Reactions on mineral dust.

Titanium Dioxide Photocatalysis in Atmospheric Chemistry

Atmospheric aerosols can be categorized into primary particles, which are directly emitted by their sources, and secondary particles, generated in the atmosphere from gaseous inorganic and organic precursors. For example, atmospheric oxidation of sulfur containing compounds leads to sulfuric acid and its salts, which represent a major secondary inorganic component of atmospheric aerosols. Likewise, oxidation of nitrogen oxides leads to nitric acid or its salts, which are also abundant in aerosols. The ocean surface, which covers three-quarters of the planet, offers a remarkably dynamic and chemically complex surface for interfacial reactions in the marine boundary layer. The porous nature of permanent or perennial snowpacks adds a tremendous amount of surface area, with which the atmosphere interacts. In short, solar radiation can provide the energy to initiate reactions while atmospherically available surfaces or condensed phases may act to reduce the required energy for a given photochemical pathway, for instance, by allowing a longer wavelength for reaction of species associated with a surface or bulk phase environment.

Heterogeneous photochemistry in the atmosphere.

The presence of oxygenated organic compounds in the troposphere strongly influences key atmospheric processes. Such oxygenated species are, for example, carriers of reactive nitrogen and are easily photolysed, producing free radicals1,2,3—and so influence the oxidizing capacity and the ozone-forming potential of the atmosphere4,5,6—and may also contribute significantly to the organic component of aerosols7,8. But knowledge of the distribution and sources of oxygenated organic compounds, especially in the Southern Hemisphere, is limited. Here we characterize the tropospheric composition of oxygenated organic species, using data from a recent airborne survey9 conducted over the tropical Pacific Ocean (30° N to 30° S). Measurements of a dozen oxygenated chemicals (carbonyls, alcohols, organic nitrates, organic pernitrates and peroxides), along with several C2–C8 hydrocarbons, reveal that abundances of oxygenated species are extremely high, and collectively, oxygenated species are nearly five times more abundant than non-methane hydrocarbons in the Southern Hemisphere. Current atmospheric models are unable to correctly simulate these findings, suggesting that large, diffuse, and hitherto-unknown sources of oxygenated organic compounds must therefore exist. Although the origin of these sources is still unclear, we suggest that oxygenated species could be formed via the oxidation of hydrocarbons in the atmosphere, the photochemical degradation of organic matter in the oceans, and direct emissions from terrestrial vegetation.

Evidence from the Pacific troposphere for large global sources of oxygenated organic compounds

from 1.05 � 10 � 11 to 23.1 � 10 � 11 mg particle � 1 min � 1 in the presence of � 5 ppb glyoxal. Uptake coefficients (g) of glyoxal varied from 8.0 � 10 � 4 to 7.3 � 10 � 3 with a median g =2 .9� 10 � 3 , observed for (NH4)2SO4 seed aerosols at 55% relative humidity. Increased g values were related to increased particle acidity, indicating that acid catalysis played a role in the heterogeneous mechanism. Experiments conducted at very low relative humidity, with the potential to be highly acidic, resulted in very low reactive uptake. These uptake coefficients indicated that the heterogeneous loss of glyoxal in the atmosphere is at least as important as gas phase loss mechanisms, including photolysis and reaction with hydroxyl radicals. Glyoxal lifetime due to heterogeneous reactions under typical ambient conditions was estimated to be thet = 5–287 min. In rural and remote areas the glyoxal uptake can lead to 5–257 ng m � 3 of secondary organic aerosols in 8 hours, consistent with recent ambient measurements.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2004JD005113

Reactive uptake of glyoxal by particulate matter

The heterogeneous reactivity of volatile organic compounds (VOCs) on oxide particles has been investigated. Oxide particles composed of the most abundant elements present in the Earth's crust were used to identify potentially important reactions of VOCs on mineral dust. In particular, heterogeneous reactions of acetaldehyde, acetone, and propionaldehyde on α-Al2O3, α-Fe2O3, TiO2, CaO, and SiO2 particle surfaces have been investigated. Fourier transform infrared and UV/visible spectroscopic measurements show that these carbonyl compounds weakly and reversibly adsorbed on SiO2. However, on the more basic and acidic oxides investigated, the data show that these carbonyl compounds irreversibly adsorb and can, in part, react on the surface to form larger molecular weight compounds. The kinetics of the heterogeneous reaction of acetaldehyde, acetone, and propionaldehyde on α-Al2O3, α-Fe2O3, TiO2, CaO, and SiO2 were measured with a Knudsen cell reactor at a gas concentration of 6×1010 molecules/cm3. Using the Brunauer-Emmett-Teller surface area of the powdered sample, initial uptake coefficients were determined to be in the 10−4 to 10−6 range for the adsorption of carbonyl compounds on the oxides at 298 K. By using these values for the uptake coefficient, it is found that the heterogeneous loss of these trace volatile organic compounds is comparable to that due to photolysis and reaction with OH in the middle to upper troposphere. Although surface adsorption can occur, the measured second-order pressure dependence to form higher molecular weight compounds suggests that these reaction products will less likely form under atmospheric conditions.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JD900573

Heterogeneous reactions of volatile organic compounds on oxide particles of the most abundant crustal elements: Surface reactions of acetaldehyde, acetone, and propionaldehyde on SiO2, Al2O3, Fe2O3, TiO2, and CaO

V. H. Grassian

Papers

Heterogeneous reactions of volatile organic compounds on oxide particles of the most abundant crustal elements: Surface reactions of acetaldehyde, acetone, and propionaldehyde on SiO2, Al2O3, Fe2O3, TiO2, and CaO