Changes in the Dominant Contributions of Nitrate Formation and Sources During Haze Episodes: Insights From Dual Isotopic Evidence

Inorganic nitrate (NO₃⁻), a crucial component of fine particulate matter (PM_2.5), has not shown a consistent decrease, despite an obvious decrease of nitrogen oxide (NOx) and PM_2.5. The atmospheric oxidation process for nitrate formation has been deemed a key factor in pollution; however, the changes of sources and oxidation pathways during a particular haze episode require further investigation. Here, daily dual isotopes (δ¹⁵N and δ¹⁸O) were used to quantify the sources and oxidation pathways of nitrate formation in Qingdao, a port city in Northern China, from September 2017 to February 2018. This study also includes a detailed introduction to two haze episodes. δ¹⁵N and δ¹⁸O results show that both fractions of nocturnal oxidation for nitrate formation and NOx from coal combustion were lower in warmer season and higher in colder season. The fractions increased with increasing PM_2.5 under low PM_2.5 concentration while the fractions were not significantly changed under higher PM_2.5 concentration, dominated by nocturnal oxidation (70.6% ± 9.7%) and coal combustion (66.1% ± 18.2%), respectively. The haze episode 1 was attributed to smoke transported over long distances, which provided a large amount of aerosol particles to absorb more locally formed gaseous HNO₃ or N₂O₅. In this episode, meteorological and air quality factors, nitrate sources, and formation mechanism did not obviously change. Haze episode 2 was caused by unfavorable meteorological factors that enhanced local nitrate accumulation. As pollution worsened, the oxidation pathway shifted from OH oxidation to N₂O₅ hydrolysis, and the primary source changed from coal combustion to more vehicle exhaust.