Abstract
Abstract. The impacts of clouds and atmospheric aerosols on the terrestrial carbon cycle at semi-arid Loess Plateau in Northwest China are investigated, by using the observation data obtained at the SACOL (Semi-Arid Climate and Environment Observatory of Lanzhou University) site. Daytime (solar elevation angles of larger than 50°) net ecosystem exchange (NEE) of CO2 obtained during the midgrowing season (July–August) are analyzed with respect to variations in the diffuse radiation, cloud cover and aerosol optical depth (AOD). Results show a significant impact by clouds on the CO2 uptake by the grassland (with smaller LAI values) located in a semi-arid region, quite different from areas covered by forests and crops. The light saturation levels in the canopy are low, with a value of about 434.8 W m−2. Thus, under overcast conditions of optically thick clouds, the CO2 uptake increases with increasing clearness index (the ratio of global solar radiation received at the Earth surface to the extraterrestrial irradiance at a plane parallel to the Earth surface), and a maximum CO2 uptake and light use efficiency of vegetation occur with the clearness index of about 0.37 and lower air temperature. Under other sky conditions, CO2 uptake decreases with cloudiness but light use efficiency is enhanced, due to increased diffuse fraction of PAR. Additionally, under cloudy conditions, changes in the NEE of CO2 also result from the interactions of many environmental factors, especially the air temperature. In contrast to its response to changes in solar radiation, the carbon uptake shows a slightly negative response to increased AOD. The reason for the difference in the response of the semi-arid grassland from that of the forest and crop lands may be due to the difference in the canopy's architectural structure.
Highlights
Solar radiation is a major factor that influences the CO2 exchange in the biosphere
The seasonally averaged carbon flux exceeds −5.0 μmol m−2 s−1, but the amount of carbon uptake is still much less than those observed in forests and croplands (Niyogi et al, 2004)
For our site we have found that the light use efficiency of vegetation under cloudy and overcast conditions is significantly enhanced by a large fraction of diffuse photosynthetically active radiation (PAR) (>0.6), and the CO2 uptake during the midgrowing season reaches a maximum under overcast conditions of optically thick clouds, though the total solar radiation itself decreases significantly
Summary
Solar radiation is a major factor that influences the CO2 exchange in the biosphere. Several researchers have suggested that the diffuse radiation proportion in the solar radiation can result in higher light use efficiency than direct radiation (Goudriaan, 1977; Gu et al, 2002, 2003; Roderick et al, 2001). In forests, higher light use efficiencies and carbon uptake have been demonstrated by field observations for cloudy and high aerosols conditions (Price and Black, 1990; Hollinger et al, 1994; Fitzjarrald et al, 1995; Gu et al, 1999; Freedman et al, 1998, 2001; Niyogi et al, 2004). Under cloudy conditions and with aerosol loading, there is broad consensus on the impact of diffuse radiation on terrestrial ecosystem carbon cycle (Fitzjarrald et al, 1995; Gu et al, 1999; Roderick et al, 2001; Niyogi et al, 2004; Oliveira et al, 2007), especially for the North American forests. Changes in vapor pressure deficit (VPD), air and soil temperature
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