Particulate-bound alkyl nitrate pollution and formation mechanisms in Beijing, China.

Fine particulate matter (PM_2.5) samples were collected between November 2020 and October 2021 at the Minzu University of China in Beijing and the n -alkyl nitrate concentrations in the PM_2.5 samples were determined to investigate n -alkyl nitrate pollution and formation mechanisms. C₉–C₁₆ n -alkyl nitrate standards were synthesized and the n -alkyl nitrate concentrations in PM_2.5 were determined by gas chromatography triple quadrupole mass spectrometry. Temporal trends in and correlations between particulate-bound n -alkyl nitrate, ozone, PM_2.5, and nitrogen dioxide concentrations were investigated to assess the relationships between particulate-bound n -alkyl nitrate concentrations and photochemical reactions and identify the n -alkyl nitrate formation mechanisms. The n -alkyl nitrate concentrations in the PM_2.5 samples were 9.67–2730 pg/m³, and the mean was 578 pg/m³. The n -alkyl nitrate homologue group concentrations increased as the carbon chain length increased, i.e, long-chain n -alkyl nitrates contributed more than short-chain n -alkyl nitrates to the total n -alkyl nitrate concentrations in PM_2.5. The n -alkyl nitrate concentrations clearly varied seasonally and diurnally, the concentrations decreasing in the order winter > spring > autumn > summer and the mean concentrations being higher at night than in the day. The particulate-bound n -alkyl nitrate and ozone concentrations significantly negatively correlated despite gas-phase alkyl nitrate and ozone concentrations previously being found to positively correlate. This indicated that long-chain alkyl nitrates may not be produced during photochemical reactions. The particulate-bound n -alkyl nitrate concentrations followed the same trends as and significantly positively correlated with the PM_2.5 and nitrogen dioxide concentrations. Nitrogen dioxide is an important contributor of nitrates in particulate matter. This indicated that particulate-bound n -alkyl nitrates may form through reactions between alkanes and nitrates on particulate matter surfaces. Particulate-bound n -alkyl nitrates are important components of PM_2.5 during haze events and strongly affect atmospheric visibility. Particulate-bound n -alkyl nitrates are secondary pollutants that strongly influence haze pollution. [ABSTRACT FROM AUTHOR]