Denitrification by large NAT particles: the impact of reduced settling velocities and hints on particle characteristics

Abstract

Abstract. Vertical redistribution of HNO 3 through large HNO 3 -containing particles associated with polar stratospheric clouds (PSCs) plays an important role in the chemistry of the Arctic winter stratosphere. During the RECON-CILE (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 . 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 (T NAT ). 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 HNO 3 . Utilising CLaMS simulations, we investigate the impact of reduced settling velocities of NAT particles on vertical HNO 3 redistribution and compare the results with observations of gas-phase HNO 3 by the airborne Fourier transform spectrometer MIPAS-STR associated with two REC-ONCILE 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 HNO 3 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 HNO 3 redistribution.

A multi-wavelength classification method for polar stratospheric cloud types using infrared limb spectra

“…For NAT clouds Woiwode et al (2014) found indications that aspherical particles might modify the spectral shape of the characteristic spectral NAT feature at 820 cm −1 in balloon-based IR measurements (Fig. 1).…”

Development of a Polar Stratospheric Cloud Model within the Community Earth System Model using constraints on Type I PSCs from the 2010–2011 Arctic winter

“…As a result of the cold bias, the NAT particles cannot evaporate fast enough to return the nitric acid back to the gas phase around 400 K. Instead, the NAT particles fall out of the stratosphere. However, the sensitivity tests by Woiwode et al [2014] indicate that the renitrification at the lower altitude is not very sensitive to 61 K temperature bias. Instead, the falling velocity of NAT is important for simulating the renitrification of HNO 3 .…”

“…However, the sensitivity tests by Woiwode et al . [] indicate that the renitrification at the lower altitude is not very sensitive to ±1 K temperature bias. Instead, the falling velocity of NAT is important for simulating the renitrification of HNO 3 .…”

Interannual variations of early winter Antarctic polar stratospheric cloud formation and nitric acid observed by CALIOP and MLS

“…13a), probably indicating sedimentation of NAT and permanent denitrification. However, since Woiwode et al (2014) have reported the presence of NAT with larger apparent sizes compared to compact spherical particles and with concomitant reduced settling rates, we must also consider the possibility that the NAT radius grew larger and subsequently evolved into sub-visible NAT. Höpfner et al (2009), and even with longer-term averaging, over the entire May-October period, Spang et al (2016) concluded that MIPAS has higher sensitivity to PSCs since, for the cases where CALIOP detects no cloud, about 60 % correspond to cloudy scenes for MIPAS.…”

“…The morphology of NAT particles is still an open question, as is the compactness of the particles Woiwode et al, 2014Woiwode et al, , 2016. Light scattering studies have repeatedly shown that detailed particle morphology cannot be deduced from the depolarization; e.g., Nousiainen et al (2012) investigated simple and complex shapes with size parameters (ratio of the particle circumference to the wavelength) in the range 2-12 and real refractive indices in the range 1.55-1.603 representative of silicate particles, and noted similar depolarization ranges for the fifteen different shapes (regular and irregular) that were analyzed.…”