Decreased Aviation Leads to Increased Ice Crystal Number and a Positive Radiative Effect in Cirrus Clouds

Abstract

Travel restrictions in the wake of the COVID‐19 pandemic resulted in an unprecedented decrease of 73% in global flight mileage in April–May 2020 compared to 2019. Here we examine the CALIPSO satellite observations and find a significant increase in ice crystal number concentrations (Ni) in cirrus clouds in the mid‐latitudes of the Northern Hemisphere, which we attribute to an increase in homogeneous freezing when soot from aircraft emissions is reduced. A relatively small positive global average radiative effect of 21 mW m−2 is estimated if a decrease in aircraft traffic continues, with an average of up to 64 mW m−2 over the area where aviation is most active. We infer from this analysis that the worldwide adoption of biofuel blending in aircraft fuels that lead to smaller soot emissions could lead to a significant change in the microphysical properties of cirrus clouds but a rather small positive radiative effect.

The Role of Mineral Dust Aerosol Particles in Aviation Soot‐Cirrus Interactions

“…Clearly, it is important that the improved process understanding presented here regarding dust‐aviation soot‐cirrus interactions is incorporated into global model parameterizations. Although mineral dust INPs are included in global climate models (Righi et al., 2021; Zhu et al., 2022), predicted aviation soot effects are not discussed in relation to simulated dust and soot particle number concentrations.…”

“…As a result, the average reduction of homogeneously nucleated ICNCs, which is the basis for the large radiative effect ascribed to aviation soot reductions in Zhu et al (2022), will be overestimated. We recall that for a typical mean updraft speed value σ w = 15 cm s −1 , the reduction in total ICNCs in case Soot relative to case Hom amounts only to 3.5-22.5%, depending on the assumed PSD (Table 2).…”

“…For example, the global climate model parameterization used in Penner et al (2018) and Zhu et al (2022) specify s = 0.35 as a single ice supersaturation threshold for aviation soot particles with fewer than 3 monolayers of sulfate coating to nucleate ice at cirrus temperatures. This causes aviation soot to be more ice-active than mineral dust.…”

“…The large spread of RF is mainly caused by the underlying parameterizations of the ice activity of aviation soot, which rely on disparate assumptions. For example, some climate models parameterize ice formation from aviation soot at ice supersaturation considerably lower than that needed for homogeneous freezing (Penner et al., 2018; Righi et al., 2021; Zhu et al., 2022), allowing aviation soot to readily contribute to the formation of cirrus clouds. The corresponding model simulations predict large RF values from aviation‐soot cirrus interactions.…”

Impacts of Simulated Contrail Processing and Organic Content Change on the Ice Nucleation of Soot Particles

“…Quaas et al (2021) reported that the COVID-19 pandemic related aviation reduction in the Northern Hemisphere leads to a decrease in cirrus cloud cover and reduces the global net-warming radiative forcing. In contrast, Zhu et al (2022) reported a small increase in global radiative forcing because air traffic reduction results in increased cirrus ice crystal number trapping more long-wave radiation in the troposphere. It is suggested that complex and poorly constrained soot-cloud interactions result in inter-model discrepancies and disagreements (Xu et al, 2019).…”