Water-soluble ions in PM2.5 during spring haze and dust periods in Chengdu, China: Variations, nitrate formation and potential source areas.

Hourly concentrations of water-soluble inorganic ions (Na+, NH4+, K+, Mg2+, Ca2+, Cl-, NO3- and SO42-) in PM2.5 and related reactive gases were measured with a Gas and Aerosol Collector combined with Ion Chromatography (GAC-IC) in urban Chengdu from April 17 to May 27, 2017, during which both haze and dust episodes occurred frequently. Nitrate was the most abundant ion in PM2.5 and substantially increased during haze pollution with the NO3-/SO42- mass ratio increasing from 0.78 during clean period to 1.1 during haze period. Aerosols in Chengdu were generally ammonium-rich, wherein ammonium nitrate was primarily formed through homogeneous gas-phase reactions and limited by the availability of HNO3, indicating that preferentially reducing the emissions of NOx could make for mitigating spring haze pollution in Chengdu. Backward trajectory clustering coupled with measured species and a potential source contribution function (PSCF) for PM2.5, PM10/PM2.5, sulfate, nitrate, ammonium, and Ca2+ indicated that regionally transported pollutants from the southern and southeastern Sichuan Basin strongly contributed to springtime PM2.5 pollution in Chengdu, but long-distance transport from northwestern China also contributed to dust pollution. Moreover, the treatment of urban fugitive dust in southern Sichuan is also important for reducing coarse particles in Chengdu. Therefore, the improvement of air quality in Chengdu, even in the Sichuan Basin, requires the regional joint emission reduction of particles and gaseous precursors across the entire Sichuan Basin, especially for cities located in southeastern Sichuan Basin.