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 (PM2.5). 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 PM2.5 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.