Possible effects of climate change of wind on aerosol variation duringwinter in Shanghai, China

Several data sets were introduced to investigate the possible effects of climate-change-related variation of wind on aerosol concentration during winter in Shanghai, China. These data sets included the daily wind speed, wind direction, visibility, and precipitation from 1956 to 2010, hourly PM10 concentration from 2008 to 2010, and the NCEP/NCAR reanalysis data of global atmospheric circulation from 1956 to 2010. The trend of aerosol concentration and its correlations with wind speed and wind direction in winter were analyzed. Results indicated that there was an increase in the number of haze days in winter of 2.1 days/decade. Aerosol concentration, represented by PM10 in this study, was highly correlated to both wind speed and direction in winter. The PM10 concentration increased as wind speed decreased, reaching maximum values under static wind conditions. The PM10 concentration was relatively lower under easterly winds and higher under westerly winds. The analysis showed that weaker East Asia winter monsoons have resulted in a reduction of wind speed, increase in static wind frequency, and decline in the frequency of northerly winds since the 1980s. Moreover, the rapid expansion of urban construction in Shanghai has changed the underlying surface considerably, which has led to a reduction in wind speed. Finally, a wind factor was defined to estimate the combined effects of wind speed and wind direction on aerosol concentrations in Shanghai. The analysis of this factor indicated that changes in atmosphere circulation and urbanization have had important effects on the number of winter haze days in Shanghai

PM2.5 from the Guanzhong Plain: Chemical composition andimplications for emission reductions

Atmospheric particulate matter (PM) affects important environmental issues including air quality, regional and global climates, and human health. A one-year sampling campaign for PM2.5 was conducted at six locations in Guanzhong Plain, including the cities of Xi&#39;an, Weinan and Baoji, from March 2012 to March 2013. The 24-h average PM2.5 mass concentration was 134.7&nbsp;&mu;g&nbsp;m&minus;3, that substantially exceeds the National Ambient Air Quality Standard level of 35&nbsp;&nbsp;&mu;g&nbsp;m&minus;3. The highest loadings of both organic and elemental carbon (OC and EC) occurred in winter: EC co-varied with OC but showed less variability, presumably due to more stable emissions. The greatest contributions of secondary inorganic ions (SO42&minus;, NO3&minus; and NH4+) to the total quantified ions also were seen in winter, presumably due to gaseous precursors from coal combustion and biomass burning. Two high PM episodes occurred, one in the autumn and the other in winter. During the autumn episode, regional pollution from biomass burning raised the concentrations of secondary ions while coal combustion was a strong influence during the winter episode. Modeling simulations suggest that the control measures on both primary emissions and secondary aerosol precursors including SO2, NOx, and NH3 are needed to reduce the PM levels of the region.</p

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

Analysis of the causes of heavy aerosol pollution in Beijing, China:A case study with the WRF-Chem model

The causes and variability of a heavy haze episode in the Beijing region was analyzed. During the episode, the PM2.5 concentration reached a peak value of 450 g/kg on January 18, 2013 and rapidly decreased to 100 g/kg on January 19, 2013, characterizing a large variability in a very short period. This strong variability provides a good opportunity to study the causes of the haze formation. The in situ measurements (including surface meteorological data and vertical structures of the winds, temperature, humidity, and planetary boundary layer (PBL)) together with a chemical/dynamical regional model (WRF-Chem) were used for the analysis. In order to understand the rapid variability of the PM2.5 concentration in the episode, the correlation between the measured meteorological data (including wind speed, PBL height, relative humidity, etc.) and the measured particle concentration (PM2.5 concentration) was studied. In addition, two sensitive model experiments were performed to study the effect of individual contribution from local emissions and regional surrounding emissions to the heavy haze formation. The results suggest that there were two major meteorological factors in controlling the variability of the PM2.5 concentration, namely, surface wind speed and PBL height. During high wind periods, the horizontal transport of aerosol particles played an important role, and the heavy haze was formed when the wind speeds were very weak (less than 1 m/s). Under weak wind conditions, the horizontal transport of aerosol particles was also weak, and the vertical mixing of aerosol particles played an important role. As a result, the PBL height was a major factor in controlling the variability of the PM2.5 concentration. Under the shallow PBL height, aerosol particles were strongly confined near the surface, producing a high surface PM2.5 concentration. The sensitivity model study suggests that the local emissions (emissions from the Beijing region only) were the major cause for the heavy haze events. With only local emissions, the calculated peak value of the PM2.5 concentration was 350 g/kg, which accounted for 78% of the measured peak value (450 g/kg). In contrast, without the local emissions, the calculated peak value of the PM2.5 concentration was only 100 g/kg, which accounted for 22% of the measured peak value

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