On the partitioning of nitric acid and water in cirrus clouds – results from observations with the high altitude research aircraft Geopysica

Abstract

Uptake of reactive nitrogen species (NOy) in cirrus clouds has extensively been studied during field missions in the last decade with the aim to understand the interactions of trace gases with ice crystals, cirrus cloud formation and the perturbation of the composition of the tropopause region by clouds. Gao et al. (2004) suggest that the formation of clusters of nitric acid trihydrate (NAT) on ice at low temperatures might explain the observations of enhanced relative humidity over ice (RHI) in cirrus clouds (Delta-ice hypothesis). Here we evaluate in situ-observations of reactive nitrogen species, ice water content (IWC) and relative humidity in tropical, sub-tropical and polar cirrus clouds performed with the high altitude research aircraft Geophysica. Above 205 K, the fraction of total HNO3 bound to cirrus ice crystals and the nitric acid (HNO3) content of the ice crystals, expressed in HNO3/H2O molar ratios in ice, increase with decreasing temperature. The observations can be explained by uptake of HNO3 in ternary aerosol serving as freezing nuclei and subsequent trapping of HNO3 during ice crystal growth. A particularly cold cirrus cloud has been detected on 29 November 2005 for almost two hours by in situ-instruments onboard the Geophysica and by the Lidar on the DLR-Falcon during the SCOUT campaign from Darwin. The cirrus particles were measured at potential temperatures up to 390 K in the lowest stratosphere in connection to a large-scale convective system. At temperatures below 200 K, a high nitric acid content of the ice crystals coincides with enhanced values of RHI within the cirrus cloud. A recent compilation of NOy observations in cirrus clouds (Voigt et al., 2006) will be extended to low temperatures and with observations of RHI to test the Delta-ice hypothesis. Further the impact of experimental uncertainties and integration times on the data interpretation will be discussed.