Direct observations of the effects of aerosol loading on net ecosystem CO₂ exchanges over different landscapes

Abstract

[1] We present the first direct, multisite observations in support of the hypothesis that atmospheric aerosols affect the regional terrestrial carbon cycle. The daytime growing season (summer) CO 2 flux observations from six sites (forest, grasslands, and croplands) with collocated aerosol and surface radiation measurements were analyzed for high and low diffuse radiation; effect of cloud cover; and effect of high and low aerosol optical depths (AOD). Results indicate that, aerosols exert a significant impact on net CO 2 exchange, and their effect may be even more significant than that due to clouds. The response appears to be a general feature irrespective of the landscape and photosynthetic pathway. The CO 2 sink increased with aerosol loading for forest and crop lands, and decreased for grassland. The cause for the difference in response between vegetation types is hypothesized to be canopy architecture.

The effects of clouds and aerosols on net ecosystem CO<sub>2</sub> exchange over semi-arid Loess Plateau of Northwest China

“…Our different results for a semi-arid grassland ecosystem show the importance of canopy structure of an ecosystem and its photosynthetic interaction with the radiation field. Niyogi et al (2004) did analyze the effects of aerosol loading on NEE over different ecosystem landscapes, and the results for the grasslands also show a negative response to the aerosols, which is consistent with our results.…”

“…Previous studies on the effect of aerosol loading on CO 2 uptake have suggested that aerosols exert a positive function on the terrestrial carbon cycle, at least for forest and cropland ecosystems (e.g., Gu at al., 2003;Niyogi et al, 2004). Our different results for a semi-arid grassland ecosystem show the importance of canopy structure of an ecosystem and its photosynthetic interaction with the radiation field.…”

“…The measurement errors may be caused by the instrument surface heat exchange affecting the open-path CO 2 flux measurements (Burba et al, 2008). In summer and autumn, the seasonally averaged carbon flux exceeds −5.0 µmol m −2 s −1 (in July and August, the monthly averaged NEE can reach around −6.5 µmol m −2 s −1 , not shown), but the amount of carbon uptake is still much less than those observed in forests and croplands (Niyogi et al, 2004). It is also less than that observed in the semi-arid area in Tongyu (44 • 25 N, 122 • 52 E) and in the typical steppe prairie in Inner Mongolia (44 • 08 31 N, 116 • 18 45 E) both covered by grassland and can reach −18 µmol m −2 s −1 and −14 µmol m −2 s −1 (Gao et al, 2009) in summer, respectively.…”

“…Additionally, the diffuse fraction of the solar radiance incident at the earth's surface has increased substantially in many regions as a consequence of increases in both cloudiness and the concentration of aerosols in the atmosphere (Suraqui et al, 1974;Abakumova et al, 1996). 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, 1998Freedman et al, , 2001Niyogi 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. However, the relationship between clouds and ecosystem CO 2 exchange can be more complicated, due to changes in other environmental factors that could influence the ecosystem carbon cycle.…”