Mechanical Analyses And Numerical Simulations On The Aerosol Pollution Over The Pearl River Delta

This study were carried out in five aspects.1) The coupled CMAQ / MM5/SMOKE air quality modeling system was successfully transplanted and localized over southern China and thus provides an advanced tool to make studies on the formation, transport, dispersion, transformation and removal of the atmospheric pollutants; 2) Full estimation of various kinds of emissions was made over southern china to form the gridded emissions data for air quality modeling;3) A statistical analysis was conducted on the relation between tropical cyclone (TC) and the haze in the Pearl River Delta (PRD); 4) A numerical simulation for an extreme haze event in PRD related to severe tropical storm MELOR was conducted. Results from the simulation revealed the impact and the influencing mechanism of the tropical cyclone on the haze; 5)Process analyses were performed for all terms in the dispersion equation of air pollutants to indicate the contribution of each process to the concentrations of several aerosol components.Major results of this study are summarized below.1. The CMAQ/MM5/SMOKE air quality modeling system was a combination of three complicated numerical systems. It comprises emission processing model, mesoscale meteorological model and chemical transport model. These systems are mutually dependent and interactive. The processes for transplanting and running the systems are rather complicated. It was the first time to localize successfully in the PRD this air quality modeling system which was then applied to make successful study and prediction of the haze.2. It was the first time to utilize SMOKE model to estimate the emissions of point source, area source, biogenic source and mobile source over the southern China. The high resolution emissions data with full consideration of various emissions for use in air quality modeling in the PRD were obtained for the first time. This provides an important foundation for air quality prediction.3. Statistical analyses revealed that tropical cyclone could produce haze in the PRD. But the number of haze days related to TC was quite small. Most TCs that could bring haze in the PRD were those appeared in summer and autumn. These TCs frequently made turning over the sea east of Philippines. TCs appeared in the Bashi Strait and in the northeast of South China Sea were conducive to producing high PM10 in the PRD.4. Simulations for severe tropical storm MELOR indicated that TC causes haze in the PRD by both thermal and dynamic processes. During the activity of MELOR, lower troposphere in the PRD was controlled by high pressure ridge with obvious flow divergence. While in the upper troposphere, the convergence of air flow was also significant. Thus the strong systematic subsidence dominated the PRD. As MELOR approaching, the ascending motion at the center of the TC forced a compensating downward motion that intensified the downdraft in the PRD. The downdraft in the PRD directly lowered the height of planetary boundary layer (PBL), causing flow stagnation and further confining the diffusion and transport of aerosols. The downdraft also generated significant adiabatic heating by a warming of 2-4℃daily and drying in the lower troposphere, intensifying greatly the lower layer static stability. This stable air layer acted like a warm and dry cover that mantled the boundary layer in the PRD, causing accumulation of aerosol particles and thus the formation of high PM10 concentrations at the surface. The TC caused clear sky in the PRD, producing no rain to clear away the aerosols. On the other hand, the daytime photochemical processes were also in favor of the generation of photochemical smog and the formation of secondary aerosols. During the TC, nocturnal inversion in the surface layer was enhanced through radioactive cooling along with the subsidence warming in the upper air, causing aerosol concentrations to increase at night.5. Process analyses of concentrations of aerosols indicated that the concentrations of secondary aerosols varied similar to PM10, implying the important effect of severe tropical storm MELOR on the formation of secondary aerosols. The concentrations of secondary organic aerosols, nitrates and ammonium varied consistently with surface humidity. This may indicates that water vapor may play an important role in the gas-particle conversion. Research also revealed that different components of aerosols might experience different processes for their concentrations formation, but in general, the vertical diffusion, the horizontal transport and gas-particle conversion are the major processes. This implies that atmospheric stratification, largescale motion and aerosol formation are the important factors for the variation of aerosols concentrations.