Detection and attribution of an anomaly in terrestrial photosynthesis in Europe during the COVID-19 lockdown

Carbon dioxide (CO2) uptake by plant photosynthesis, referred to as gross primary production (GPP) at theecosystem level, is sensitive to environmental factors, including pollutant exposure, pollutant uptake, andchanges in the scattering of solar shortwave irradiance (SWin) - the energy source for photosynthesis. The 2020spring lockdown due to COVID-19 resulted in improved air quality and atmospheric transparency, providing aunique opportunity to assess the impact of air pollutants on terrestrial ecosystem functioning. However,detecting these effects can be challenging as GPP is influenced by other meteorological drivers and managementpractices. Based on data collected from 44 European ecosystem-scale CO2 flux monitoring stations, we observedsignificant changes in spring GPP at 34 sites during 2020 compared to 2015-2019. Among these, 14 sites showedan increase in GPP associated with higher SWin, 10 sites had lower GPP linked to atmospheric and soil dryness,and seven sites were subjected to management practices. The remaining three sites exhibited varying dynamics,with one experiencing colder and rainier weather resulting in lower GPP, and two showing higher GPP associatedwith earlier spring melts. Analysis using the regional atmospheric chemical transport model (LOTOS-EUROS)indicated that the ozone (O3) concentration remained relatively unchanged at the research sites, making itunlikely that O3 exposure was the dominant factor driving the primary production anomaly. In contrast, SWinincreased by 9.4 % at 36 sites, suggesting enhanced GPP possibly due to reduced aerosol optical depth andcloudiness. Our findings indicate that air pollution and cloudiness may weaken the terrestrial carbon sink by upto 16 %. Accurate and continuous ground-based observations are crucial for detecting and attributing subtlechanges in terrestrial ecosystem functioning in response to environmental and anthropogenic drivers.