New Estimation of the NO_x Snow‐Source on the Antarctic Plateau

Abstract

To fully decipher the role of nitrate photolysis on the atmospheric oxidative capacity in snow‐covered regions, NOx flux must be determined with more precision than existing estimates. Here, we introduce a method based on dynamic flux chamber measurements for evaluating the NOx production by photolysis of snowpack nitrate in Antarctica. Flux chamber experiments were conducted for the first time in Antarctica, at the French‐Italian station Concordia, Dome C (75°06'S, 123°20’E, 3233 m a.s.l) during the 2019–2020 summer campaign. Measurements were gathered with several snow samples of different ages ranging from newly formed drifted snow to 6‐year‐old firn. Contrary to existing literature expectations, the daily average photolysis rate coefficient, JNO3¯, did not significantly vary between differently aged snow samples, suggesting that the photolabile nitrate in snow behaves as a single‐family source with common photochemical properties, where a JNO3¯ = (2.37 ± 0.35) × 10−8 s−1 (1σ) has been calculated from December 10th 2019 to January 7th 2020. At Dome C summer daily average NOx flux, FNOx, based on measured NOx production rates was estimated to be (4.3 ± 1.2) × 108 molecules cm−2 s−1, which is 1.5–7 times less than the net NOx flux observed previously above snow at Dome C using the gradient flux method. Using these results, we extrapolated an annual continental snow sourced NOx budget of 0.017 ± 0.003 Tg·N y−1, ∼2 times the nitrogen budget, (N‐budget), of the stratospheric denitrification previously estimated for Antarctica. These quantifications of nitrate photolysis using flux chamber experiments provide a road‐map toward a new parameterization of the σNO3−(λ,0.25emT)ϕ(T,0.25empH) product that can improve future global and regional models of atmospheric chemistry.

Modeling the Complete Nitrogen and Oxygen Isotopic Imprint of Nitrate Photolysis in Snow

“…For each time step, j for the dominant pathway (R1) is determined via Equation using the absorption cross section of Chu and Anastasio (2003) and a quantum yield calculated from the same work for Dome A summer temperature of −33°C (ϕ = 0.0017). The quantum yield estimated here is close to the value deduced from the field observations of NO x flux at Dome C (ϕ = 0.0013 ± 0.0003) (Barbero et al., 2021).…”

“…Yields measured in ice and liquid appear continuous across the freezing point (Chu & Anastasio, 2003;Dubowski et al, 2001), suggesting that these are aqueous-phase reactions predominantly occurring within a disordered interface (DI) at the ice crystal surface (Bartels-Rausch et al, 2014). Under most conditions, NO 2(g) released from the DI is the major product emitted from snow (Barbero et al, 2021;Frey et al, 2013;Grannas et al, 2007). Frey et al (2009) was the first to examine the isotopic effects of NO 3 − photolysis in snow with a theoretical approach developed by Yung and Miller (1997) for stratospheric N 2 O.…”

Nitrate Photolysis at the Air–Ice Interface of Nature-Identical Snow

“…While faster reaction rate constants on ice surfaces could have important implications for nitrate photodegradation rates in the environment, calculations by McFall et al 30 suggest most snow nitrate is present in LLRs, rather than at the air−ice interface. Furthermore, recent Antarctic measurements of NO x fluxes found the daily average nitrate photolysis rate constant did not vary significantly among snow samples of varying ages, suggesting nitrate is present primarily in one compartment, 51 which is most likely LLRs. While our work shows nitrate is more readily photolyzed to nitrite at the air−ice interface (Channel 2) than in aqueous solution, our results do not give insights into whether or not the reaction mechanism is the same in each compartment.…”

Validating HONO as an Intermediate Tracer of the External Cycling of Reactive Nitrogen in the Background Atmosphere

“…To identify the external cycling in polar areas, several key arguments, such as the distributions and budgets of HONO and NO x , mechanism analyses, and atmospheric perturbations of external cycling, have been comprehensively discussed in various studies lasting from the 1990s to the present. − ,− The polar scenario suggests a revision of atmospheric photochemistry by the external cycling mechanism within the snowpack. , Specifically, an external source of nitrate photolysis on snow/ice surfaces promotes HONO and NO x abundances and increases HO x to levels even exceeding those found in tropical marine boundaries . Herein, we follow this lead to explore the scientific questions raised above.…”