Evolution of the Aerosol Daily Direct Radiative Efficiency Using the Aeronet Database

Environmental rules, laws and policies have been established around the world to reduce the aerosol concentrations and their climate impact as much as possible. The AERONET network provides instantaneous Aerosol Radiative Impact and the Aerosol Radiative Efficiency (the efficiency is the impact normalized by the aerosol concentration). Thus, we determined the Aerosol Daily Radiative Efficiency (ADRE) using data of each month. While the Aerosol Daily Radiative Impact is directly linked to how much the aerosol impacts the climate, the ADRE is linked to the intrinsic composition of the aerosol, i.e., its chemical composition, and its variability may be relied on the changes in the nature of aerosol emissions (anthropic and/or natural).

We decided to focus on the yearly trend of ADRE within the atmosphere (ATM) and at the bottom of the atmosphere (BOA) We selected the decade before the COVID-19 pandemic 2008-2019. From AERONET, we were able to determine the yearly trend for 247 sites over the world. These sites didn't give a representation at the global scale, but it gives a good idea.

While we observe a small increasing of the yearly trend for the aerosol Single Scattering Albedo (meaning a lower proportion of absorption), we found for ADRE_ATM (Figure 1) and ADRE_BOA respectively a small decreasing of about -1.0%/year and about -0.5%/year over our sites. It means, we have, in average, an ability of aerosols to slightly slow down the atmospheric heating and the cooling at surface. This average stability hides geographical and economic variabilities with negative yearly trends (when policies look to be applied) and positive ones (some of them are relied to natural events (increasing of wildfires and dust storms).

We looked at relationships between ADRES yearly trends and the aerosol microphysical properties. The most significant result is the good correlation with the imaginary part of the refractive index (R2=0.75), itself linked to absorption. It's not a real surprise as the Black Carbon is largely responsible of the Daily Aerosol Radiative Efficiency