Science Seminars

The lingering ozone hole – Polar stratospheric clouds and volcanic aerosols

Speaker: Yunqian Zhu (LASP)
Date: Thursday, Oct 25, 2018
Time: 4:00 PM
Location: SPSC W120

Seminar Abstract:
Polar stratospheric clouds (PSCs) are critical elements of  Arctic and Antarctic ozone depletion, yet most models use simple parameterizations for them. I built a PSC microphysics model that is coupled with the NCAR Community Earth System model for atmospheric chemistry and climate. This PSC model includes detailed microphysics of sulfuric aerosols and three types of PSCs: supercooled ternary solution (STS); nitric acid trihydrate (NAT); and ice. This presentation will discuss how I built this model step by step and how I tested it against observations. Simulated characteristics of STS particles, which are dominated by thermodynamics, compare well with observations. My model includes two major NAT formation mechanisms, both of which are essential to reproduce the PSC features observed by Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). Homogeneous nucleation of NAT from STS produces NAT particles with sizes near 8 μm, which are important to properly simulate denitrification and the gas phase HNO3 observed by the Microwave Limb Sounder (MLS). Heterogeneous nucleation of NAT on ice particles or ice particles on NAT and subsequent evaporation of the ice produces NAT particles with sizes from sub-micrometers to a few micrometers. These particles account for the large backscattering ratio from NAT observed by the CALIPSO satellite, especially in the midwinter season.
Data show that the area of the ozone hole is wildly varying from year to year, with the maximum size of the ozone hole occurring in 2015 despite the long-ago ban on chlorine containing compounds that should have resulted in a shrinking ozone hole.  In the second part of the talk, I investigate the impact of the 2015 Mt. Calbuco eruption and previous eruptions on stratospheric aerosols, PSCs, and ozone depletion. The modeled volcanic sulfate aerosol size distribution agrees with balloon observation within the error bars. Both the observed and simulated backscatter show volcanic sulfate aerosol from the Mt. Calbuco eruption was transported from mid-latitude towards the Antarctic, and slowly descended during transport. They also indicate that the SO2 emission into the stratosphere from Mt. Calbuco is 0.2-0.4 Tg. The modeled number density indicates that the volcanic sulfate aerosol from the Mt. Calbuco eruption penetrated into the Antarctic polar vortex in May 2015 and thereafter. The volcanic sulfate aerosol increases the ozone hole area and the ozone depletion in September especially around 100 hPa and 70˚S, relative to a case without any volcanic eruptions because it adds reactive surface area to warm regions of the Antarctic vortex, as originally suggested by Susan Solomon.​