Nitric acid concentrations near the tropical tropopause: Implications for the properties of tropical nitric acid trihydrate clouds

In situ measurements of NOy , NOx, and temperature confirm that nitric acid trihydrate (NAT) particles could form at the tropical tropopause. The HNO3 mixing ratio near the tropical tropopause is typically no larger than about 0.2– 0.3 ppbv, and the corresponding equilibrium mass of nitric acid trihydrate (NAT) is no larger than 0.3 mg m-3. Considerably larger NAT condensed masses are required to explain the HALOE extinctions; however, localized regions of enhanced HNO3 produced by oxidation of lightning-generated NO might exist. NAT layers would only be identified as clouds by SAGE II if the particle diameters are in the optimum range of about 0.6 to 2 mm and the condensed NAT mass is larger than about 0.2 mg m-3. The SAGE II extinction ratio measurements (0.5 mm/1.0 mm) cannot distinguish NAT clouds from mixtures of optically thin ice clouds and background aerosols.