A long-term chemical characteristics and source apportionment of atmospheric rainfall in a northwest megacity of Xi'an, China.

A long-term measurement on rainfall was conducted in urban Xi’an, China, from 2009 to 2016. The seasonal and annual variations of major inorganic components and their chemical properties in the rainfall were studied. The annual rainfall ranged from 165.3 to 916.3 mm. The pH value of the rainfall ranged from 6.36 to 7.19, with an average value of 6.70. The electric conductivity (EC) in the rainfall was in a range of 55.91 to 227.44 μS·cm−1. Ammonium (NH4+), calcium (Ca2+), nitrate (NO3−), and sulfate (SO42−) were the four major components, accounting for 88.5% of the total quantified inorganic ion concentration. Neutralization factors were determined for Ca2+ (1.03), NH4+ (0.57), Mg2+ (0.10), Na+ (0.06), and K+ (0.04). The high abundance of NH4+ that formed from its precursor of ammonia gas (NH3) suggested the contribution of agricultural fertilization. Ca2+ in the rainfall was mainly from natural sources such as soil dust, while anions of NO3− and SO42− originated from fossil fuel combustion. Source apportionment was conducted with positive matrix factorization (PMF) which identified that secondary inorganic formation, crustal dust, coal combustion, and biomass burning are the contributors to the rainfall. In between, secondary inorganic formation was the largest contributor, which accounted for 27.8–58.1% of the total sources, followed by crustal dust of 0.4–42.6%. The results of this long-term study demonstrated the decreasing trends of contributions from coal combustion and biomass burning under a series of air pollution control measures implemented by the government. However, continuous urbanization and development of the city caused substantial increases of the construction activities, inducing more crustal dusts to the environment in urban Xi’an.