Chemical nature of PM2.5 and PM10 in Xi'an, China: Insights into primary emissions and secondary particle formation

Highlights: • 33% and 42% of ambient SOC and sulfate in winter were from primary emissions. • Sulfate, OC, and EC have decreased during wintertime since 2003. • The ratio of NO₃^-/SO₄^2- increased from 0.4 in 2006 to 1.3 in 2014. In Xi'an, a city that frequently experiences serious PM pollution in northern China, 1476 PM₁₀ and 1464 PM_2.5 valid daily filter samples were collected at six sites from December 2014 to November 2015 and analyzed for 29 species. The annual mean PM₁₀ and PM_2.5 concentrations were 149.4 ± 93.1, 108.0 ± 70.9 μg/m³, respectively. Organic carbon (OC) is the predominant PM_2.5 component while crustal material predominated in PM₁₀. Sulfate concentrations, which was the largest component in Xi'an PM in previous studies, were lower than nitrate. Winter sulfate, OC, and elemental carbon (EC) have decreased since 2003, while nitrate remained constant in recent years and the ratio of NO₃⁻/SO₄²⁻ increased from 0.4 in 2006 to 1.3 in 2014. This result suggests that the motor vehicle contribution to PM has increased relative to coal-fired power plant emissions over the past decade. The mass fractions of crustal material, sulfate, and EC in PM_2.5 decreased as the PM_2.5 concentrations increased from "clean" days (<50 μg/m³) to the highest values, while nitrate significantly increased. Despite forming through secondary reactions, the high concentrations of SOC and SO₄²⁻ in winter are attributed to primary emissions and particularly to residential heating and cooking with coal. Primary SOC and SO₄²⁻ accounted for 33% and 42% of their total PM_2.5 concentrations in winter, respectively. Therefore, control measures applied to these primary sources can substantially improve air quality.