[Characterization and Formation Mechanism of Water-soluble Inorganic Ions in PM2.5 and PM10 in Summer in the Urban Agglomeration of the Ili River Valley].

Abstract

The simultaneous observation and analysis of atmospheric particles on a regional scale is an important approach to developing control strategies for air pollution. To study the spatial distribution characteristics of particulate matter and water-soluble inorganic ions in the Ili Valley Urban agglomeration, PM_2.5 and PM₁₀ samples were synchronously collected from July 19 to July 29, 2021 in Yining City and the surrounding three counties, and then nine types of water-soluble inorganic ions (WSIIs) were analyzed. The spatial distribution characteristics, existence form of WSIIs, and influencing factors were discussed in depth. The results showed that the average ρ(PM_2.5) and ρ(PM₁₀) in the Ili River Valley urban agglomeration in summer were 23 μg·m^-3 and 59 μg·m^-3, respectively. The emission of local industrial and mobile sources in Yining City was higher than that of the surrounding three counties, resulting in the highest ρ(PM_2.5) in the region (25 μg·m^-3). Due to the influence of dust sources and topography, the ρ(PM₁₀) in Yining county was the highest in the region (63 μg·m^-3). Huocheng county is located upwind of the region, and these favorable diffusion conditions resulted in the lowest ρ(PM_2.5) and ρ(PM₁₀) (20 μg·m^-3 and 49 μg·m^-3, respectively). The concentrations of WSIIs in PM_2.5 and PM₁₀ ranged from 28.2%-29.9% and 16.0%-20.2%, respectively. The four main ions (SO₄^2-, NO₃^-, NH₄⁺, and Ca²⁺) accounted for approximately 90% of WSIIs mass concentrations. The concentration order of the four main ions in PM_2.5 was SO₄^2->Ca²⁺>NH₄⁺>NO₃^- and SO₄^2->Ca²⁺>NO₃^->NH₄⁺ in PM₁₀. The results of correlation analysis showed that the similar SO₄^2- concentrations in the four cities were mainly caused by regional transport. Ca²⁺ was the highest-concentration ion in PM₁₀ of Yining City and Qapqal Xibe Autonomous county, and the proportion of Ca²⁺ was significantly higher than that in most cities in China, which reflected that the cities in the core area of the Ili Valley were greatly affected by the dust sources. The ratios of n(NO₃^-)/n(SO₄^2-) in PM_2.5 and PM₁₀ were 0.78 and 0.76, respectively, indicating that the influence of stationary sources was greater than that of mobile sources. The ratio of n(NO₃^-)/n(SO₄^2-) in Yining City>Huocheng county>Yining county>Qapqal Xibe Autonomous county, which was consistent with the motor vehicle populations of the four cities, reflecting that Yining City was affected by motor vehicle sources more than the surrounding three counties. The secondary components mainly existed in the form of (NH₄)₂SO₄, NH₄HSO₄, and NH₄NO₃. There was excess ammonia after the reaction between NH₄⁺ and SO₄^2- in each city. NH₄NO₃ mainly existed in Yining City, which was mainly related to high NO₂ in Yining City. The NOR of the four cities were 0.03-0.10 and 0.03-0.16 in PM_2.5 and PM₁₀, respectively, and the secondary transformation of NO₃^- was weak due to the influence of high temperatures in summer. The SOR were 0.21-0.41 and 0.23-0.44, respectively. The SOR of Qapqal Xibe Autonomous county was the highest due to the relatively high humidity, whereas the SOR of Huocheng county was higher than that of the three sites in Yining City due to the influence of regional transportation. The formation mechanisms showed that SO₄^2- in Qapqal Xibe Autonomous county and Yining City were mainly produced by the heterogeneous reaction, and in Yining county it was mainly formed via the homogeneous reaction. However, the formation mechanism in Huocheng county was complex and was affected by both homogeneous and heterogeneous reactions.