Enhanced Heterogeneous Nitrates Photolysis on Ice and Potential Impacts on NOx Emissions

Abstract:

Nitrates photolysis plays a key role in the chemistry of the polar boundary layer and of the lower troposphere over snow-covered areas (1). Using a combination of vibrational (2) and photo-absorption spectroscopies (3), we show that nitric acid is mostly dissociated upon its adsorption onto, and its dissolution within ice at temperatures as low 20K. Using amorphous solid water as a model substrate for the disordered surface layer at the interstitial air-ice interface, UV irradiation in the environmentally relevant n-π* transition uncovers the fact that the photolysis rates are significantly higher for surface-bound nitrates compared to those dissolved within the bulk. The complex coupled interfacial transport and reaction kinetics result in the formation of a thin photochemically active layer at the surface of ice which may magnify the impact of surface-enhanced nitrates photolysis rates on ice thereby providing a significant contribution to the intense photochemical NO_xfluxes observed to emanate from the sunlit snowpack upon polar sunrise.(4)

(1) F. Dominé, P.B. Shepson, Science, 297, 1506-1510 (2002).

(2) P. Marchand, G. Marcotte, and P. Ayotte, Spectroscopic Study of HNO₃ Dissociation on Ice, J. Phys. Chem. A 116, 12112-12122 (2012).

(3) G. Marcotte, P. Ayotte, A. Bendounan, F. Sirotti, C. Laffon and P. Parent, J. Phys. Chem. Lett. 4, 2643-2648 (2013).

(4) G. Marcotte, P. Marchand, S. Pronovost, P. Ayotte, C. Laffon and P. Parent, J. Phys. Chem. A 119, 1996-2005 (2015).