E. J. Dunlea

Evolution of Organic Aerosols in the Atmosphere

Framework for Change Organic aerosols make up 20 to 90% of the particulate mass of the troposphere and are important factors in both climate and human heath. However, their sources and removal pathways are very uncertain, and their atmospheric evolution is poorly characterized. Jimenez et al. (p. 1525; see the Perspective by Andreae) present an integrated framework of organic aerosol compositional evolution in the atmosphere, based on model results and field and laboratory data that simulate the dynamic aging behavior of organic aerosols. Particles become more oxidized, more hygroscopic, and less volatile with age, as they become oxygenated organic aerosols. These results should lead to better predictions of climate and air quality. Organic aerosols are not compositionally static, but they evolve dramatically within hours to days of their formation. Organic aerosol (OA) particles affect climate forcing and human health, but their sources and evolution remain poorly characterized. We present a unifying model framework describing the atmospheric evolution of OA that is constrained by high–time-resolution measurements of its composition, volatility, and oxidation state. OA and OA precursor gases evolve by becoming increasingly oxidized, less volatile, and more hygroscopic, leading to the formation of oxygenated organic aerosol (OOA), with concentrations comparable to those of sulfate aerosol throughout the Northern Hemisphere. Our model framework captures the dynamic aging behavior observed in both the atmosphere and laboratory: It can serve as a basis for improving parameterizations in regional and global models.

Measurement of the rate coefficient for the reaction of O(1D) with H2O and re-evaluation of the atmospheric OH production rate

The rate coefficient for the reaction of O(1D) with H2O (Reaction 1), whose value is critical for calculating production rates of the OH radical in the atmosphere, was measured over a range of atmospherically relevant temperatures. The temporal profile of O(3P) following photolytic production of O(1D) in the presence of water vapor was used to determine a temperature dependent value for k1 of (1.45 ± 0.34) × 10−10 exp((89 ± 65)/T) cm3 molecule−1 s−1, where the quoted errors are at the 95% confidence level, include estimated systematic errors and σA = AσlnA. In addition, ratios of rate coefficients for quenching of O(1D) by N2 and O2 (k2 and k3) to that for Reaction 1 were determined to be k2/k1 = (0.13 ± 0.04) and k3/k1 = (0.19 ± 0.09) by measuring the relative OH concentration produced from Reaction 1 in the presence of various concentrations of N2 or O2. Combining our results of this work with previous measurements of k1, we recommend a value for k1 = (1.62 ± 0.27) × 10−10 exp((65 ± 50)/T) cm3 molecule−1 s−1, where the quoted errors are 95% confidence level, include estimated systematic errors and σA = AσlnA. Based on these results, the uncertainty in the calculated atmospheric OH production rate due to the uncertainty in these rate coefficients is reduced from ±50% to ±20%.

Temperature-dependent quantum yields for O(3P) and O(1D) production from photolysis of O3 at 248 nm

A pulsed laser photolysis–resonance fluorescence technique was employed independently by two laboratories to measure ΦO3P, the quantum yield for production of O(3P) from O3 photolysis at 248 nm, between 196 and 427 K. The agreement between the two studies is very good, and the combined results are adequately represented by the function ΦO3P= (0.115 ± 0.030) × exp((35 ± 60)/T) where the uncertainties are 2σ. Within experimental uncertainties, the new results are in agreement with previously reported room temperature results as well as with the single previous temperature dependence study, and greatly reduce the uncertainties in ΦO3P(T) and ΦO1D(T) (=1− ΦO3P(T)) especially at temperatures other than room temperature. The yield of O(3P) in the reaction of O(1D) with O3 is shown to be greater than unity at room temperature and below, and to increase slightly with decreasing temperature.