Observations of The Effects of Aerosol Loading on Carbon and Water Cycles Over Various Landscapes

Abstract

I present multi-site observational evidence that atmosphere aerosols affect regional terrestrial carbon and water cycle. Past studies have indicated that increase in diffuse irradiance due to cloudiness and aerosols could increase net ecosystem exchanges. Though the effect of clouds on terrestrial CO2 and LHF exchanges have been reported, there have been no field scale, direct observations relating aerosol loading and CO2 /LHF fluxes. We present first direct observations in support of the hypothesis that atmospheric aerosols affect the regional terrestrial carbon cycle. Observations from six CO2 flux and latent heat flux monitoring sites (forest, grasslands, and croplands) with collocated aerosol and surface radiation measurements were analyzed. The daytime (10AM to 4PM) growing season (summer, June to August) CO2 flux observations were subject to three clustering: (1) high and low diffuse radiation fraction (DRF) of the global radiation; (2) DRF changes with and without cloud cover; and (3) high DRF, no-cloud cover regimes for high and low aerosol optical depths (AOD). Results suggest that, aerosols exert a significant impact on potentially increasing the CO2 fluxes, and their effect may be even more advantageous than that due to clouds (which reduced the total radiation). For the data analyzed, the response of increasing aerosol loading on landscape CO2 fluxes, appears to be a general feature irrespective of the landscape (forest, crops, or grasslands) and photosynthesis pathway (C3 or C4). The CO2 sink increased with aerosol loading for forest and crop lands, and decreased for grassland. The slope of the AOD - CO2 flux correlation however, was wavelength dependent and appeared to affect the woody trees more than the crops or grasslands. The analysis of the direct field measurements indicate; aerosol loading could play a significant role in the variability of the regional terrestrial carbon exchange by altering the amount of diffuse solar radiation.

As for water cycle, we examined latent heat flux (LHF) using the same time periods and locations. But instead of analyzing DRF and CO2 correlation, we were more focus on (1) heat fluxes and aerosol-loading correlation; (2) whether leaf area would affect the heat fluxes during aerosol loading; (3) soil moisture effect and (4) air temperature effect on the heat fluxes. Results indicate: (1) for corn, soybean croplands and forest sites, aerosol loadings had little impact on latent heat fluxes, but it had more impact in winter wheat and grassland sites; (2) after accounting for the leaf area index (LAI), our analysis results showed that aerosol loading had more significant impact in the heat fluxes for agriculture sites, where there were still no obvious trend for forest sites; (3) for agricultural site, latent heating tend to be affected more under low LAI condition; (4) grassland sites are more sensitive to soil moisture changes than agricultural sites; (5) agricultural sites have more obvious heat flux changes when air temperature changes than grassland site.