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

Research Project , Research Report 2007

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

The measurements of EC in fine particulate matter were made at hourly intervals from 25 November 2005 through 31 October 2006 at Peking University in Beijing, China using a Sunset Laboratory’s semi-continuous carbon analyzer. CO and CO2 were also measured simultaneously. EC concentrations generally decreased with increase in the wind speeds, indicating the importance of dilution by vertical mixing and horizontal transport in controlling their near surface concentrations. For this reason, we focused the data in the condition of weak wind speed (2 m s-1). EC and CO were well correlated throughout the measurement period due to a similarity in sources. CO and CO2 were also well correlated in autumn and winter, indicating that both CO and CO2 are good tracers of EC. CO and CO2 with EC were introduced as a useful parameter to detect those emissions, because those ratios are not affected by dilution in the atmosphere.
The average EC concentrations for each season ranged between about 6 and 8 μg m-3. CO showed minimum values of about 0.8 ppmv in spring-summer and maximum of about 1.6 ppmv in winter. Slightly higher EC concentrations were identified in fall and winter, while severely advanced CO in winter occurred by domestic combustion for heating. CO2 in summer showed the lowest level (423 ppmv) related to the vegetative activities. Investigation of diurnal cycles in EC measurements over the entire study period revealed that EC concentrations showed a consistent diurnal behavior, characterized by not a typical pattern of rush hour peaks occurring in the morning or evening but a noticeable pattern of nocturnal advanced EC. CO showed a similar diurnal pattern with EC. In the case of CO2, the concentrations decrease during the daytime, and show the lowest level in summer because of taken up by photosynthesis.
The ΔEC/ΔCO ratios showed a similar diurnal pattern with EC, increasing at night and peaking the maximum values during the nighttime after the midnight, because of increasing an inflow of diesel trucking into the city after 2200 LT under the traffic policy in Beijing. This is supported by strong dependency of the nocturnal ΔEC/ΔCO ratios on the ambient temperatures. This suggests that diesel exhaust is the dominant local source of EC emissions. The diurnal pattern of ΔEC/ΔCO2 ratios also similar cycles with higher nocturnal values in summer and fall related to the increase of the EC emissions. During the winter periods, however, the diurnal pattern of ΔEC/ΔCO ratio was different from that in other seasons, because large amounts of coal in winter were consumed in Beijing due to residential heating occurring extremely high CO concentrations. An interesting finding was that no weekend effect indicates for all measured species in Beijing. In addition, the diurnal patterns of the ΔEC/ΔCO ratios did not change between weekdays and weekends.
Our observed surface concentrations of EC, CO, and CO2 were compared with the emission inventory data proposed by Streets et al., [2003] to estimate the emission rates using their correlation relationships. The results of this comparison reveal the possibility of overestimation in EC and CO emission rates estimated by Streets et al., [2003] around Beijing region.

Keywords:

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