Alexandre Kukui

Water Vapor Effect on the HNO3 Yield in the HO2 + NO Reaction: Experimental and Theoretical Evidence

The influence of water vapor on the production of nitric acid in the gas-phase HO(2) + NO reaction was determined at 298 K and 200 Torr using a high-pressure turbulent flow reactor coupled with a chemical ionization mass spectrometer. The yield of HNO(3) was found to increase linearly with the increase of water concentration reaching an enhancement factor of about 8 at [H(2)O] = 4 x 10(17) molecules cm(-3) ( approximately 50% relative humidity). A rate constant value k(1bw) = 6 x 10(-13) cm(3) molecule(-1) s(-1) was derived for the reaction involving the HO(2)xH(2)O complex: HO(2)xH(2)O + NO --> HNO(3) (1bw), assuming that the water enhancement is due to this reaction. k(1bw) is approximately 40 times higher than the rate constant of the reaction HO(2) + NO --> HNO(3) (1b), at the same temperature and pressure. The experimental findings are corroborated by density functional theory (DFT) calculations performed on the H(2)O/HO(2)/NO system. The significance of this result for atmospheric chemistry and chemical amplifier instruments is briefly discussed. An appendix containing a detailed consideration of the possible contribution from the surface reactions in our previous studies of the title reaction and in the present one is included.

Pressure and temperature dependence of methyl nitrate formation in the CH3O2 + NO reaction.

The branching ratio β = k(1b)/k(1a) for the formation of methyl nitrate, CH(3)ONO(2), in the gas-phase CH(3)O(2) + NO reaction, CH(3)O(2) + NO → CH(3)O + NO(2) (1a), CH(3)O(2) + NO → CH(3)ONO(2) (1b), has been determined over the pressure and temperature ranges 50-500 Torr and 223-300 K, respectively, using a turbulent flow reactor coupled with a chemical ionization mass spectrometer. At 298 K, the CH(3)ONO(2) yield has been found to increase linearly with pressure from 0.33 ± 0.16% at 50 Torr to 0.80 ± 0.54% at 500 Torr (errors are 2σ). Decrease of temperature from 300 to 220 K leads to an increase of β by a factor of about 3 in the 100-200 Torr range. These data correspond to a value of β ≈ 1.0 ± 0.7% over the pressure and temperature ranges of the whole troposphere. Atmospheric concentrations of CH(3)ONO(2) roughly estimated using results of this work are in reasonable agreement with those observed in polluted environments and significantly higher compared with measurements in upper troposphere and lower stratosphere.

Pressure and temperature dependence of ethyl nitrate formation in the C(2)H(5)O(2) + NO reaction.

The branching ratio beta = k(1b)/k(1a) for the formation of ethyl nitrate, C(2)H(5)ONO(2), in the gas-phase C(2)H(5)O(2) + NO reaction, C(2)H(5)O(2) + NO --> C(2)H(5)O + NO(2) (1a), C(2)H(5)O(2) + NO --> C(2)H(5)ONO(2) (1b), was determined over the pressure and temperature ranges 100-600 Torr and 223-298 K, respectively, using a turbulent flow reactor coupled with a chemical ionization mass spectrometer. At 298 K the C(2)H(5)ONO(2) yield was found to increase linearly with pressure from about 0.7% at 100 Torr to about 3% at 600 Torr. At each pressure, the branching ratio of C(2)H(5)ONO(2) formation increases with the decrease of temperature. The following parametrization equation has been derived in the pressure and temperature ranges of the study: beta(P,T) (%) = (3.88 x 10(-3).P (Torr) + 0.365).(1 + 1500(1/T - 1/298)). The atmospheric implication of the results obtained is briefly discussed, in particular the impact of beta on the evolution of ethyl nitrate in urban plumes.

Evidence of atmospheric nanoparticle formation from emissions of marine microorganisms

Earth, as a whole, can be considered as a living organism emitting gases and particles into its atmosphere, in order to regulate its own temperature. In particular, oceans may respond to climate change by emitting particles that ultimately will influence cloud coverage. At the global scale, a large fraction of the aerosol number concentration is formed by nucleation of gas-phase species, but this process has never been directly observed above oceans. Here we present, using semicontrolled seawater-air enclosures, evidence that nucleation may occur from marine biological emissions in the atmosphere of the open ocean. We identify iodine-containing species as major precursors for new particle clusters’ formation, while questioning the role of the commonly accepted dimethyl sulfide oxidation products, in forming new particle clusters in the region investigated and within a time scale on the order of an hour. We further show that amines would sustain the new particle formation process by growing the new clusters to larger sizes. Our results suggest that iodine-containing species and amines are correlated to different biological tracers. These observations, if generalized, would call for a substantial change of modeling approaches of the sea-to-air interactions.