HYDROCHLORIC-ACID AND THE CHLORINE BUDGET OF THE LOWER STRATOSPHERE

Concentrations of HC1 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, CIO, and CIONO2, the latter inferred from measurements of CIO and NO2, 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 CIONO2 may exceed those of HC1 near 20 km altitude, consistent with a slower photolysis rate for CIONO2 than calculated using recommended cross sections. Comparison of profiles of HC1 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 HC1 to Cl(y) ratio, consistent with a factor of 3-10 reduction in the photolysis rate for CIONO2 at ER-2 altitudes. However, the diurnal variation of CIO is well-simulated by models using the recommended photolysis rate, and simultaneous measurements of ClONO2 and HCl at mid-latitudes by ATMOS and MARKIV report HC1/HCl + CIONO2) ratios greater-than-or-equal-to 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.