Impacts Of Aerosol Direct Radiation On Terrestrial Gross Primary Product In China

In recent years,accelerated industrialization and urbanization in China have caused a significant increase in aerosol content.Aerosols not only affect regional climate,the environment and human health,but also affect the carbon cycle process of the ecosystem.The increase of aerosol particles reduces the total solar radiation and increases the proportion of scattered radiation,thus affecting the photosynthesis of vegetation.At present,the aerosol data used in the assessment of the effects of aerosol radiation on terrestrial ecological carbon cycle are mainly model simulation data,and there is a lack of long-term analysis of the effects of aerosol in China.Therefore,the effects of aerosols on carbon cycling in long-term terrestrial ecosystems in China still need to be further explored.In view of the large errors in the aerosol data simulated by the current model,this paper uses the high-precision MCD19A2 aerosol observation data,inputs other atmospheric and surface data,and combines the libradtran radiation transfer model and the Artificial Neural Net（ANN）method to simulate photosynthetically active radiation（PAR）products with 1km resolution in China.The retrieved PAR is validated by the observation data of 39 CERN sites,the R2,RMSE and Bias are 0.56,31.64 W/m2 and-18.36 W/m2,respectively.For the lack of long-term time series impact analysis of aerosol,we input the PAR results simulated in the previous step into revised Eddy Covariance-Light Use Efficiency（EC-LUE）model to obtain the Gross Primary Productivity（GPP）of terrestrial ecosystems in China from 2000 to 2018.The simulated GPP is verified by the observation data of 9 FLUXNET flux sites,the R2,RMSE,regression coefficient are 0.537,2.716 gC · m^-2d^-1,and 0.978,respectively.In this paper,aerosol optical thickness is not equal to/equal to 0 into the model,can be simulated with/without aerosol PAR and GPP results.The effect of aerosols is obtained by making a difference between GPP products with and without aerosols.It can be found that in the absence of clouds,aerosols mainly promote GPP,with the highest promotion degree near Hubei and Shaanxi as well as the eastern part of Northeast,which is close to 1 gC·m^-2d^-1（accounting for more than 18%）.In spring,summer and autumn,the effects of aerosol on GPP are obvious in southern China,while in summer,the effects of aerosol on GPP are most obvious in southern Shaanxi Province and around Sichuan Basin,with the maximum value approaching 2 gC·m^-2d^-1（40%）.There is slight suppression in Hunan and Jiangxi in winter.The order of GPP promotion degree of aerosol to different vegetation is in the descending degree as follows:evergreen broad-leaved forest,deciduous broad-leaved forest,evergreen needle forest,mixed forest,savanna,cropland,wetland,shrubland and grassland.Except for deciduous broad-leaved forest and grassland,the change trends of other vegetation types over time pass the significance test of p<0.1.Except for shrubs,all other vegetation types show an upward trend over time,with the highest promotion degree in June and July,and the lowest promotion degree near January.In the real case,the inhibitory effect of aerosol on GPP is concentrated in central and southeastern China,with a range of 0.5 gC·m-2 d-1,accounting for 8%of the total GPP.In the eastern part of Northeast China and the southern part of Yunnan Province,there is a slight promoting effect(0.1 gC · m^-2d^-1).The inhibition of aerosol on GPP in spring,autumn and winter is concentrated in the southeast.In summer,the inhibition is concentrated in North China and has a slight promoting effect in the eastern part of Northeast China.The effect of aerosol on GPP is not significant(lower than 0.3 gC · m^-2d^-1).Evergreen broad-leaved forest and savanna have the greatest inhibition degree,followed by evergreen needle forest,mixed forest,wetland and cropland.Grassland and shrubland have no obvious effect,while deciduous broad-leaved forest has a promoting effect.The inhibiting effect of aerosols on GPP in evergreen broad-leaved forest,evergreen needle forest,mixed forest and wetland passes the significance test（p<0.1）,and the inhibiting effect is more obvious with time.The aerosol effect on GPP of evergreen broad-leaved forest and deciduous broad-leaved forest show a slight promotion trend in July,August and September.