Comment on egusphere-2023-744

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> The oxygen (𝛥¹⁷O) and nitrogen (𝛿¹⁵N) isotopic compositions of atmospheric nitrate (NO₃-) are widely used as tracers of its formation pathways, precursor (nitrogen oxides NO_x = nitric oxide NO + nitrogen NO₂) emission sources, and physico-chemical processing. However, the critical lack of observations on the multi-isotopic composition of NO₂ maintains significant uncertainties regarding the links between the isotopic composition of NO_x and NO₃-, which may bias estimates of the NO₃- formation processes and the distribution of sources. We report here on the first simultaneous atmospheric observations of 𝛥¹⁷O and 𝛿¹⁵N in NO₂ and NO₃-. The measurements were carried out at sub-daily (ca. 3 h) resolution over two non-consecutive days in an Alpine city in February 2021. Important diurnal variabilities are observed in both NO₂ and NO₃- multi-isotopic composition. 𝛥¹⁷O of NO₂ and NO₃- range from 19.6 to 40.8 &permil; and 18.7 to 26 &permil;, respectively. During both daytime and nighttime, the variability of 𝛥¹⁷O(NO₂) is mainly driven by the oxidation of NO by ozone, with a substantial contribution from peroxy radicals in the morning. NO₃- local mass balance equations, constrained by observed 𝛥¹⁷O(NO₂), suggest that during the first day of sampling NO₃- was formed locally from the oxidation of NO₂ by hydroxyl radicals during the day, and via heterogeneous hydrolysis of dinitrogen pentoxide during the night. For the second day, calculated and observed 𝛥¹⁷O(NO₃-) do not match, particularly daytime values. The effects on 𝛥¹⁷O(NO₃-) of a Saharan dust event that occurred during the second day and winter boundary layer dynamics are discussed. 𝛿¹⁵N of NO₂ and NO₃- ranged from -10.0 to 19.7 &permil; and -4.2 to 14.8 &permil;, respectively. Consistent with theoretical predictions of N isotope fractionation, the important variability of 𝛿¹⁵N(NO₂) is explained by significant post-emission equilibrium N fractionation. After accounting for this effect, vehicle exhaust is found to be the primary source of NO_x emissions at the sampling site. 𝛿¹⁵N(NO₃-) is closely linked to 𝛿¹⁵N(NO₂) variability, which bring further evidence of fast and local processing, but uncertainties on current N fractionation factors during NO₂ to NO₃- conversion are underscored. Overall, this detailed investigation highlights the potential and the necessity to use 𝛥¹⁷O and 𝛿¹⁵N in NO₂ and NO₃- to trace quantitatively the sources and formation chemistry of NO₃-, particularly in urban environments in winter.