Spatial and diurnal distributions of n-alkanes and n-alkan-2-ones on PM 2.5 aerosols in the Lower Fraser Valley, Canada

Abstract

Ninety daytime/nighttime PM 2.5 aerosol samples were collected at 5 sites in forest, tunnel, urban, rural, and mixed forest/urban areas in the Lower Fraser Valley during the Pacific 2001 Air Quality Study. Solvent-extractable organic matter, such as n-alkanes (C 14–C 33), n-alkan-2-ones (C 10–C 31), and 6, 10, 14- trimethylpentadecan-2-one on the fine aerosols, were quantified. The concentrations of total n-alkanes from primary sources were 45.5–112 ng m −3 at the tunnel site, 3.3–34.6 ng m −3 at the urban site, 0.6–18.1 ng m −3 at the rural site, and 1.7–16.9 ng m −3 at the forest and the mixed areas. The homologue distributions of the n-alkanes displayed different patterns at the 5 sites, showing day–night differences and reflecting their primary source types and impacts of episodes. The carbon preference index (CPI) values of the n-alkanes showed highest value (average of 2.39±0.47) at the forest, lowest (1.15±0.11) at the tunnel. The CPI showed higher values in night samples at all sites except the urban site which was impacted by specific episodes such as biomass burning and/or fuel burning occurring during the nighttimes, the higher nighttime values of CPI, with consistent lower n-alkane concentrations, suggested that weaker anthropogenic emissions during night were a more likely cause . The total n-alkan-2-ones on the aerosols were 1.8–12.6 ng m −3 at the tunnel site and 0.2–7.2 ng m −3 at the other 4 sites. Low-molecular weight n-alkan-2-ones (<C 22) were observed at all sites with the highest level at the tunnel. High molecular weight n-alkan-2-one (>C 23) were consistently higher at the tunnel but varied with date at the forest site. The n-alkan-2-ones, both low and high molecular weight, could have multiple sources including vehicular emissions and oxidation processes. The branched ketone, 6, 10, 14- trimethylpentadecan-2-one, an oxidative product of phytol on the fine aerosols, was enriched in the forest with the average of 7.6±7.1 ng m −3.