Long-Term Observations Reveal Enhanced Nighttime Formation of Nitrated PAHs Following Primary Emission Reduction

Nitrated polycyclic aromatic hydrocarbons (NPAHs) are some of the most mutagenic and carcinogenic substances in ambient fine particulate matter (PM_2.5). Due to their origin from both combustion processes and complex atmospheric nitrations of PAHs, it remains unknown how regulations, mainly on controlling primary emissions, impact NPAH levels and sources. Here, we analyzed long-term trends of 12 NPAHs in 415 PM_2.5 samples collected in Beijing from 2014 to 2021 during Chinese clean air actions. ∑₁₂NPAHs decreased by 89.2% (2350 pg m^–3 to 254 pg m^–3), a slower decline than that of ∑₁₆PAHs determined using the same samples. The stable 2-nitropyrene/pyrene (2-NPYR/PYR) ratio suggests unchanged daytime gas-phase nitration, while the increasing 2-nitrofluoranthene (2-NFLT) contribution (26.1% to 46.2%) and rising 2-NFLT/FLT ratio indicate enhanced nighttime nitration. Based on estimations using the isomeric 2-NPYR/2-NFLT ratio, the nighttime reaction increased from 40.3% during the Air Pollution Prevention and Control Action Plan to 47.5% in 2021. Especially, nighttime reactions are negligible during cold seasons from 2014 to 2016 but contributed 10.3% in 2021. This evidence suggests that reducing primary emissions may intensify nighttime nitration, requiring targeted strategies to mitigate NPAHs. This study provides pivotal data for exposure assessment and informs air pollution control policies by addressing the secondary formation of key toxic pollutants.