| Inorganic fine particulate matter (PM2.5), also called inorganic fine aerosol, is playing the most vital role in the regional air pollution across China. Focusing on two representative inland megacities in the northern and mid-western China, Beijing and Chongqing, a long-term paralleled PM2.5 study was carried out at the distance of 1500km, including both field observations and model simulations on the primary mineral components and secondary inorganic species, as well as the meteorology and air mass histories. Based on these datasets, both common and different features of the inorganic fine aerosol in the two cities were characterized, and seasonal impact on the fine mineral components from the sources outside the cities, mainly Asian dust, was evaluated. In addition, several key factors leading to the seasonal variability of particulate acidity, which is the key parameter influencing the atmospheric chemistry and physics, were also identified.Due to the variable emission characteristics and meteorological factors, PM2.5 level at Chongqing was significantly higher than Beijing, with more even spatial distribution between urban and rural sites. As indicated from the PM2.5 chemical signatures, the contribution from coal and biomass burning played a more important role at Chongqing, whereas the vehicle exhaust showed stronger regional influence at Beijing. For the fine mineral components, however, similar concentrations and proportions (10%–25%) in PM2.5 were found in spring for all the sites, which were distinctively higher than in other seasons. Meanwhile, high content of secondary inorganic species (sulfate, nitrate and ammonium, SNA) also showed up at each site, contributing more than 30% of PM2.5 mass in all seasons.In spring, weekly concentration of fine mineral components at Chongqing reached or beyond the level of Beijing under the influence of Asian dust, dominated by different source regions, however. Chongqing's dust was contributed from more distant western deserts (<105?E), with highly enriched Ca and Mg, while Beijing's dust was more contributed from eastern deserts (>105?E) after short transport distance, with less enriched Ca and Mg. Regulated by the patterns of synoptic system, both high and low pathways were characterized for the dust transported from the deserts to Chongqing. Significant influence of Asian dust on the spring air quality in Chongqing was identified based on five criteria, including SO2/PM10, NO2/PM10, surface wind speed, air mass history and dust simulation results. For days of PM10 violation during the spring of 2005, it was estimated that 7 days (20%) and 21 days (54%) in Chongqing and Beijing might be associated with the dust incursions, respectively. The depletion of Si/Al ratio during the periods of national dust incursion in spring validated the primary results from the initial identification. Its seasonal evolution also suggested that external sources, mostly in association with Asian dust, could also be very important for the fine mineral particles of Beijing and Chongqing. Its contribution generally increased from spring to summer, and then decreased from fall to winter, averagely above 50% for the whole studied period.In contrast with the weak neutralization of mineral dust, the availability of ammonium determined the acidity of PM2.5 for both Beijing and Chongqing, which was mainly contributed by sulfate. Extensive seasonal anomalies of PM2.5 acidity were found at both cities, with synchronous variation during spring and summer of 2005 and opposite trend for springs of 2005 and 2006. The anomalies were identified in relation with the behaviors of synoptic systems. The differences of Asian dust transport and precipitation could well explain the opposite spring anomalies of PM2.5 acidity for Beijing and Chongqing, respectively. Heteregenous reaction may become one of the major pathways for nitrate formation at both cities under the condition of high aerosol acidity.
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