Differences in Meteorological Conditions between Days with Persistent and Non-Persistent Pollution in Beijing, China

We compared the regional synoptic patterns and local meteorological conditions during persistent and non-persistent pollution events in Beijing using US NCEP–Department of Energy reanalysis outputs and observations from meteorological stations. The analysis focused on the impacts of high-frequency（period 〈 90 days） variations in meteorological conditions on persistent pollution events（those lasting for at least 3 days）. Persistent pollution events tended to occur in association with slow-moving weather systems producing stagnant weather conditions, whereas rapidly moving weather systems caused a dramatic change in the local weather conditions so that the pollution event was short-lived. Although Beijing was under the influence of anomalous southerly winds in all four seasons during pollution events, notable differences were identified in the regional patterns of sea-level pressure and local anomalies in relative humidity among persistent pollution events in different seasons. A region of lower pressure was present to the north of Beijing in spring, fall, and winter, whereas regions of lower and higher pressures were observed northwest and southeast of Beijing, respectively, in summer. The relative humidity near Beijing was higher in fall and winter, but lower in spring and summer. These differences may explain the seasonal dependence of the relationship between air pollution and the local meteorological variables. Our analysis showed that the temperature inversion in the lower troposphere played an important part in the occurrence of air pollution under stagnant weather conditions.Some results from this study are based on a limited number of events and thus require validation using more data.

Influences of Meteorological Conditions on Interannual Variations of Particulate Matter Pollution during Winter in the Beijing–Tianjin–Hebei Area

To investigate the interannual variations of particulate matter （PM） pollution in winter, this paper examines the pollution characteristics of PM with aerodynamic diameters of less than 2.5 and 10 μm （i.e., PM2.5 and PM10）, and their relationship to meteorological conditions over the Beijing municipality, Tianjin municipality, and Hebei Province--an area called Jing-Jin-Ji （JJJ, hereinafter）-in December 2013-16. The meteorological conditions during this period are also analyzed. The regional average concentrations of PM2.5 （PM10） over the JJJ area during this period were 148.6 （236.4）, 100.1 （166.4）, 140.5 （204.5）, and 141.7 （203.1） μg m^-3, respectively. The high occurrence frequencies of cold air outbreaks, a strong Siberian high, high wind speeds and boundary layer height, and low temperature and relative humidity, were direct meteorological causes of the low PM concentration in December 2014. A combined analysis of PM pollution and meteorological conditions implied that control measures have resulted in an effective improvement in air quality. Using the same emissions inventory in December 2013-16, a modeling analysis showed emissions of PM2.5 to decrease by 12.7%, 8.6%, and 8.3% in December 2014, 2015, and 2016, respectively, each compared with the previous year, over the JJJ area.

Regional Meteorological Patterns for Heavy Pollution Events in Beijing

The present study investigates meteorological conditions for the day-to-day changes of particulate matter （PM） concentration in Beijing city during the period 2008-2015. The local relationship of PM concentration to surface air temperature, pressure, wind speed, and relative humidity displays seasonal changes and year-to-year variations. The average correlation coefficient with PMI0 in spring, summer, fall, and winter is 0.45, 0.40, 0.38, and 0.30 for air tem- perature; -0.45, -0.05, -0.40, and -0.45 for pressure; 0.13, 0.04, 0.53, and 0.50 for relative humidity; and -0.18, -0.11, -0.45, and -0.33 for wind speed. A higher correlation with wind speed is obtained when wind speed leads by halfa day. The heavily polluted and clean days, which are defined as the top and bottom 10% of the PM values, show obvious differences in the regional distribution of air temperature, pressure, and wind. Polluted days correspond to higher air temperature in all the four seasons, lower sea level pressure and anomalous southerly winds to the south and east of Beijing in spring, fall, and winter, and a northwest-southeast contrast in the pressure anomaly and anom- alous southerly winds in summer. Higher relative humidity is observed on polluted days in fall and winter. The pol- luted days are preceded by an anomalous cyclone moving from the northwest, accompanied by lower pressure and higher air temperature, in all four seasons. This feature indicates the impacts of moving weather systems on local meteorological conditions for day-to-day air quality changes in Beijing.

Observed Impact of the South Asian Summer Monsoon on the Local Meteorology in the Himalayas

The South Asian summer monsoon（SASM） is the most important climate system in Asia.Using observational data from the HEST2006（Himalayan Exchange between the Surface and Troposphere 2006） campaign and large-scale grid data,this paper analyzed the SASM impact on local meteorological parameters including radiation,temperature,humidity,and wind in the Himalayas.The SASM experienced one active and one break period during the HEST2006 campaign.The local meteorological parameters exhibit great differences between the active period and the break period of the SASM.The radiation fluxes are greater in the break period than in the active period.The air temperature and specific humidity are lower,but soil temperature and wind speed are higher in the break period than in the active period.Further analysis indicates that the SASM greatly affects the meteorological features of the Himalayan region.