Hydrochloric acid and the chlorine budget of the lower stratosphere

Abstract

Concentrations of HCl measured in the lower stratosphere in 1993 by the ALIAS instrument on the ER‐2 aircraft reveal that only 40% of inorganic chlorine (Cl y , inferred from in situ measurements of organic chlorinated source gases) is present as HCl, significantly lower than model predictions. Although the sum of measured HCl, ClO, and ClONO 2 , the latter inferred from measurements of ClO and NO 2 , equals Cl y to within the uncertainty of measurement, it is systematically less than Cl y by 30–50%. This discrepancy suggests that concentrations of ClONO 2 may exceed those of HCl near 20 km altitude, consistent with a slower photolysis rate for ClONO 2 than calculated using recommended cross sections. Comparison of profiles of HCl measured during 1992 and 1993 at mid‐latitudes by balloon (BLISS and MARKIV), space shuttle (ATMOS), and satellite (HALOE) instruments with the aircraft data reveal an apparent pressure dependence to the HCl to Cl y ratio, consistent with a factor of 3–10 reduction in the photolysis rate for ClONO 2 at ER‐2 altitudes. However, the diurnal variation of ClO is well‐simulated by models using the recommended photolysis rate, and simultaneous measurements of ClONO 2 and HCl at mid‐latitudes by ATMOS and MARKIV report HCl/(HCl+ClONO 2 ) ratios ≥ 50%. Preliminary measurements by ALIAS in the southern hemisphere report HCl/Cl y values of about 75%. It is unclear to what extent elevated aerosol surface area, uncertainties in the estimation of Cl y , or in the photolysis rates, or missing reactive pathways contribute to our inability to balance the budget of inorganic chlorine using the aircraft measurements.