Uptake of NO3 on Water Solutions: Rate Coefficients for Reactions of NO3 with Cloud Water Constituents

Oxidation of Catechols at the Air-Water Interface by Nitrate Radicals

Abundant substituted catechols are emitted to, and created in, the atmosphere during wildfires and anthropogenic combustion and agro-industrial processes. While ozone (O3) and hydroxyl radicals (HO•) efficiently react in a 1 μs contact time with catechols at the air-water interface, the nighttime reactivity dominated by nitrate radicals (NO3) remains unexplored. Herein, online electrospray ionization mass spectrometry (OESI-MS) is used to explore the reaction of NO3(g) with a series of representative catechols (catechol, pyrogallol, 3-methylcatechol, 4-methylcatechol, and 3-methoxycatechol) on the surface of aqueous microdroplets. The work detects the ultrafast generation of nitrocatechol (aromatic) compounds, which are major constituents of atmospheric brown carbon. Two mechanisms are proposed to produce nitrocatechols, one (equivalent to H atom abstraction) following fast electron transfer from the catechols (QH2) to NO3, forming NO3- and QH2•+ that quickly deprotonates into a semiquinone radical (QH•). The second mechanism proceeds via cyclohexadienyl radical intermediates from NO3 attack to the ring. Experiments in the pH range from 4 to 8 showed that the production of nitrocatechols was favored under the most acidic conditions. Mechanistically, the results explain the interfacial production of chromophoric nitrocatechols that modify the absorption properties of tropospheric particles, making them more susceptible to photooxidation, and alter the Earth's radiative forcing.

Reactive uptake kinetics of NO3 on multicomponent and multiphase organic mixtures containing unsaturated and saturated organics

We investigated the reactive uptake of NO(3) (an important night-time oxidant in the atmosphere) on binary mixtures containing an unsaturated organic (methyl oleate) and saturated molecules (diethyl sebacate, dioctyl sebacate, and squalane) which we call matrix molecules. These studies were carried out to better understand the reactivity of unsaturated organics in multicomponent and multiphase atmospheric particles. For liquid binary mixtures the reactivity of methyl oleate depended on the matrix molecule. Assuming a bulk reaction, H(matrix)√(D(matrix)k(oleate)) varied by a factor of 2.7, and assuming a surface reaction H(matrix)(S)K(matrix)(S)k(oleate)(S) varied by a factor of 3.6, where H(matrix)√(D(matrix)k(oleate) and H(matrix)(S)K(matrix)(S)k(oleate)(S) are constants extracted from the data using the resistor model. For solid-liquid mixtures, the reactive uptake coefficient depended on exposure time: the uptake decreased by a factor of 10 after exposure to NO(3) for approximately 90 min. By assuming either a bulk or surface reaction, the atmospheric lifetime of methyl oleate in different matrices was estimated for moderately polluted atmospheric conditions. For all liquid mixtures, the lifetime was in the order of a few minutes (with an upper limit of 35 min). These lifetimes can be used as lower limits to the lifetimes in semi-solid mixtures. Our studies emphasize that the lifetime of unsaturated organics (similar to methyl oleate) is likely short if the particle matrix is in a liquid state.

Temperature Dependence of the Mass Accommodation Coefficients of 2-Nitrophenol, 2-Methylphenol, 3-Methylphenol, and 4-Methylphenol on Aqueous Surfaces

The uptake of 2-nitrophenol, 2-methylphenol, 3-methylphenol, and 4-methylphenol on aqueous surfaces was investigated between 278 and 303 K, using the wetted-wall flow tube technique coupled with UV absorption spectroscopic detection. The uptake coefficients gamma were found to be independent of the aqueous phase composition and of the gas-liquid contact times. In addition, the uptake coefficients and the derived mass accommodation coefficients alpha show a negative temperature dependence in the temperature range studied. The mass accommodation coefficients decrease from 5.2 x 10(-3) to 8.3 x 10(-4), from 5.0 x 10(-3) to 3.1 x 10(-4), from 6.7 x 10(-3) to 7.3 x 10(-4), and from 1.2 x 10(-2) to 5.9 x 10(-4) for 2-nitrophenol, 2-methylphenol, 3-methylphenol, and 4-methylphenol, respectively. These results are used to discuss the incorporation of these species into the liquid using the nucleation theory. These data combined with the Henry's law constants were used to estimate the partitioning of the phenolic compounds between gaseous and aqueous phases and the corresponding atmospheric lifetimes under clear sky (tau(gas)) and cloudy conditions (tau(multiphase)) have then been derived.