Hydrocarbons in the atmospheric gas phase of a coastal city in Tunisia: Levels, gas-particle partitioning, and health risk assessment.

Many studies have focused on aliphatic hydrocarbons and polycyclic aromatic hydrocarbons (AHs and PAHs) in different environmental compartments, especially atmospheric particles (aerosols), due to their adverse effects on the environment and human health. However, much less information is currently available on the content of AHs and PAHs in the atmospheric gas phase, which is a major reservoir of volatile and photoreactive compounds. Here, for the first time, we assessed the levels, gas-particle partitioning, human health risks and seasonal variations of AHs and PAHs in the atmospheric gas-phase of Bizerte city (Tunisia, North Africa) over a one-year period (March 2015-January 2016). ΣPAH concentration in the gas phase over the period ranged from 6.7 to 90.6 ng m and on average was 2.5 times higher in the cold season than in the warm season. ΣAH concentration in the gas phase over the period ranged from 14.0 to 35.9 ng m, with no clear seasonal variations. In the gas phase, hydrocarbons were dominated by low-molecular-weight (LMW) compounds, i.e. 3- and 4-ring for PAHs and < n-C for AHs. Gas-phase concentrations of PAHs and AHs accounted for up to 80 % of the total (gas + particle phases) atmospheric concentrations of PAHs and AHs. Further analysis of gas-particle partitioning showed that LMW hydrocarbons preferential accumulated in the gas phase, and that gas-particle partitioning was not in equilibrium but dominated by absorption processes into the aerosol organic matter. Benzo[a]pyrene toxic equivalency quotient (BaP-TEQ) in the gas phase represented on average 37 % of the total atmospheric BaP-TEQ concentration, which was always higher in the cold season. Atmospheric gas is a significant factor in the risks of cancer associated with inhalation of ambient air. The Monte Carlo simulation-based exposure assessment model predicted that outdoor air exposure to PAHs does not pose a cancer risk to infants, but the children, adolescent, and adult populations may face a lower cancer risk during the warm season and a higher risk in the cold season.