Influence of different weather events on concentrations of particulate matter with different sizes in Lanzhou,China

The formation and development of weather events has a great impact on the diffusion, accumulation and transport of air pollutants, and causes great changes in the particulate pollution level. It is very important to study their influence on particulate pollution. Lanzhou is one of the most particulate-polluted cities in China and even in the world. Particulate matter （PM） including TSP, PM〉10, PMzs-10, PM2.5 and PM1.0 concentrations were simultaneously measured during 2005-2007 in Lanzhou to evaluate the influence of three kinds of weather events - dost, precipitation and cold front - on the concentrations of PM with different sizes and detect the temporal evolution. The main results are as follows： （1） the PM pollution in Lanzhou during dust events was very heavy and the rate of increase in the concentration of PM2.5-10 was the highest of the five kinds of particles. During dust-storm events, the highest peaks of the concentrations of fine particles （PM2.5 and PM1.0） occurred 3 hr later than those of coarse particles （PM〉10 and PM/.5-10）. （2） The major effect of precipitation events on PM is wet scavenging. The scavenging rates of particles were closely associated with the kinds of precipitation events. The scavenging rates of TSP, PM〉10 and PMa.5-10 by convective precipitation were several times as high as those caused by frontal precipitation for the same precipitation amount, the reason being the different formation mechanism and precipitation characteristics of the two kinds of precipitation. Moreover, there exists a limiting value for the scavenging rates of particles by precipitation. （3） The major effect of cold-front events on particles is clearance. However, during cold-front passages, the PM concentrations could sometimes rise first and decrease afterwards, which is the critical difference in the influence of cold fronts on the concentrations of particulate pollutants vs. gaseous pollutants.

Analysis of the Structure of Different Tibetan Plateau Vortex Types

Knowledge of the structure of the Tibetan Plateau vortex （TPV） is of considerable importance for understanding the generation and development mechanisms of this mesoscale system. However, our understanding of vortex struc- tures and our ability to classify them on a physical basis is limited due to insufficient observations. The high- resolution new-generation NCEP-CFSR （Climate Forecast System Reanalysis） dataset is used in the present paper to investigate the general structural features of various types of mature TPV through classification and composite struc- ture analysis. Results indicate that the dynamic and thermodynamic structures show regional and seasonal depend- ency, as well as being influenced by attributes of translation, associated precipitation, and the South Asian high （SAH）. The common precipitating TPV （type I）, frequently occurring in the west-east-oriented zonal region between 33° and 36°N, is a notably low-level baroclinic and asymmetric system. It resides within a large-scale confluent zone and preferentially travels eastward, potentially moving out of the plateau. The heavy rain vortex （type II） corresponds to a deep vortex circulation occurring in midsummer. The low-level baroclinic sub-category （type IIa） is associated with a low-level jet and mainly originates in the area 32°-35°N, 86°-94°E, preferentially moving east of 90°E and even away from the plateau; meanwhile, the nearly upright sub-category （type IIb）, which has a cold center at low levels and a warm center at mid-upper levels, is a quasi-stationary and quasi-symmetric system favorably occurring west of 92°E. A western-pattern SAH exists in the upper troposphere for these two sub-categories. The springtime dry vortex in the western plateau （type III） is warm and shallow （approximately 100 hPa deep）, and zonal circulation dominates the large-scale environmental flows in the middle and upper troposphere. The precipitating vortex in the southern plateau occurring during July-August （type IV） is not affected by northerly flow at low levels. It is vertically aligned and controlled by a banded SAH.