Spatial and diurnal variability in reactive nitrogen oxide chemistry as reflected in the isotopic composition of atmospheric nitrate: Results from the CalNex 2010 field study

W. C. Vicars (1) , S. Morin (2) , J. Savarino (1) , N. L. Wagner , J. Erbland (1) , E. Vince (3) , J. M. F. Martins (3) , B. M. Lerner , P. K. Quinn , D. J. Coffman , E. J. Williams , S. S. Brown

1 LGGE - Laboratoire de glaciologie et géophysique de l'environnement
2 CNRM - Centre national de recherches météorologiques
3 LTHE - Laboratoire d'étude des transferts en hydrologie et environnement

we present measurements of the size-resolved concentration and isotopic composition of atmospheric nitrate (NO₃^-) collected during a cruise in coastal California. Significant differences in air mass origin and atmospheric chemistry were observed in the two main regions of this cruise (South and Central Coast) with corresponding differences in NO₃^- concentration and isotope ratios. Measurements of the ¹⁷O-excess (Δ¹⁷O) of NO₃^- suggest that nocturnal chemistry played an important role in terms of total NO₃^- production ( 50%) in the coastal Los Angeles region (South Coast), where NO₃^- concentrations were elevated due to the influence of sea breeze / land breeze recirculation and Δ¹⁷O(NO₃^-) averaged (25.3 ± 1.6)‰. Conversely, Δ¹⁷O(NO₃^-) averaged (22.3 ± 1.8)‰ in the Central Coast region, suggesting that the daytime OH + NO₂ reaction was responsible for 60-85% of NO₃^- production in the marine air sampled in this area. A strong diurnal signal was observed for both the Δ¹⁷O and δ¹⁵N of NO₃^-. In the case of Δ¹⁷O, this trend is interpreted quantitatively in terms of the relative proportions of daytime and nighttime production and the atmospheric lifetime of NO₃^-. For δ¹⁵N, which had an average value of (0.0 ± 3.2)‰, the observed diurnality suggests a combined effect of isotopic exchange between gas-phase precursors and variability in reactive nitrogen sources. These findings represent a significant advance in our understanding of the isotope dynamics of nitrate and its precursor molecules, with potentially important implications for air quality modeling.

Mots clés: nitrogen cycle; stable isotopes; nocturnal tropospheric chemistry; non-mass dependent fractionation; ozone

Domaines: Planète et Univers [physics]

W. C. Vicars, S. Morin, J. Savarino, N. L. Wagner, J. Erbland, et al.. Spatial and diurnal variability in reactive nitrogen oxide chemistry as reflected in the isotopic composition of atmospheric nitrate: Results from the CalNex 2010 field study. Journal of Geophysical Research: Atmospheres, 2013, 118, pp.10,567-10,588. ⟨10.1002/jgrd.50680⟩. ⟨insu-03620910⟩