The chemical composition of fine particles and quantitative relationship between the mass concentration and meteorological condition in Beijing

The recent year's monitor results of Beijing indicate that the pollution level of fine particles PM2.5 shows an increasing trend. To understand pollution characteristics of PM2.5 and its relationship with the meteorological conditions in Beijing, a one-year monitoring of PM2.5 mass concentration and correspondent meteorological parameters was performed in Beijing in 2001. The PM2.5 levels in Beijing were very high, the annual average PM2.5 concentration in 2001 was 7 times of the National ambient air quality standards proposed by US EPA. The major chemical compositions were organics, sulfate, crustals and nitrate. It was found that the mass concentrations of PM2.5 were influenced by meteorological conditions. The correlation between the mass concentrations of PM2.5 and the relative humidity was found. And the correlation became closer at higher relative humidity. And the mass concentrations of PM2.5 were anti-correlated to wind speeds, but the correlation between the mass concentration of PM2.5 and wind speed was not good at stronger wind. The results also showed that precipitation had great washout effect on mass concentration of fine particles, but the drizzle had some washout effects to remove fine particles. And the synoptic regime had relatively great influence upon the mass concentration of fine particles.

The monitor results showed that PM2.5 had higher concentration in summer and winter. In summer time, the PM2.5 may mainly come from secondary reaction that makes serious pollution at regional scale, while in winter AEBCMA at southern part of Beijing had much higher level of PM2.5 because of more coal burning in that area.

The chemical compositions of PM2.5 showed very similar pattern at different site in Beijing. The annual average chemical composition in 2001 at PKU and DS indicated that the organics and crustal elements at both sites were the largest contributors to the PM2.5 mass, sulfate and nitrate had also significant portion. The nearly identical chemical composition in figure 2 hinted that the PM2.5 pollution was a regional issue.

However, the chemical composition of PM2.5 seemed to change with season. Using the data measured at AEBCMA as an example, the major chemical species in PM2.5 showed different contribution to PM2.5 in summer, autumn and winter. Organics, sulfate and nitrate were main species in PM2.5, the organics (OC) was the largest contributor also. The ratio of OC to EC in atmosphere was used as an indicator for secondary pollution in the air, the ratios of OC/EC were 7 to 11, much larger than the critical value of 2, showing the existence of secondary pollution in Beijing.

Beijing city in Spring is dry and windy, it is favorable for the out-spreading of pollutants. A good correlation between the mass concentrations of PM2.5 and the relative humidity was found in these days.

Beijing had more precipitation and smog in summer than in other seasons, the relative humidity was therefore higher in summer. In no precipitation days in summer, the relative humidity was low, and particles diffusion was efficient, consequently the mass concentration of PM2.5 was low. The relative humidity and the mass concentrations of PM2.5 in the light rainy days were comparable to that in smog days. But the relative humidity in light rainy days was a little higher than that in smog days, and the mass concentrations of PM2.5 was a little lower than that in smog days, hinting played a role in wash-out of PM2.5. Though relative humidity became very higher in heavy rain, the mass concentrations of PM2.5 were significantly low. The mass concentrations of PM2.5 dropped rapidly in the heavy rain, then the mass concentrations of PM2.5 went up after heavy rain.

In the fall, the humidity was still high in Beijing. From the results of PM2.5 mass concentrations and relative humidity in AEBCMA in foggy days in autumn of 2001 by using Anderson's CAMMS real time monitor, we find that correlation between the mass concentrations of PM2.5 and relative humidity seemed to have dependency on relative humidity itself, and the correlation was getting closer while relative humidity became higher.

The PM2.5 pollution in winter time was influenced by three major factors: (1) coal burning in winter made the primary emission of fine particles increase; (2) atmospheric inverse layer was the lowest in a year, and formed earlier but destructed later in a day, therefore fine particles could accumulate to higher concentrations in winter; (3) cold air of large scale from north of Beijing brought dry air with strong wind in Beijing. In this case the PM2.5 could be very low.

The factors above made the PM2.5 pollution level in winter varies greatly.

Keyword: Meteorological conditions; quantitative relation; fine particles (PM2.5); Beijing