Extrinsic Influence of Haze Episodes in the Central City Group of Liaoning Province

The city group of central Liaoning (CL) is one of the major air pollution areas in Northeast China. Based on the MODIS (MODerate-resolution Imaging Spectroradiometer) satellite data, air quality monitoring data, and ground meteorological data for Shenyang (one of the cities in the CL city group) in 2009, this paper, using the backward trajectory method, analyzes the weather patterns and haze pollution sources of 30 haze pollution days on which particulate matter (PM10) was the main pollutant and the Air Pollution Index (API) level was ≥3. The results show that there were four different weather patterns that mainly triggered the heavy air pollution episodes: A uniform high-pressure field pattern, such as rear high-pressure, high-pressure in front, or high-pressure inside; the Changbai Mountains small high-pressure pattern; a weak gradient low-pressure field pattern; and a topographic trough pattern. The polluted days were mainly influenced by the southwest airflows from the Beijing-Tianjin-Hebei (BTH) region and the northerly airflows from Inner Mongolia-Changchun. Most (19) of the polluted days were generated under the southwest airflows conditions. The external haze pollutants from the BTH region were transported to the CL city group via Bohai Bay and the Liaoxi Corridor, and were mostly influenced by the near-surface southwesterly airflow from the uniform pressure field pattern to the rear of the high pressure and the topographic trough pattern. The two patterns resulted in eight polluted days, which accounted for 26.7% of the total number of haze days (30) in Shengyang during 2009. This demonstrates that cross-regional impacts of haze pollution cannot be ignored. The improvement of air quality in the CL city group is closely related to that over its neighboring regions, especially the BTH region. This poses a challenge for air pollution regional control strategies in China in the future.