Vortex‐Wide Detection of Large Aspherical NAT Particles in the Arctic Winter 2011/12 Stratosphere

W Woiwode,M L Santee,M Höpfner,L Bi,F Khosrawi

doi:10.1029/2019gl084145

Abstract

AbstractMicron‐sized HNO3‐containing particles in polar stratospheric clouds are known to denitrify the polar winter stratosphere and support chemical ozone loss. We show that populations of nitric acid trihydrate (NAT) particles with volume‐equivalent median radii of 3–7 μm can be detected vortex‐wide by means of infrared limb sounding. Key for detection are the applied optical characteristics of highly aspherical particles consisting of the β‐NAT phase. Spectroscopic signatures and ambient conditions of detected populations show that these particles play a key role in denitrification of the Arctic winter stratosphere. Complementary gas‐phase HNO3 observations indicate collocated highly efficient HNO3 sequestration within days and are consistent with measured spectral signals of populations of large NAT particles. High amounts of condensed gas‐phase equivalent HNO3 exceeding 10 ppbv and long persistence of detected populations, despite expected gravitational settling, imply that our understanding of the particles is incomplete.

Highlights

Large HNO3‐containing particles in polar stratospheric clouds (PSCs) sediment and denitrify the polar winter stratosphere and support chemical ozone loss (Fahey et al, 1990; Peter, 1997; Poole & McCormick, 1988; Salawitch et al, 1989; Solomon, 1999; Toon et al, 1986; Waibel et al, 1999)
While extensive denitrification is common in the cold Antarctic winter stratosphere, the Arctic winter stratosphere is mostly warmer, and extensive denitrification is linked to individual cold winters (Fahey et al, 1990; Toon et al, 1990)
The detected populations of large nitric acid trihydrate (NAT) particles follow the regions depleted in gas‐phase HNO3 consistently, whereas temperatures remain below temperature of NAT (TNAT) and above Tice (Figure 2o)

Summary

Introduction

Large HNO3‐containing particles in polar stratospheric clouds (PSCs) sediment and denitrify the polar winter stratosphere and support chemical ozone loss (Fahey et al, 1990; Peter, 1997; Poole & McCormick, 1988; Salawitch et al, 1989; Solomon, 1999; Toon et al, 1986; Waibel et al, 1999). In a case study involving airborne in situ and infrared limb observations, large highly aspherical β‐NAT particles with equivalent radii exceeding 10 μm were detected (Molleker et al, 2014; Woiwode et al, 2016). State‐of‐the art model parameterizations were not able to reproduce the observed combinations of large particle sizes and high HNO3 content inferred from these case studies (Grooß et al, 2014; Zhu et al, 2015). While state‐of‐the‐art parameterizations of large NAT particles are successful in reproducing major patterns of denitrification and the state at the end of the winter, large discrepancies remain in direct comparisons with rare Arctic particle field observations (Grooß et al, 2014; Molleker et al, 2014; Woiwode et al, 2016; Zhu et al, 2015). Vortex‐ wide observational capabilities sensitive to large NAT particles that allow model parameterizations to be tested are lacking

New Method to Detect Large NAT Particles

Sensitivity

Complementary MIPAS and MLS Observations

Conclusions

Data Availability Statement