Abstract
Abstract. The detection of nitric acid trihydrate (NAT, HNO3×3H2O) particles in the tropical transition layer (TTL) harmonizes our understanding of polar stratospheric cloud formation. Large reactive nitrogen (NOy) containing particles were observed on 8 August 2006 by instruments onboard the high altitude research aircraft M55-Geophysica near and below the tropical tropopause. The particles, most likely NAT, have diameters less than 6 μm and concentrations below 10-4 cm−3. The NAT particle layer was repeatedly detected at altitudes between 15.1 and 17.5 km over extended areas of 9.5 to 17.2° N and 1.5° W to 2.7° E above the African continent. Satellite observations suggest that the NAT particles could have nucleated on ice fed by convective activity. Once nucleated, the NAT particles can slowly grow within the TTL for days, while being transported over long distances. Their in-situ detection combined with global model simulations of the NAT supersaturation near the tropical tropopause indicate the potential for a tropical tropopause NAT particle belt.
Highlights
IntroductionThe thermodynamical possibility of the existence of solid nitric acid aerosol in the tropical tropopause layer has early been proposed by Hamill and Fiocco (1988), the observational evidence was missing for almost two decades
The composition of the tropopause region is modified by ice crystals and HNO3 containing aerosol through uptake of wa-ter (H2O) and nitric acid (HNO3) (Voigt et al, 2006) with subsequent particle sedimentation
We suggest that higher concentrations of smaller NOy containing particles could be present, which cannot be identified as peaks in the NOy,t signal due to their low HNO3 content
Summary
The thermodynamical possibility of the existence of solid nitric acid aerosol in the tropical tropopause layer has early been proposed by Hamill and Fiocco (1988), the observational evidence was missing for almost two decades. Only recently Popp et al (2006) detected NAT particles in the tropical lowermost stratosphere south of Costa Rica with an aircraft-borne particle mass spectrometer. Cold tropical cirrus clouds have been shown to contain enhanced concentrations of HNO3 (Voigt et al, 2007; Popp et al, 2007) at temperatures below the NAT saturation threshold (T
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