Title: Heterogeneous Chemistry of Calcium Carbonate Aerosols with Relevant Stratospheric Gases and Its Potential Impact on Stratospheric Ozone
Description: Given the rapid rise in global mean temperature as a direct consequence of increasing levels of greenhouse gases (GHGs) emissions, various climate geoengineering techniques, for example, solar radiation management (SRM), have been suggested. Often criticized as a distraction from global efforts of removing and reducing GHGs, most notably carbon dioxide (CO2), SRM involves both marine cloud brightening (MCB) and stratospheric aerosol injection (SAI), both of which are based on increasing the Earth’s albedo by seeding aerosols in the marine boundary layer and in the lower stratosphere, respectively. SAI has been explored more extensively in various modeling studies following observations of major volcanic eruptions. A significant loading of sulfate particles, a byproduct of the eruptions, were monitored to cool the Earth’s surface temperature temporarily, albeit with some significant consequences including increasing stratospheric ozone (O₃) depletion and reducing precipitation. For our studies, we solely focused on the application of SAI by studying relevant heterogeneous chemistry of alternative aerosols to sulfate, specifically, calcite (CaCO₃) aerosols, to better understand the aerosols’ unintended impact on stratospheric O3 level.

CaCO₃ aerosols, often serve as an idealized proxy for calcium-rich mineral dust, have been modeled to have minimal negative impact on both stratospheric O₃ level, through heterogeneous chemistry, and stratospheric temperature. However, only a few laboratory studies have been done on the heterogeneous chemistry of CaCO₃ aerosols with relevant stratospheric trace gases, such as HNO3 and HCl. These gases play a significant role in O₃ catalytic loss cycles in the stratosphere. Since HNO₃ is a common oxidation product of nitrogen oxides which contribute significantly to urban air pollution, a handful of ambient laboratory studies of CaCO₃ heterogeneous reaction with HNO₃ have been conducted. However, very little is known about CaCO₃ heterogeneous chemistry with HCl. Thus, the modeled impact of CaCO₃ aerosols on stratospheric O₃ so far may not be reliable given the lack of experimentally measured kinetics data. Here we report the results of an experimental study of the uptake of HNO₃ and HCl onto submicron CaCO₃ particles in two different flow reactors.

Products and reaction kinetics were observed by impacting aerosolized CaCO₃ 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)(HCO₃ termination layer was detected in the form of a HCO₃¯ peak in the FTIR spectra, indicating a hydrated surface even under dry conditions. The results demonstrate the reaction of HNO₃ with Ca(OH)(HCO₃) to produce Ca(NO₃)2, water, and CO₂. HCl reacted with Ca(OH)(HCO₃) to produce CaCl₂ and also water and CO₂. The depletion of the Ca(OH)(HCO₃)/Ca(CO₃) signal due to reaction with HNO3₃ or HCl followed pseudo-first order kinetics. From the FTIR analysis, the reactive uptake coefficient for HNO₃ 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 CaCO₃ aerosols was also studied in an aerosol flow tube (AFT) 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.

However, the heterogeneous chemistry of CaCO₃ aerosols 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 the same aerosol generation and characterization method, we coupled a low-temperature flow tube with the 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. Following the most recent modeling studies, our initial kinetic results suggest that the reactions of HCl and HNO₃ with CaCO₃ in the stratosphere could lead to a O₃ column change between -5% and +25%. This wide range of O₃ impact highlights the high uncertainties in estimating the true atmospheric impact of CaCO₃ aerosols, the most well-studied proposed SAI aerosols after sulfate, upon its release into the stratosphere.

Nevertheless, our kinetic study establishes a good experimental standard for studying airborne aerosol heterogeneous chemistry under stratospheric conditions as a necessary step to evaluate SAI as a realistic method to battle global warming.