Experimental and theoretical studies of novel HOx-NOx reactions in the atmosphere
Final Report Abstract
A variety of pulsed laser based experiments were conducted to investigate some novel atmospheric reactions that couple the HOx and NOx families. The formation of OH in the reactions of electronically excited NO2 and NO3 with H2O was investigated by preparation of large concentrations of NO2* and NO3* by non-dissociative absorption of laser light at several wavelengths in the presence of water vapour, followed by sensitive detection of OH by laser induced fluorescence. In neither reaction could OH be detected and upper limits were derived for its yield. We show that the reaction of NO2* with H2O is not, as previously believed, a significant source of OH under any atmospheric conditions and rule out a major role for OH formation from NO3*. In order to test for the recently proposed formation of HNO3 at sub-percent levels in the reaction of HO2 with NO, which has a major impact on global atmospheric radical budgets, experiments were conducted in which the reaction was initiated by pulsed photolytic generation of HO2 in the presence of NO. The HNO3 product was to be detected by photofragment emission spectroscopy (PES). The first part of this work was to establish the sensitivity and selectivity of the 193 nm detection scheme to HNO3. We discovered that there is excellent intrinsic sensitivity but that the HNO3 signal is perturbed by interfering emissions. The source of the interferences was investigated using wavelength-resolved PES, which required set-up of a new detector system. We report the resolved emission spectra of HNO3, NO and NO2 and also investigated the potential contribution of several other trace gases. Further characterisation and improvement of the PES scheme is required before we can report HNO3 yields in the HO2 + NO reaction. The rate coefficient (k4) of the reaction of HNO3 with OH, an important mechanism for conversion of NOy to NOx, increases dramatically at low temperature and high pressures. We have performed extensive quantum chemical and statistical rate theory calculations to rationalise this behaviour and to examine the molecular mechanism. A model was developed which is capable to quantitatively describe the temperature and pressure dependence of k4 as well as the isotope effect. We have shown that tunnelling plays a crucial role. We have further shown that, in pulsed photolysis experiments, rate coefficients calculated from HNO3 measurements integrated over the reaction volume are too large as HNO3 shows a strong concentration gradient between the centre of the reactor and the walls. Our results imply that previous measurements may have overestimated the rate coefficient and its pressure dependence at low temperatures. Measurements of k4 employing PES measurement of [HNO3] in the reaction volume only overcome such problems and are being undertaken at low temperatures. These measurements are to be supplemented by complementary experiments to determine the high-pressure limiting values of k4, which provides a broader database for a further validation of the proposed model.
Publications
- Is OH produced from NO2* + H2O or NO3* + H2O?, 22nd International Symposium on Gas Kinetics, Boulder (USA) 2012
T. J. Dillon, J. N. Crowley
- Laboratory studies of key gas-phase HOx –NOx coupling reactions, European Geosciences Union Annual Congress, Vienna (Austria) 2013
T. J. Dillon, K. Dulitz, J. N. Crowley
- Consecutive chemical activation steps in the OH-initiated atmospheric degradation of isoprene: An analysis with coupled master equations, Int. J. Chem. Kinet. 46, 231-244 (2014)
M. Pfeifle, M. Olzmann
(See online at https://doi.org/10.1002/kin.20849) - The rate coefficient of the OH + HNO3 reaction over a wide temperature and pressure range, 23rd International Symposium on Gas Kinetics and Related Phenomena, Szeged (Hungary) 2014
C. Bänsch, J. N. Crowley, T. J. Dillon, K. Dulitz, M. Olzmann, M. Pfeifle, M. Szöri
