Direct Radiative Effect of Aerosols on Net Ecosystem Carbon Exchange in the Pearl River Delta Region

Abstract

The environmental impact of aerosols is currently a hot issue that has received worldwide attention. Lacking simultaneous observations of aerosols and carbon flux, the understanding of the aerosol radiative effect of urban agglomeration on the net ecosystem carbon exchange (NEE) is restricted. In 2009-2010, an observation of the aerosol optical property and CO₂ flux was carried out at the Dongguan Meteorological Bureau Station (DMBS) using a sun photometer and eddy covariance systems. The different components of photosynthetically active radiation (PAR), including global PAR (GPAR), direct PAR (DPAR), and scattered PAR (FPAR), were calculated using the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model. The effects of PAR on the NEE between land-atmosphere systems were investigated. The results demonstrated that during the study period the aerosol optical depth (AOD) reduced the DPAR by 519.28±232.89 μmol photons · m⁻² s⁻¹, but increased the FPAR by 324.93±169.85μmol photons · m⁻² s⁻¹, ultimately leading to 194.34±92.62 μmol photons·m⁻² s⁻¹ decrease in the GPAR. All the PARs (including GPAR, DPAR, and FPAR) resulted in increases in the NEE (improved carbon absorption), but the FPAR has the strongest effect with the light use efficiency (LUE) being 1.12 times the values for the DPAR. The absorption of DPAR by the vegetation exhibited photo-inhibition in the radiation intensity > 600 photons · m⁻² s⁻¹; in contrast, the absorptions of FPAR did not exhibit apparent photo-inhibition. Compared with the FPAR caused by aerosols, the DPAR was not the primary factor affecting the NEE. On the contrary, the increase in AOD significantly increased the FPAR, enhancing the LUE of vegetation ecosystems and finally promoting the photosynthetic CO₂ absorption.