Aerial distribution, temperature-dependent seasonal variation, and sources of polycyclic aromatic hydrocarbons in pine needles from the Houston metropolitan area, Texas, USA

To investigate the aerial distribution, seasonal variation, and sources of polycyclic aromatic hydrocarbons (PAHs), pine needles were collected from 18 sites in the Houston metropolitan area, Texas, USA. Total PAHs ranged from 209 to 2,226 ng/g (dry wt.), which were similar to those found in other urban areas. The highest and lowest concentrations were found in samples from the inner city of Houston and the outer edge of suburban area, respectively. Aerial distribution of PAHs in pine needles was closely correlated to the proximity to densely occupied residential area and traffic volumes around sampling sites. Seasonal changes of PAHs in pine needles were inversely correlated with ambient temperature with lower levels in warmer months. Calculated concentrations of 3-ring PAHs in the air had a positive correlation with ambient temperature with higher levels in warmer months. Ratios of PAHs in pine needles to PAHs in the air decreased as ambient temperature increased because fugacity (escaping tendency) of PAHs in pine needles is grater in warmer months. The PAH patterns and ratios were fairly constant in all samples, indicating that the whole study area was influenced by the same emission sources, most importantly gasoline vehicle emission. Phenanthrene was the predominant PAH and 3- and 4-ring PAHs were the most abundant in all samples, accounting for 79 to 97% of the total PAHs. The present study provides evidence that pine needle monitoring can be used as a good screening method to assess the atmospheric PAH contamination quickly and at low cost.