Effects of meteorology and secondary particle formation on visibilityduring heavy haze events in Beijing, China

The causes of haze formation in Beijing, China were analyzed based on a comprehensive measurement, including PBL (planetary boundary layer), aerosol composition and concentrations, and several important meteorological parameters such as visibility, RH (relative humidity), and wind speed/direction. The measurement was conducted in an urban location from Nov. 16, 2012 to Jan. 15, 2013. During the period, the visibility varied from N20 km to less than a kilometer, with a minimum visibility of 667 m, causing 16 haze occurrences. During the haze occurrences, the wind speeds were less than 1 m/s, and the concentrations of PM2.5 (particle matter with radius less than 2.5 &mu;m) were often exceeded 200 &mu;g/m3. The correlation between PM2.5 concentration and visibility under different RH values shows that visibility was exponentially decreased with the increase of PM2.5 concentrations when RH was less than 80%. However, when RH was higher than 80%, the relationship was no longer to follow the exponentially decreasing trend, and the visibility maintained in very low values, even with low PM2.5 concentrations. Under this condition, the hygroscopic growth of particles played important roles, and a large amount of water vapor acted as particle matter (PM) for the reduction of visibility. The variations of meteorological parameters (RH, PBL heights, and WS (wind speed)), chemical species in gas-phase (CO, O3, SO2, and NOx), and gas-phase to particle-phase conversions under different visibility ranges were analyzed. The results show that from high visibility (N20 km) to low visibility (b2 km), the averaged PBL decreased from 1.24 km to 0.53 km; wind speeds reduced from 1 m/s to 0.5 m/s; and CO increased from 0.5 ppmv to 4.0 ppmv, suggesting that weaker transport/diffusion caused the haze occurrences. This study also found that the formation of SPM (secondary particle matter) was accelerated in the haze events. The conversions between SO2 and SO4 _ as well as NOx to NO3 &minus; increased, especially under high humidity conditions. When the averaged RH was 70%, the conversions between SO2 and SO4 _ accounted for about 20% concentration of PM2.5, indicating that formation of secondary particle matter had important contribution for the haze occurrences in Beijing.</p

Evolution of planetary boundary layer under different weather conditions, and its impact on aerosol concentrations

A field experiment was conducted in Tianjin, China from September 9-30,2010, focused on the evolution of Planetary Boundary Layer (PBL) and its impact on surface air pollutants. The experiment used three remote sensing instruments, wind profile radar (WPR), microwave radiometer (MWR) and micro-pulse lidar (MPL); to detect the vertical profiles of winds, temperature, and aerosol backscattering coefficient and to measure the vertical profiles of surface pollutants (aerosol, CO, SO2, NOx), and also collected sonic anemometers data from a 255-m meteorological tower. Based on these measurements, the evolution of the PBL was estimated. The averaged PBL height was about 1000-1300 m during noon/afternoon-time, and 200-300 m during night-time. The PBL height and the aerosol concentrations were anti-correlated during clear and haze conditions. The averaged maximum PBL heights were 1.08 and 1.70 km while the averaged aerosol concentrations were 52 and 17 mu g/m(3) under haze and clear sky conditions, respectively. The influence of aerosols and clouds on solar radiation was observed based on sonic anemometers data collected from the 255-m meteorological tower. The heat flux was found significantly decreased by haze (heavy pollution) or cloud, which tended to depress the development of PBL, while the repressed structure of PBL further weakened the diffusion of pollutants, leading to heavy pollution. This possible positive feedback cycle (more aerosols -&gt; lower PBL height -&gt; more aerosols) would induce an acceleration process for heavy ground pollution in megacities.</p

Variability of SO2 in an intensive fog in North China Plain: Evidence of high solubility of SO2

A field experiment was conducted in an intensive fog event between November 5 and November 8, 2009, in a heavily SO2-polluted area in North China Plain (NCP), to measure SO2 and other air pollutants, liquid water content (LWC) of fog droplets, and other basic meteorological parameters. During the fog period, the concentrations of SO2 showed large variability, which was closely related to the LWC in the fog droplets. The averaged concentration of SO2 during non-fog periods was about 25 ppbv, while during the fog period, it rapidly reduced to about 4-7 ppbv. Such large reduction of SO2 suggested that a majority of SO2(about 70%-80%) had reverted from gas to aqueous phase on account of the high solubility of SO2 in water in the fog droplets. However, the calculated gas to aqueous phase conversion was largely underestimated by merely using the Henry&#39;s Law constant of SO2, thus suggesting that aqueous reaction of SO2 in fog droplets might play some important role in enhancing the solubility of SO2. To simplify the phenomenon, an &quot;effective solubility coefficient&quot; is proposed in this study. This variability of SO2 measurement during the extensive fog event provides direct evidence of oxidation of SO2 in fog droplets, thus providing important implications for better understanding of the acidity in clouds, precipitation, and fogs in NCP, now a central environmental focus in China due to its rapid economic development.</p