Experimental reaction rates constrain estimates of ozone response to calcium carbonate geoengineering

Abstract

Stratospheric solar geoengineering (SG) would impact ozone by heterogeneous chemistry. Evaluating these risks and methods to reduce them will require both laboratory and modeling work. Prior model-only work showed that CaCO3 particles would reduce, or even reverse ozone depletion. We reduce uncertainties in ozone response to CaCO3 via experimental determination of uptake coefficients and model evaluation. Specifically, we measure uptake coefficients of HCl and HNO3 on CaCO3 as well as HNO3 and ClONO2 on CaCl2 at stratospheric temperatures using a flow tube setup and a flask experiment that determines cumulative long-term uptake of HCl on CaCO3. We find that particle ageing causes significant decreases in uptake coefficients on CaCO3. We model ozone response incorporating the experimental uptake coefficients in the AER-2D model. With our new empirical reaction model, the global mean ozone column is reduced by up to 3%, whereas the previous work predicted up to 27% increase for the same SG scenario. This result is robust under our experimental uncertainty and many other assumptions. We outline systematic uncertainties that remain and provide three examples of experiments that might further reduce uncertainties of CaCO3 SG. Finally, we highlight the importance of the link between experiments and models in studies of SG.

Heterogeneous Reactivity of HCl on CaCO3 Aerosols at Stratospheric Temperature

“…7 We expect the uptake of trace gases on CaCO 3 aerosols to decrease over time due to the saturation of formed products on aerosol surface leading to a smaller available surface area. 16,24 Additional experiments should be…”

“…13 However, the impact of CaCO 3 on stratospheric ozone (O 3 ) varied widely between increasing the global O 3 column by 25% and depleting it by 5%, depending on the assumptions made about the kinetics of CaCO 3 heterogeneous reactions. 14,16 It is, therefore, essential to quantify the heterogeneous uptake of stratospherically relevant gases on the surface of CaCO 3 aerosol in the context of SAI in order to assess the potential environmental and climate impact of these activities.…”

“…With the goal of limiting the rise in global temperature to less than 1.5 °C, scientists have explored various climate engineering approaches, including stratospheric aerosol injection (SAI). − On the basis of observations − and modeling studies − of the impact of sulfate aerosols formed in the stratosphere following major volcanic eruptions, solid and high-refractive-index materials including calcium carbonate (CaCO 3 ) aerosols have been studied for SAI application. − Simulated models showed that an injection rate of 5.6 Tg yr –1 CaCO 3 aerosols, ∼20% of the peak sulfate level post Pinatubo eruption, resulted in a low level of stratospheric heating (∼0.2 K) compared to other studied materials including sulfate . However, the impact of CaCO 3 on stratospheric ozone (O 3 ) varied widely between increasing the global O 3 column by 25% and depleting it by 5%, depending on the assumptions made about the kinetics of CaCO 3 heterogeneous reactions. , It is, therefore, essential to quantify the heterogeneous uptake of stratospherically relevant gases on the surface of CaCO 3 aerosol in the context of SAI in order to assess the potential environmental and climate impact of these activities.…”

“…There is significant spread in the reported uptake values, which vary over several orders of magnitude due to variations in sample preparation and measurement techniques. On the contrary, only a few studies have been done on the interaction between HCl gas and CaCO 3 particles, , which is significant for the stratospheric context. As a reservoir species of the ClO x catalytic cycle, HCl uptake on the surfaces of polar stratospheric clouds (PSCs) perturbs the reactive chlorine budget in the polar winter that leads to stratospheric O 3 depletion. − When the background sulfate aerosol surface area was significantly elevated from roughly 1 to 20 μm 2 cm –3 after the eruption of Mt.…”

Potential implications of solar radiation modification for achievement of the Sustainable Development Goals

“…Sulfate aerosols could furthermore delay the recovery of the ozone layer, with important implications for health (SDG 3;Nowack et al, 2016;Pitari et al, 2014) and food production (Barnes et al, 2019). Calcite aerosols on the other hand could counter acidification (SDG 2, 14, 15) and might avoid most negative effects on tropospheric pollution and stratospheric ozone (SDG 3, 15;Dai et al, 2020;Keith et al, 2016). There is, however, potential for secondary effects on the ozone layer and cloud formation that could arise from aerosol interaction (Cziczo et al, 2019).…”