Extreme NAT supersaturations in mountain wave ice PSCs: A clue to NAT formation

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There is evidence for the existence of nitric acid trihydrate (NAT) in polar stratospheric clouds (PSCs) and for its role in sedimentary denitrification enhancing chlorine‐catalyzed ozone reduction. However, the scientific understanding of NAT formation mechanisms is vague. We show that high cooling rates in mountain waves, though causing only moderate NAT saturation ratios (SNAT ≲ 30) within supercooled ternary solution (STS) droplets, lead to SNAT ≳ 500 within the gas phase. Evidence from previous laboratory measurements suggests that such extreme supersaturations promote NAT deposition nucleation on ice or other solid surfaces, as long as they are not coated by STS. On the basis of airborne lidar PSC observations in three Arctic winters and a combined microphysical/optical model, we derive a simple parameterization for a NAT deposition nucleation rate which depends only on NAT supersaturation and temperature. The results indicate that deposition nucleation of NAT on ice surfaces exposed directly to the gas phase driven by extreme NAT supersaturations is able to explain the formation of NAT particle number densities up to ∼10 cm−3.