Abstract
Abstract. Vertical redistribution of HNO3 through large HNO3-containing particles associated with polar stratospheric clouds (PSCs) plays an important role in the chemistry of the Arctic winter stratosphere. During the RECONCILE (Reconciliation of essential process parameters for an enhanced predictability of Arctic stratospheric ozone loss and its climate interactions) campaign, apparently very large NAT (nitric acid trihydrate) particles were observed by the airborne in situ probe FSSP-100 (Molleker et al., 2014). Our analysis shows that the FSSP-100 observations associated with the flight on 25 January 2010 cannot easily be explained assuming compact spherical NAT particles due to much too short growing time at temperatures below the existence temperature of NAT (TNAT). State-of-the-art simulations using CLaMS (Chemical Lagrangian Model of the Stratosphere; Grooß et al., 2014) suggest considerably smaller particles. We consider the hypothesis that the simulation reproduces the NAT particle masses in a realistic way, but that real NAT particles may have larger apparent sizes compared to compact spherical particles, e.g. due to non-compact morphology or aspheric shape. Our study focuses on the consequence that such particles would have reduced settling velocities compared to compact spheres, altering the vertical redistribution of HNO3. Utilising CLaMS simulations, we investigate the impact of reduced settling velocities of NAT particles on vertical HNO3 redistribution and compare the results with observations of gas-phase HNO3 by the airborne Fourier transform spectrometer MIPAS-STR associated with two RECONCILE flights. The MIPAS-STR observations confirm conditions consistent with denitrification by NAT particles for the flight on 25 January 2010 and show good agreement with the simulations within the limitations of the comparison. Best agreement is found if settling velocities between 100 and 50% relative to compact spherical particles are considered (slight preference for the 70% scenario). In contrast, relative settling velocities of 30% result in too weak vertical HNO3 redistribution. Sensitivity simulations considering temperature biases of ±1 K and multiplying the simulated nucleation rates by factors of 0.5 and 2.0 affect the comparisons to a similar extent, but result in no effective improvement compared to the reference scenario. Our results show that an accurate knowledge of the settling velocities of NAT particles is important for quantitative simulations of vertical HNO3 redistribution.
Highlights
Irreversible vertical redistribution of HNO3 through denitrification plays an important role in Arctic ozone depletion chemistry (Solomon, 1999, and references therein)
We consider the hypothesis that the new saturation-dependent parameterisation of heterogeneous NAT nucleation rates applied in CLaMS (Grooß et al, 2014) represents the masses of the largest NAT particles in a realistic way, but that these particles have larger apparent sizes compared to compact spherical particles, e.g. due to aspheric shape and/or noncompact morphology
This study analyses the combination of airborne in situ particle observations, airborne remote sensing observations of gas-phase HNO3, temperature and polar stratospheric clouds (PSCs) coverage as well as Lagrangian simulations of vertical HNO3 redistribution by NAT particles associated with the Arctic winter stratosphere at the end of January 2010
Summary
Irreversible vertical redistribution of HNO3 through denitrification plays an important role in Arctic ozone depletion chemistry (Solomon, 1999, and references therein). The formation of liquid and solid polar stratospheric cloud (PSC) particles and the reactive surface capable of chlorine activation depend on HNO3 availability in the gas phase (Grooß et al, 2005, and references therein). Particles capable of denitrification are assumed to be composed of NAT (nitric acid trihydrate), ice-coated NAT (or vice versa) and potentially further metastable phases composed of HNO3 and H2O, with NAD (nitric acid dihydrate) being one of the most likely candidates (Hanson and Mauersberger, 1988; Worsnop et al, 1993; Peter and Grooß, 2012, and references therein). Measurements of large HNO3-containing particles with significant potential to denitrify the lower stratosphere were reported by Fahey et al (2001)
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