Studies on the impact of carbonaceous aerosols on atmospheric environment in large-cities in China

Research Project , Research Report 2008

Yutaka Kondo Professor , Research Center for Advanced Science and Technology,The University of Tokyo
Nobuyuki Takegawa Associate Professor , Research Center for Advanced Science and Technology,The University of Tokyo

Abstract

Abstract Concentrations of elemental carbon (EC), carbon monoxide (CO), and carbon dioxide (CO2) were measured every hour at Peking University (PKU) site near the fourth ring road in Beijing, China, between winter 2005 and summer 2008. EC was measured every hour with a semi-continuous thermal-optical analyzer. Simultaneously, gaseous components, CO and CO2, meteorological data, and other properties were measured. The observed data was found to represent the values in the region within 50 km from the PKU site. The annual mean of EC concentration was 6.9 μgC m-3, which was about 1.5-4 times higher than the level in other Asian urban areas, such as Seoul and Tokyo. The concentrations of the observed species decreased with increasing near-surface wind speed (WS). The concentrations reached low and stable levels at a WS of about 4 m s-1. The concentrations at stronger winds were considered to the background levels, and those in southerly air were much higher than northerly air.Under low-wind speed conditions (WS ? 2.0 m s-1) of each species are interpreted to be more strongly controlled by regional-scale air mass affected by emissions from Beijing. The slopes of the CO-CO2, EC-CO2, and EC-CO correlations in the weak-wind regime (WS ? 2.0 m s-1) are used to estimate major EC and CO sources in Beijing. The median EC, ΔEC/ΔCO2, and ΔEC/ΔCO (except for winter) increased in the late evening and remained high until early morning. These diurnal variations indicate regular sources of EC throughout the year. According to the traffic volume data measured on the North-fourth ring road, the total vehicles were high during daytime, while the numbers of heavy-duty diesel trucks (HDDTs) and their fraction to total vehicles increased at 2200 LT and remained high during nighttime. The nighttime increase in the traffic of HDDTs is due to the traffic regulations, which allow trucks to enter the city only during nighttime (2200-0600 LT). These results indicate dominant contribution of exhaust from diesel vehicles to the nighttime enhancement of EC. In winter, the CO showed higher concentrations compared with those in other seasons. Especially, the nighttime CO and ΔCO/ΔCO2 ratio were about two times higher than those in the other seasons. CO emissions associated with coal or biofuel combustion for domestic heating are not the main cause of the high CO in winter, considering the lack of expected increases in EC in winter above the levels in the other seasons. Instead, the increase in the CO emissions from the exhaust of gasoline vehicles at low temperature is likely the dominant cause. The time required for catalysts to be heated to temperatures high enough for efficient CO removal is elongated at low temperatures, leading to higher CO emissions. The similarity in ΔEC/ΔCO2 between winter and the other seasons indicates no significant additional EC emissions in winter. These results suggest that the vehicular sector is one of the most dominant sources to impact on the diurnal and seasonal variation of CO and EC in Beijing. EC, CO, and CO2 showed no significant decrease on the weekend. In addition, the diurnal variations of EC, CO, CO2, and ΔEC/ΔCO were similar between weekdays and weekends, confirming the absence of the differences in the diurnal variation of emission activity. This finding shows no significant weekend effect in Beijing. A series of aggressive measures was launched by the Chinese government to reduce pollutant emissions from Beijing and surrounding areas during the Olympic Games. Observations showed significant decreases in concentrations of EC and CO during the Olympic season 2008, relative to the summer seasons in 2006 and 2007. The median concentration of EC was 2.22 μgC m-3 in summer 2008; about 2.5 and 3.9 μgC m-3 (53 and 63%) lower than for summer in 2007 and 2006, respectively. The median concentration of CO in summer 2008 was 660 ppbv, which was reduced about 16 and 33% compared with those in summer 2006 and 2007, respectively. Under weak-wind speed (WS ? 1.0 m s-1) conditions, large decrease for EC concentrations during the Olympics would have to be related to the significant reductions of EC emissions in the Beijing urban area. Especially, large reduction of the nighttime EC concentrations and the change in the diurnal variation of ΔEC/ΔCO ratios without the nighttime enhancement in summer 2008 strongly reflected the decrease in the traffic of nighttime heavy-duty diesel trucks by traffic restrictions. These indicate that control measures for Olympics targeted at the vehicle would sector were effective in reducing EC emissions in Beijing. The observed slopes of the CO-CO2-EC correlations for one year, four seasons in 2005-2006, were used to evaluate the emission ratios of these species derived from the emission inventory by Streets et al. [2003] in the Beijing area. The comparison suggests that Streets et al. [2003] largely overestimate the contributions of the emissions from domestic sectors with high CO/CO2 and EC/CO2 emission ratios and/or underestimate those from non-domestic sectors, especially vehicle emissions, in the Beijing study area. These results indicate that the observed slopes are useful parameters in assessing the reliability of emission inventories.

Keywords:

carbonaceous aerosol, elemental carbon, organic aerosol, carbon monoxide, carbon dioxide, Beijing