Aerosol optical depth thresholds as a tool to assess diffuse radiation fertilization of the land carbon uptake in China

Xu Yue,Nadine Unger

doi:10.5194/acp-17-1329-2017

Abstract

Abstract. China suffers from frequent haze pollution episodes that alter the surface solar radiation and influence regional carbon uptake by the land biosphere. Here, we apply combined vegetation and radiation modeling and multiple observational datasets to assess the radiative effects of aerosol pollution in China on the regional land carbon uptake for the 2009–2011 period. First, we assess the inherent sensitivity of China's land biosphere to aerosol pollution by defining and calculating two thresholds of aerosol optical depth (AOD) at 550 nm, (i) AODt1, resulting in the maximum net primary productivity (NPP), and (ii) AODt2, such that if local AOD < AODt2, the aerosol diffuse fertilization effect (DFE) always promotes local NPP compared with aerosol-free conditions. Then, we apply the thresholds, satellite data, and interactive vegetation modeling to estimate current impacts of aerosol pollution on land ecosystems. In the northeast, observed AOD is 55 % lower than AODt1, indicating a strong aerosol DFE on local NPP. In the southeastern coastal regions, observed AOD is close to AODt1, suggesting that regional NPP is promoted by the current level of aerosol loading, but that further increases in AOD in this region will weaken the fertilization effects. The North China Plain experiences limited enhancement of NPP by aerosols because observed AOD is 77 % higher than AODt1 but 14 % lower than AODt2. Aerosols always inhibit regional NPP in the southwest because of the persistent high cloud coverage that already substantially reduces the total light availability there. Under clear-sky conditions, simulated NPP shows widespread increases of 20–60 % (35.0 ± 0.9 % on average) by aerosols. Under all-sky conditions, aerosol pollution has spatially contrasting opposite sign effects on NPP from −3 % to +6 % (1.6 ± 0.5 % on average), depending on the local AOD relative to the regional thresholds. Stringent aerosol pollution reductions motivated by public health concerns, especially in the North China Plain and the southwest, will help protect land ecosystem functioning in China and mitigate long-term global warming.

Highlights

Atmospheric aerosols scatter and absorb solar radiation, while plants rely on sunlight for photosynthesis
High gross primary productivity (GPP) and net primary productivity (NPP) are simulated in the northeast, the southwest, and the southeastern coastal regions, where deciduous broadleaf forest (DBF) and evergreen needleleaf forest (ENF) trees dominate (Fig. S1 in the Supplement)
We examine Plant functional types (PFTs)-specific GPP responses to diffuse fraction (DF) for clear and all-sky conditions (Fig. 4)

Summary

Introduction

Atmospheric aerosols scatter and absorb solar radiation, while plants rely on sunlight for photosynthesis. Observations have demonstrated that aerosols can enhance canopy photosynthesis and light-use efficiency (LUE = GPP/PAR; GPP is gross primary productivity and PAR is photosynthetically active radiation) by increasing diffuse radiation in the lower canopy (Gu et al, 2003; Rap et al, 2015; Strada et al, 2015). This aerosol diffuse fertilization effect (DFE) is subject to the aerosol loading and sky conditions (Cohan et al, 2002; Oliphant et al, 2011) because the potential benefit of increased diffuse radiation in the lower canopy can be offset or even reversed by the concomitant reductions in direct sunlight.

Methods

Results

Conclusion