Diurnal variations of atmospheric polycyclic aromatic hydrocarbons (PAHs) during three sequent winter haze episodes in Beijing, China.

Gas- and particle-phase concentrations of 18 atmospheric polycyclic aromatic hydrocarbons (PAHs) were respectively measured during daytime and nighttime at an urban site of Beijing around the New Year's Day of 2015. The average concentration of total atmospheric PAHs (ΣPAHs) during three haze episodes (PM>75μg/m) was 1473.1ng/m, which was 2.6 times higher than that (405.1ng/m) during normal periods (PM<75μg/m). Significant diurnal variations in the ΣPAH concentrations, homologue pattern and gas-particle partitioning were observed during haze episodes. There was a significantly negative correlation between ΣPAH concentrations and planetary boundary layer heights. During haze episodes, PAHs in daytime atmosphere should mostly originate from the vehicle emission, while the main sources shift to coal combustion in the nighttime. The gas-particle distribution behavior of PAHs was decisively affected by air temperature and relative humidity, and generally simulated by Junge-Pankow model. During haze episodes, the average benzo[a]pyrene equivalent concentration of atmospheric PAHs in the nighttime were 0.7-fold higher than that in the daytime, indicating that people staying out more during haze episode nighttime would pose a considerably higher cancer risk for inhalation exposure to PAHs.