Calcite (CaCO 3 ) 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 3 and HCl and therefore their potential impact on stratospheric ozone (O 3 ). Here we report the results of an experimental study of the uptake of HNO 3 and HCl onto submicron CaCO 3 particles in two different flow reactors. Products and reaction kinetics were observed by impacting aerosolized CaCO 3 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 3 ) termination layer was detected in the form of a HCO 3 − peak in the FTIR spectra, indicating a hydrated surface even under dry conditions. The results demonstrate the reaction of HNO 3 with Ca(OH)(HCO 3 ) to produce Ca(NO 3 ) 2 , water, and CO 2 . HCl reacted with Ca(OH)(HCO 3 ) to produce CaCl 2 and also water and CO 2 . The depletion of the Ca(OH)(HCO 3 )/Ca(CO 3 ) signal due to reaction with HNO 3 or HCl followed pseudo-first-order kinetics. From the FTIR analysis, the reactive uptake coefficient for HNO 3 was determined to be in the range of 0.013 ≤ γ HNO3 ≤ 0.14, and that for HCl was 0.0011 ≤ γ HCl ≤ 0.012 within the reported uncertainty. The reaction of HCl with airborne CaCO 3 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 HCl was determined to be 0.013 ± 0.001. Following previous modeling studies, these results suggest that the reactions of HCl and HNO 3 with calcite in the stratosphere could ameliorate the potential for stratospheric solar radiation management to lead to stratospheric ozone depletion.
Recently proposed as a possible alternative to sulfate particles for stratospheric solar radiation management (SSRM), calcite (CaCO3) aerosols have been modeled to have minimal negative impact on both stratospheric ozone level, through heterogeneous chemistry, and stratospheric temperature. However, the heterogeneous chemistry of CaCO3 aerosols with relevant trace gases, such as HCl, at stratospheric conditions is still underexamined. We studied the kinetics of HCl uptake on airborne CaCO3 aerosols at stratospheric temperature, 207 ± 3 K, by performing experiments under dry conditions using an aerosol flow tube coupled with a custom-built quadrupole chemical ionization mass spectrometer (CIMS) for HCl detection. The reactive uptake coefficient for HCl was measured to be 0.076 ± 0.009. This exceeds the reactive uptake coefficient of 0.013 ± 0.001 that we previously reported for this system at 296 K, consistent with the expected negative temperature dependence of gas uptake on solid surfaces. This finding suggests an initial strong reactive uptake of HCl gas on CaCO3 aerosol surfaces in the stratosphere.
Given the rise in global mean temperature as a direct consequence of increasing levels of greenhouse gases (GHG) in the atmosphere, a variety of climate engineering approaches, including stratospheric aerosol injection (SAI), have been proposed. Often criticized as a distraction from global efforts towards reducing GHG emissions, SAI aims to increase the Earth’s albedo by seeding aerosols in the lower stratosphere. SAI has been explored extensively in modeling studies based on observations of temporary cooling of the Earth’s surface following major volcanic eruptions which introduced significant loadings of sulfate particles into the stratosphere. The cooling effect is accompanied by other significant consequences including stratospheric heating, stratospheric ozone (O3) depletion, and reduced global mean precipitation. In order to understand the potential environmental and climate impacts of SAI, we review the state of the knowledge regarding these issues, starting from an aerosol science perspective. We summarize aerosol radiative properties and the role they play in defining the optimal chemical and physical aerosol characteristics for SAI, and their implications for lower stratospheric warming. We then review in depth the impacts of stratospheric aerosol heterogeneous chemistry on global O3 levels. We review SAI modeling studies as well as their uncertainties, in comparison to the observed environmental and climate impacts of volcanically derived sulfate aerosols, including impacts on global temperature, stratospheric warming, and hydrological cycle. We also discuss the current governance and economic considerations of the application of SAI and raise essential questions from both research and social standpoints that must be addressed before SAI is deployed for climate change mitigation.
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