Analysis of inorganic chlorine chemistry in the midlatitude summer stratosphere using aircraft and satellite observations

Abstract. In recent years, significant perturbations to stratospheric inorganic chlorine have been observed following wildfires or volcanic eruptions, as well as modeled in simulations with elevated water vapor from overshooting convection or organic species in biomass burning aerosol. A detailed evaluation of inorganic chlorine in the stratosphere sampled under various conditions is presented here using high precision in situ aircraft measurements of chlorine monoxide (ClO) and chlorine nitrate (ClONO₂). Data were obtained over North America during the NASA Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) mission. The vertical distributions of ClO and ClONO₂ from all 29 research flights of the 2-year mission are found to be relatively compact and lacking substantial outliers. The peak mixing ratios are approximately 30 ppt and 400 ppt for ClO and ClONO₂, respectively. No chlorine activation was observed in the lower stratosphere in the presence of low temperatures and elevated water vapor from convective injection. Steady-state calculated ClONO₂ is found to be in good agreement with measured values, suggesting that a reduction in the uncertainty of the recommended rate coefficient for ClONO₂ production is possible. HCl is calculated at high time resolution throughout the mission using satellite data, and the resulting evaluation of the inorganic chlorine budget shows excellent agreement, with the ratio of ClO + ClONO₂ + HCl to total inorganic chlorine equal to 0.95 for mixing ratios greater than 500 ppt. No evidence of inorganic chlorine activation was observed during DCOTSS when aerosol organic mass fraction and biomass burning fraction were elevated.