Impact of reduced emissions on direct and indirect aerosol radiative forcing during COVID–19 lockdown in Europe

Abstract

Abstract. Aerosols influence the Earth’s energy balance through direct radiative effects and indirectly by altering the cloud micro-physics. Anthropogenic aerosol emissions dropped considerably when the global COVID–19 pandemic resulted in severe restraints on mobility, production, and public life in spring 2020. Here we assess the effects of these reduced emissions on direct and indirect aerosol radiative forcing over Europe, excluding contributions from contrails. We simulate the atmospheric com- position with the ECHAM5/MESSy Atmospheric Chemistry (EMAC) model in a baseline (business as usual) and a reduced emission scenario. The model results are compared to aircraft observations from the BLUESKY aircraft campaign performed in May/June 2020 over Europe. The model agrees well with most of the observations, except for sulfur dioxide, particulate sulfate and nitrate in the upper troposphere, likely due to a somewhat biased representation of stratospheric aerosol chemistry and missing information about volcanic eruptions which could have influenced the campaign. The comparison with a business as usual scenario shows that the largest relative differences for tracers and aerosols are found in the upper troposphere, around the aircraft cruise altitude, due to the reduced aircraft emissions, while the largest absolute changes are present at the surface. We also find an increase in shortwave radiation of 0.327 ± 0.105 Wm−2 at the surface in Europe for May 2020, solely attributable to the direct aerosol effect, which is dominated by decreased aerosol scattering of sunlight, followed by reduced aerosol absorption, caused by lower concentrations of inorganic and black carbon aerosols in the troposphere. A further in- crease in shortwave radiation from aerosol indirect effects was found to be much smaller than its variability. Impacts on ice crystal- and cloud droplet number concentrations and effective crystal radii are found to be negligible.

Fate of the nitrate radical at the summit of a semi-rural mountain site in Germany assessed with direct reactivity measurements

“…Northern sector and the generally lower NOX levels during TO2021 may also have been a result of changes in vehicle usage in the region as a higher fraction of locally employed people worked from home as a result of the COVID-19 pandemic (Reifenberg et al, 2021). TO2021 and PARADE had the highest incidence of very humid days, with a median RH of > 80 % for TO2021 and > 75%…”

Kinetics of OH + SO2 + M: Temperature-dependent rate coefficients in the fall-off regime and the influence of water vapour

“…The model was weakly nudged in spectral space, applying Newtonian relaxation of the parameters temperature, vorticity, divergence and surface pressure to meteorological reanalysis data (Jeuken et al, 1996). The model set-up is identical to the simulation RED presented in Reifenberg et al (2021) where the model was evaluated against an aircraft campaign over Europe. In addition, the model has been evaluated on many occasions (Pozzer et al, 2012a;Pozzer et al, 2012b;Yan et al, 2019).…”

“…For additional references, see http://www.messy-interface.org. As in Reifenberg et al (2021), EMAC was used in a chemical-transport model (Deckert et al, 2011) without feedbacks between photochemistry, radiation and atmospheric dynamics. In this work, we performed three identical simulations for the year 2019 but with three different parameterisations of k1 from this work and from the IUPAC and NASA evaluations panels.…”

Distribution of hydrogen peroxide over Europe during the BLUESKY aircraft campaign

“…Emission reduction coefficients were additionally adapted to lockdown conditions in Europe based on the work of Guevara et al (2021). A detailed description of the emission submodels as well as their modifications are presented in the work of Reifenberg et al (2021). The horizontal resolution of the model in this study is T63 (i.e., roughly 1.8°×1.8°) and the vertical resolution consists of 47 levels up to 0.01 hPa.…”

“…4S). Thus, emission reduction is not considered to be responsible for the strong deviation between the simulated and observed hydrogen peroxide mixing ratios (Reifenberg et al 2021).…”