity. The results suggest that aldol condensation of the aldehydes can be important in the upper troposphere, but may not significantly contribute to secondary organic aerosol formation in the lower troposphere.

Recent environmental chamber studies suggest that acid-catalyzed reactions between alcohols and aldehydes in the condensed phase lead to the formation of hemiacetals and acetals, enhancing secondary organic aerosol (SOA) growth. We report measurements of heterogeneous uptake of butanol and decanol on liquid H2SO4 in the range of 62-84 wt % and between 273 and 296 K. Both alcohols exhibit two distinct types of uptake behaviors (partially irreversible vs totally irreversible uptake), depending on the acid concentration and temperature. For the partially irreversible uptake, a fraction of the alcohol was physically absorbed while the other fraction underwent irreversible reaction. For the totally irreversible uptake, the alcohols were completely lost onto the sulfuric acid. The Henry's law solubility constant (H*) was determined from the time-dependent uptake, while the reactive uptake coefficients were calculated from the time-independent irreversible loss. Coexistence of butanol or decanol with octanal or decanal did not show enhanced uptake of the aldehydes in the sulfuric acid. Protonation and dissolution likely account for the reversible uptake, while formation of alkyl sulfate or dialkyl sulfate explains irreversible uptake of the alcohols. The results suggest that heterogeneous uptake of larger alcohols is unlikely of significant importance in the lower atmosphere except in the case of freshly nucleated aerosols that may have high acid concentrations.