Haze and ozone pollution effects on the land carbon sink in China

Abstract:

Atmospheric pollutants have both beneficial and detrimental effects on carbon assimilation by land ecosystems. Aerosols promote carbon uptake by increasing diffuse radiation, while ozone damages leaf photosynthesis by oxidizing plant cells. As the world’s largest emitter of air pollutants, China experiences frequent haze episodes. In this study, we apply coupled chemistry-carbon-climate simulations using the Yale Interactive Terrestrial Biosphere Model that is embedded in the NASA ModelE2 global chemistry-climate model to quantify the combined effects of ozone and aerosol pollution on land carbon assimilation for the present and future world. The simulated land carbon cycle has been extensively evaluated at 145 FLUXNET sites globally. The aerosol optical depth (AOD) and surface ozone are validated with satellite data and air quality monitoring data from a network of 188 Chinese sites. In the present day, we find that air pollution in China reduces net primary productivity (NPP) by 0.47 Pg C a^-1 (10.8%), resulting from an increase of 0.13 Pg C a^-1 (3.1%) by aerosol diffuse radiation fertilization and a decrease of 0.60 Pg C a^-1 (13.9%) by ozone vegetation damage. Sensitivity simulations indicate that the effects are dominated by anthropogenic emissions. Simulations using natural precursor emissions only show minor changes in NPP. The IPCC RCP8.5 future world predicts an 18% reduction in SO₂ emissions but increases of 17% in NO_x and 15% in volatile organic compound emissions in 2030 relative to 2010. The emissions changes lead to reduced AOD but enhanced surface ozone over eastern China in 2030. For this future projection, we estimate a stronger NPP reduction of 0.62 Pg C a^-1 (12.5%) due to air pollution in 2030. The increased future damage is a consequence of the opposing sign effects of aerosol diffuse radiation fertilization (0.13 Pg C a^-1; 2.6%) and larger ozone inhibition (0.75 Pg C a^-1; 15.1%).