Variation Characteristics Of Ultraviolet/ Photosynthetically Active Radiation And The Aerosol Radiative Forcing

Solar radiation is the main energy source of the earth and atmosphere system and is the driving force of the atmosphere,hydrosphere,biosphere,lithosphere and cryosphere motion.All weather phenomenons and climates are formed attributing to the uneven surface solar radiation.Ultraviolet(UV)radiation occurs at wavelengths from 10 nm to 400 nm and it has a significant influence on human health,ecosystems,photochemical reaction and climate change.Photosynthetically active radiation(PAR)is a portion of solar radiation in the wavelength spectrums of 400~700 nm and can be utilized by plants through photosynthesis.It affects the plant productivity,the substance and energy exchange between surface and atmosphere and climate.At present,long term continuous observations of UV radiation and PAR with large scale regions are very scarce and the datasets of UV radiation and PAR with high spatial resolution and long time scales are urgent needed.Efficient models for estimating UV radiation and PAR under various sky conditions were developed based on the limited observations during 2005-2014 from the Chinese Ecosystem Research Network(CERN).These estimation models were then combined with the hybrid model to obtain the historical dataset of daily UV radiation and PAR with high special resolution and long time scales.Based on the aerosol optical parameters obtained from AERONET at Beijing station,SBDART radiative transfer model was used to simulate the composite aerosol and black carbon radiative forcing in recent ten years.The main contents and results are as follows:(1)The spatial distribution and temporal variation of UV radiation were discussed based on the observed radiation at stations belonging to CERN.Multi-annual mean UV radiation and PAR are 0.55 MJ·m-2·d-1 and 23.70 mol·m-2·d-1 respectively with the highest values occurred at Lhasa station.The UV radiation and PAR are 20.46 MJ·m-2·d-1 and 36.314 mol·m-2·d-1 respectively at Lhasa station.TP region has the highest radiation and southwest region has the lowest values,the radiation decreased from south to north at the west of 100°E and the radiation increased from south to north at the east of 100°E.Variation trends of solar radiation,UV radiation and PAR in most climate regions are negative.For the entire country,the decreaseing trends of solar radiation,UV radiation and PAR are-0.051 MJ·m-2·d-1·yr-1,-0.740 KJ·m-2·d-1·yr-1 and-0.363 mol·m-2·d-1·yr-1 respectively.The distribution of the ratio of UV radiation to solar radiation is consistent with the water vapor content and is opposite with clearness index.The annual variation characteristics of the ratio of UV radiation to solar radiation and the ratio of PAR to solar radiation are opposite with the clearness index and consistent with water vapor content.(2)China was divided into 8 climate zones and a typical station was chosen to establish the UV and PAR estimation formula at each climate region.The availability of the model for other sites in the same climate region was tested.The average values of R2,MABE and RMSE for UV radiation are 0.95,10.89% and 14.31% respectively and the maximum MABE and RMSE vales for PAR are less than 15% and 17% respectively.Daily accumulated UV radiation and PAR at the 724 CMA routine weather stations from 1961 to 2014 were reconstructed based on the hybrid model and UV/PAR estimation models.Generally,the north of China has more radiation than the south of China,while the east of China has less radiation than the west of China.Multi-annual mean daily UV radiation and PAR values are 0.49 MJ·m-2·d-1 and 22.29 mol·m-2·d-1.UV radiation and PAR at all climate regions decreased from 1961 to 2014 except for Qinghai-Tibetan Plateau,which increased with an amplitude of 0.184 KJ·m-2·d-1·yr-1.UV radiation in the Southeast China,North China and Eastern China decreased significantly with linear trends of-0.630 KJ·m-2·d-1·yr-1,-0.583 KJ·m-2·d-1·yr-1 and-0.588 KJ·m-2·d-1·yr-1 and decreasing trend for the entire China is-0.275 KJ·m-2·d-1·yr-1.PAR in the Qinghai-Tibetan Plateau showed an increasing trend from 1961 to 2014 with an amplitude of 0.08 mol·m-2·d-1·yr-1 while PAR over the entire China decreased at a rate of-0.015mol·m-2·d-1·yr-1.PAR in the Southwest China,Eastern China and North China plain decreased significantly with linear trends of-0.033 mol·m-2·d-1·yr-1,-0.031 mol·m-2·d-1·yr-1 and-0.029 mol·m-2·d-1·yr-1.The decreasing trends of UV radiation and PAR for a wide range of China are mainly due to the increase of aerosol concentration.The in creasing trend of UV radiation and PAR in Qinghai-Tibetan Plateau is due to the decrease of the aerosol concentration,cloud and water vapor content.Clouds,AOD,water vapor content and ozone attenuated approximately 18.13%,7.59%,6.20% and 1.12%,respectively,of the UV radiation that would reach the Earth's surface without the attenuation of the four factors.The corresponding attenuations of PAR are 21.59%,8.19%,6.72% and 1.21% of the PAR that would reach the Earth's surface withouthe four factors attenuations.(3)Based on the aerosol optical parameters obtained from AERONET Beijing station,the radiation forcing caused by the composite aerosol and BC during 2005~2014 was simulated using SBDART radiative transfer model.In summer,AOD and ?ngstr?m exponent were higher than those in other seasons.Because of the large number of dust events,?ngstr?m exponent value was small in spring.The aerosol volume concentration has two peaks with the highest value of fine mode in summer and the the highest values of coarse mode in spring.The asymmetry factor was high in summer and low in winter at 440 nm and 675 nm wavelengths,and the high value of asymmetry factor was also found in spring at 870 nm and 1020 nm wavelength.The real part of complex refractive index was high in summer and low in winter.Radiative forcing of composite aerosol at the top of the atmosphere,at the earth surface and in the atmosphere was-25.21±23.18 W·m-2,-66.29±49.88 W·m-2 and 41.08±34.11 W·m-2,and the radiative forcing of BC at the top of the atmosphere,at the earth surface and in the atmosphere was 4.39±4.72 W·m-2,-15.47±15.23 W·m-2 and 19.86±19.69 W·m-2.The ratio of BC radiative forcing to composite aerosol radiative forcing at the top of the atmosphere,at the earth surface and in the atmosphere was 17.30%±20.44%,23.93%±13.38% and 54.89%±24.68 respectively.