Observational evidence on the effects of clouds and aerosols on net ecosystem exchange and evapotranspiration

Clouds and aerosols control variations in the ratio of diffuse to total incident solar radiation ( R d / R s ) and so determine to a large extent variations in both Light‐Use Efficiency ( LUE ) and in the Evaporative Fraction ( EF ). Five years (2002–2007) of continuous ground measurements collected at twenty‐three sites in the U.S. were analyzed to quantify this effect. Our results show that LUE is 19.4% and 203% larger for patchy clouds, and thick clouds than those for clear skies while LUE is about −6% for aerosols or thin clouds than those for clear skies. EF is 15.4%, 17.9% and 23.2% larger for aerosols or thin clouds, patchy clouds, and thick clouds than those for the clear sky when Eddy Covariance ( ECOR ) data are used, respectively. The values are 9.0%, 11.3% and 23.3% when Energy Balance Bowen Ratio ( EBBR ) data are used. This is the first systematic observational study on effects of R d / R s on EF and further study is needed.