Analysis Of The Characteristics And Influencing Factors Of Carbon Flux Changes In 2011 And 2018

Carbon dioxide(CO₂)emissions produced in urban areas promote the increase of global atmospheric CO₂ concentration and lead to global warming,making cities a key area for global CO₂ reduction.Therefore,it is of great significance to carry out research on the long-term carbon cycle in urban areas and strengthen the understanding of the source and sink dynamics of CO₂ in the complex urban environment.Eddy covariance technology(EC)can measure the turbulent exchange of matter,energy and momentum between the earth's surface and the atmosphere at a certain ecosystem scale.It is an important tool for continuous monitoring of dynamics in CO₂ flux and has been widely used in the world.In this study,EC data of 2011 and 2018 in Shanghai Fengxian University Town were used to analyse the characteristics of meteorological factors,carbon flux characteristics,flux contribution source area distribution,and the impact factors on CO₂ flux,respectively.This study might be useful in understanding the CO₂exchange process within the city,controlling greenhouse gas emissions and providing scientific support to promote green and low-carbon development.The main research conclusions are as the following:(1)The change characteristics of meteorological factors are consistent with the overall characteristics of the subtropical monsoon climate.In 2011 and 2018,the monthly average temperature change curve showed an inverted"V"shape,ranging from 0-30?.The monthly average temperature in 2018 was significantly higher than that in 2011.In 2011 and 2018,the main wind direction was easterly,and the southeast wind was generally prevailing,and the wind speed was mostly in the range of 1 m·s^-1-4 m·s^-1.The average monthly average PAR in 2018 is higher than that in 2011,but the seasonal trend is consistent.Winter is small,spring rises rapidly,summer reaches its highest in July,and then begins to decline.At certain times around noon in summer,PAR can reach 2000?mol m²s^-1 or more.(2)CO₂ flux has obvious seasonal and diurnal variation characteristics.The CO₂flux values in 2011 and 2018 were concentrated in the range of-10?mol m²s^-1 to 10?mol m²s^-1.The CO₂ flux in the study area in 2018 was lower than that in 2011.The seasonal difference of daily average flux is obvious,winter>spring>autumn>summer.In summer,CO₂ fluxes are mostly low negative values,while in winter,the flux values during the day are lower than those at night in all seasons.The lowest value of CO₂ flux in a day occurs during the period of strong vegetation photosynthesis around noon,and the highest value occurs in the morning,evening and night.(3)The distribution of the flux contribution source area is significantly affected by the degree of atmospheric stability and wind direction,and has obvious seasonal and diurnal characteristics.The underlying surface covered by the flux source area is about4 km² around the flux observation tower,mainly on the campuses of Shanghai Normal University and Shanghai Institute of Technology.The flux contribution area in 2011was slightly larger than that in 2018,and the flux contribution source area in both years increased with the atmospheric stability.In addition,the 80%flux contribution source area can include different underlying surface types such as green spaces,teaching buildings,dormitories,canteens,and roads.The area of flux contribution source areas in different seasons is arranged in the following order:autumn>winter>spring>summer in 2011;autumn>summer>winter>spring in 2018.The distribution of flux contribution sources is mainly in the north-south direction.The prevailing wind is in the same direction.Due to the strong turbulent motion during the day,the atmosphere is in an unstable stratification,so the flux contribution source area is smaller than the range under stable atmospheric conditions at night.(4)In 2011 and 2018,the traffic flow of the main roads near the study area was significantly different.The average daily traffic flow in 2018 was significantly higher than that in 2011,increasing from an average of 5,300 vehicles per day to 11,600vehicles;the two-year daily traffic flow trends were bimodal,and the average daily traffic flow on working days in 2011 was significantly greater than that of non-working vehicles.In 2018,the average traffic volume of working days was similar to that of non-working days,and the traffic volume of certain periods of non-working days was higher than that of working days.(5)Vegetation in summer is the dominant factor affecting regional CO₂flux,while in winter it depends on buildings such as canteens and dormitories.Traffic flow is an important factor affecting CO₂ flux.The peak time of traffic flow corresponds to the peak of CO₂ flux.Based on the linear regression analysis of CO₂ flux and traffic flow,the change of CO₂ flux in 2011 has a significant positive correlation with the change of traffic flow.The flux contributed 18%,but there was no significant relationship between the CO₂ flux and traffic flow in 2018.Different from other studies,the area covered by vegetation in this study area is wide.The absorption of CO₂ by vegetation in summer can completely offset regional CO₂ emissions.Vegetation is the dominant factor affecting regional CO₂ flux.Winter vegetation also plays an important role in carbon sinks,which can offset part of CO₂ emissions,but at this time CO₂ emissions from buildings such as canteens and dormitories dominate,making the entire day CO₂ flux mostly positive.