Characteristics Of Atmospheric Carbon Dioxide In Urban Area And The Influence Of The Regionally Inhomogeneously Distribution On Climate

Carbon dioxide(CO2)is an important trace gas of the atmosphere.It has a strong "greenhouse effect" after water vapor.The emission of CO2 injected into the atmosphere by human beings has increased rapidly,which leads to the quick increasing of the CO2 concentration in the atmosphere since the industrial revolution.The atmospheric CO2 influenced the global climate system through changing the radiation.China is a big source of carbon emission.Besides the urban area is where the anthropogenic activities frequently occurs.Thus it's of great significance to study on the variations,the sources,and related climate effects of atmospheric CO2 in this area.This study is focused on CO2,with the in situ observation and numerical simulation as the main methodologies.The variations of the atmospheric CO2 in the urban area was revealed by the observational data,as well as the influence of the sources and sinks of it.The spatial and temporal distribution of CO2 concentration over the metropolitan area was simulated by the numerical models,to quantify the influence of different emissions,and the characteristics of the transportation of the atmospheric CO2 in the regional scale.The radiation forcing and climate effects of dynamic inhomogeneously distributed CO2 over the metropolitan area were explored by improving the radiation scheme.Firstly,the temporal variations of atmospheric CO2 and its affecting factors in the urban area were analyzed with the observational data obtained in urban Nanjing over 2011.Results show that the annual average mixing ratio of CO2 was 406.5±20.0 ppmv over the study period.The CO2 records had different cycles including seasonal,atmospheric oscillational,quasi-biweekly,weekly and daily cycles as a result of multiple controlling factors.The seasonal and intra-seasonal fluctuations of CO2 were mainly caused by the terrestrial biospheric uptake and emission and atmospheric oscillation.The weekly variation of CO2 was largely influenced by traffic volume.The diurnal cycle of CO2 is a comprehensive result of meteorological,anthropogenic and biological influence.The seasonal mean CO2/CO correlation slope varied from 0.024 ppmv/ppbv to 0.029 ppmv/ppbv,comparable to the fossil fuel combustion emission ratio.The diurnal pattern of CO2/CO was irregular,indicating random anthropogenic emissions in urban area.The backward trajectories by HYSPLIT model showed that the local anthropogenic emissions contributed the most to the high CO2 mixing ratio in the urban areaSecondly,the temporal and spatial distribution of the sources and sinks of the atmospheric CO2 over the urban area was explored by the observational atmospheric CO2 fluxes data obtained at the urban observatory.Results show that the annual typical values of the CO2 flux are 5?10 ?mol m-2 s-1,and more than 95%of the measurements range from-12.5 ?mol m-2 s-1 to 47.5 ?mol m-2 s-1,little lower than those observed in urban Beijing and Shenyang.The seasonal cycle of CO2 flux is significant with low values in summer,largely due to the vegetation.The diurnal course of CO2 flux presented a bimodal pattern with the later peak a little higher than the earlier peak.Around 12 p.m.to 2 p.m.during the daytime,the CO2 flux in winter is the highest among the four seasons,and about 4 ?mol m-2 s-1 higher than that in summer.The monthly mean CO2 flux has a negative relationship with air temperature.CO2 flux varies with wind direction with high fluxes from the South and Northeast.The annual mean values of CO2 flux are positive,indicating that the urban area is a net source.Thirdly,the regional climate model RegCM4 was developed to be capable of simulating atmospheric CO2,and then applied to simulate the distribution of the atmospheric CO2 under the conditions of different emission sources,in order to obtain the quantified influence of the emission sources on the atmospheric CO2 over the metropolitan area and the characteristics of the regional transport of it.Results show that,the simulation of the developed model agrees well with the observation with the deviation within 3 ppmv.The spatial distribution of CO2 shows that CO2 is high in the urban area while low in the background region,and unevenly distributed near surface.The 5 main emissions in this region are from public electricity and heat production,manufacturing industries and construction,production of minerals,residential and other sectors,and transportation.In most areas of the YRD,the increase of the atmospheric CO2 mixing ratio is about 2?8 ppmv in one year caused by the anthropogenic emissions.The contributions to the atmospheric CO2 mixing ratio from the 5 main sources are 44%,25%,4%,9%,and 10%,respectively.The main processes influencing the atmospheric CO2 in the urban area in the Yangtze River Delta region are the emission progress,the horizontal advection progress,and the vertical advection progress.Fourthly,the radiation scheme was improved by using the dynamic inhomogeneously distributed CO2 concentration field with high temporal and spatial resolution instead of the assumed stable and homogenous distributed CO2 of a fixed value in the existed radiation scheme.The characteristics of radiation and the climate effects in the metropolitan area were analyzed with simulations of RegCM4 using the modified scheme.Results show the negative changes of the upward longwave radiation at surface under the condition of clear sky and total sky with-0.17 W m-2 and-0.12 W m-2 respectively.The change of the atmosphere CO2 field influences the temporal and spatial distribution of the clouds.Thus leads to the change of the longwave radiative forcing near surface with the largest change of around 10 W m-2.Especially in summer in Northeast China,the total cloud fraction and near surface longwave radiative forcing increase,with the largest change of near surface temperature exceeding 1 ?.While the situation in North China and South China are the opposite.In the northern Yangtze River Delta(YRD)region,the total precipitation and the convective precipitation increase while those in the southern YRD are the opposite.Generally,the atmospheric CO2 in the typical urban area of the YRD represented by Nanjing is largely influenced by the anthropogenic activities.The atmospheric CO2 mixing ratios in Nanjing are about 15 ppmv higher than the global background levels.The anthropogenic activities have a significant influence on the short-term cycles including weekly and diurnal variations.The high CO2 mixing ratio is contributed mainly by the local emission from Nanjing and the adjecent areas.The net CO2 flux in the urban area is always positive during four seasons with low values in summer and autumn and high values in winter and spring.The observation and simulation all indicate that the urban area in the YRD is a net source of CO2.The inhomogeneously distribution of CO2 has great impact on radiation and climate.The change of CO2 caused by anthropogenic activities influences the distribution of longwave radiative forcing and cloud fraction,thus influence air temperature and precipitation.