Size Distribution Characteristics of Elemental Carbon Emitted From Chinese Vehicles: Results and Atmospheric Implications

With the fast increase in vehicles on roads in China, element carbon (EC) becomes a more and more abundant component in urban ambient aerosols; however, size distribution characteristics of EC emissions from Chinese vehicles are unknown. In this study, we collected six sets of size segregated aerosol samples using a 10-stage MOUDI sampler (0.056-18 um) in Zhujiang tunnel, a roadway tunnel in the urban area of Guangzhou, China. The samples were analyzed for EC, organic carbon (OC), and inorganic ions (i.e., SO42-, NO3-, Cl-, NH4+, Na+, K+, Ca2+, and Mg2+). EC and OC contributed >90% of the total determined mass (PM18 >200 ug/m3 in the tunnel), consistent with the chemical characters of particles from vehicle tailpipe emissions. The OC/EC ratio was 0.67 in fine particles, indicating a dominant contribution from diesel vehicles. Both the EC and OC size distributions showed a dominant accumulation mode with a mass median aerodynamic diameter (MMAD) of 0.44 um. A positive matrix factorization analysis was used to evaluate the contribution of the background atmosphere. It was found that the background atmosphere influenced little the size distribution character of EC and OC in the fine mode. Therefore, the EC and OC size distribution characteristics in the fine mode as measured in the tunnel represented those from vehicle emissions. Studies available in the literature reported a typical accumulation mode of EC peaking at 0.1 um in vehicular emissions in North America. In comparison, EC particles from Chinese vehicles were apparently much larger. Theoretical analysis showed that these large EC particles could not result from after-emission growth processes, such as water accretion, coagulation, or vapor condensation. It is possible that high engine loads and low combustion efficiencies of diesel engines of Chinese vehicles could be the inherent reasons for producing these large agglomerates of EC. While fresh 0.1 um EC particles with little absorbed soluble materials are unlikely to act as cloud condensation nuclei (CCN), calculations showed that 0.44 um EC particles were effective CCN at atmospherically relevant critical supersaturation values of <1%. As a result, fresh EC particles from Chinese vehicle emissions could easily undergo cloud processing and form internal mixtures with sulfate in the residue droplet mode particles. This is consistent with observations that EC frequently showed a dominant droplet mode in urban atmospheres in this region. The internal mixing of EC with highly hygroscopic sulfate would facilitate its removal by wet deposition and shorten its lifetime in the atmosphere. In addition, the light adsorbing capabilities of EC particles could also be significantly enhanced due to their internal mixing with sulfate. Numerical aerosol models needs to take into consideration of these factors to better predict the corresponding behaviors and effects of Chinese urban aerosols.