Heterogeneous Chemistry of CaCO Aerosols with HNO and HCl.

Calcite (CaCO) aerosols often serve as an idealized proxy for calcium-rich mineral dust. Their use has also previously been proposed for stratospheric solar radiation management (SSRM). Little is known about the heterogeneous chemistry of calcite aerosols with trace gases HNO and HCl and therefore their potential impact on stratospheric ozone (O). Here we report the results of an experimental study of the uptake of HNO and HCl onto submicron CaCO particles in two different flow reactors. Products and reaction kinetics were observed by impacting aerosolized CaCO onto ZnSe windows, exposing them to the reagent gases at a wide range of concentrations, at 296 K and under dry conditions, and analyzing the particles before and after trace gas exposure using Fourier transform infrared spectroscopy (FTIR). A Ca(OH)(HCO) termination layer was detected in the form of a HCO peak in the FTIR spectra, indicating a hydrated surface even under dry conditions. The results demonstrate the reaction of HNO with Ca(OH)(HCO) to produce Ca(NO), water, and CO. HCl reacted with Ca(OH)(HCO) to produce CaCl and also water and CO. The depletion of the Ca(OH)(HCO)/Ca(CO) signal due to reaction with HNO or HCl followed pseudo-first-order kinetics. From the FTIR analysis, the reactive uptake coefficient for HNO was determined to be in the range of 0.013 ≤ γ ≤ 0.14, and that for HCl was 0.0011 ≤ γ ≤ 0.012 within the reported uncertainty. The reaction of HCl with airborne CaCO aerosols was also studied in an aerosol flow tube coupled with a quadrupole chemical ionization mass spectrometer (CIMS) under similar conditions to the FTIR study, and γ was determined to be 0.013 ± 0.001. Following previous modeling studies, these results suggest that the reactions of HCl and HNO with calcite in the stratosphere could ameliorate the potential for stratospheric solar radiation management to lead to stratospheric ozone depletion.