Abstract
Abstract. This study reports the first systematic measurements of nitric acid (HNO3) uptake in contrail ice particles at typical aircraft cruise altitudes. During the CIRRUS-III campaign cirrus clouds and almost 40 persistent contrails were probed with in situ instruments over Germany and Northern Europe in November 2006. Besides reactive nitrogen, water vapor, cloud ice water content, ice particle size distributions, and condensation nuclei were measured during 6 flights. Contrails with ages up to 12 h were detected at altitudes 10–11.5 km and temperatures 211–220 K. These contrails had a larger ice phase fraction of total nitric acid (HNO3ice/HNO3tot = 6%) than the ambient cirrus layers (3%). On average, the contrails contained twice as much HNO3ice as the cirrus clouds, 14 pmol/mol and 6 pmol/mol, respectively. Young contrails with ages below 1 h had a mean HNO3ice of 21 pmol/mol. The contrails had higher nitric acid to water molar ratios in ice and slightly higher ice water contents than the cirrus clouds under similar meteorological conditions. The differences in ice phase fractions and molar ratios between developing contrails and cirrus are likely caused by high plume concentrations of HNO3 prior to contrail formation. The location of the measurements in the upper region of frontal cirrus layers might account for slight differences in the ice water content between contrails and adjacent cirrus clouds. The observed dependence of molar ratios as a function of the mean ice particle diameter suggests that ice-bound HNO3 concentrations are controlled by uptake of exhaust HNO3 in the freezing plume aerosols in young contrails and subsequent trapping of ambient HNO3 in growing ice particles in older (age > 1 h) contrails.
Highlights
Heterogeneous processes such as the uptake of nitric acid (HNO3) in cirrus clouds influence the ozone budget in the tropopause region
How are the high molar ratios brought about? We argue that a high concentration of HNO3 had already entered the ice particles during contrail formation, when they are very small, leading to very high molar ratios
During the CIRRUS-III field campaign gas phase and ice phase reactive nitrogen, ice water content, and ice crystal size distributions were measured in contrails and cirrus at midlatitudes close to the tropopause
Summary
Heterogeneous processes such as the uptake of nitric acid (HNO3) in cirrus clouds influence the ozone budget in the tropopause region. Karcher et al (1996) simulate the formation of high levels of HNO3 in jet aircraft exhaust plumes prior to contrail formation These early HNO3 concentrations are determined by the NOx emission index and the engine exit plane concentration of the hydroxyl radical (OH) which acts as the primary oxidant. Gao et al (2004) measured HNO3 in ice particles of a WB-57 aircraft contrail in the subtropics at 14– 15 km altitude They demonstrated that HNO3 was present in contrail particles at temperatures below 205 K, presumably in the form of NAT, but did not explicitly report ice phase HNO3 concentrations. We present and interpret in situ observations of reactive nitrogen (NOy), H2O, as well as microphysical properties of contrails and thin frontal cirrus layers, performed over Germany, the North Sea, and the Baltic Sea during the CIRRUS-III campaign during 23–29 November 2006. These observations provide an unprecedented data set on the uptake of HNO3 in persistent contrails
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