Nitric Acid Trihydrate (NAT) formation at low NAT supersaturation in Polar Stratospheric Clouds (PSCs)

[摘要] A PSC was detected on 6 February 2003 in the Arctic stratosphereby in-situ measurements onboard the high-altitude researchaircraft Geophysica. Low number densities(~10^-4cm^-3) of small nitric acid (HNO₃)containing particles (d<6µm) were observed at altitudesbetween 18 and 20km. Provided the temperatures remain below theNAT equilibrium temperature T_NAT, these NAT particleshave the potential to grow further and to remove HNO₃ fromthe stratosphere, thereby enhancing polar ozone loss.Interestingly, the NAT particles formed in less than a day attemperatures just slightly below T_NAT(T>T_NAT-3.1K). This unique measurement of PSCformation at extremely low NAT saturation ratios(S_NAT≤10) constrains current NAT nucleationtheories. We suggest, that the NAT particles have formedheterogeneously, but for certain not on ice. Conversely,meteoritic particles may be favorable candidates for triggeringNAT nucleation at the observed low number densities.

[学科分类] 大气科学