Abstract
State Highway 73 (SH73) connects Greymouth on the West Coast with Canterbury on the East Coast of New Zealand and runs through Arthur’s Pass National Park in the Southern Alps. To study the distribution of polycyclic aromatic hydrocarbons (PAHs) from car emissions from SH73, three 10-kilometer long transects were identified in three braided river valleys extending off of SH73 (The Waimakiriri Valley extending east, the Hawdon Valley extending north and the Taramakau Valley extending east) and lichen (Usnea sp.) were sampled at 1-km intervals along each transect. Lichen has been identified as an ideal biomonitor for the measurement of PAH distribution in urban and rural environments. A selective pressurized liquid extraction method was developed to extract the PAHs from the sampled lichen and gel permeation chromatography was selected for additional clean-up of lichen extracts. The mean recovery of PAHs in spike-and-recovery experiments with this method was 73.7 ± 26.0 %.
The measured concentrations of PAHs in lichen (ng g-1 lichen) were used to assess the distribution of PAHs through each valley. In addition to the analysis of total PAH concentration (i.e. the sum of the concentrations of 16 individual PAH isomers) with respect to distance from SH73, several diagnostic ratios were used to determine the source(s) of measured PAHs in the sampled lichen. High molecular weight (HMW) (4-6 rings) PAHs are predominantly produced in high-temperature combustion conditions and are attributed to the combustion of petrol and diesel in vehicle engines. The ratio of 4-6 ringed PAHs to 2-3 ringed PAHS was calculated at each site in an attempt to identify the dominant PAH source. Other ratios, including fluoranthene/pyrene (Flu/Pyr), benz[a]anthracene/chrysene + triphenylene (BaA/Chr + Tri) and benzo[b]fluoranthene/benzo[k]fluoranthene (BbF/BkF) were calculated at each site and compared to literature values to determine the source of measured PAHs. Flu/Pyr was used to distinguish between pyrogenic and petrogenic dominant sources of measured PAHs and BaA/Chr + Tri and BbF/BkF were used to determine if the dominant source was wood, coal or petrol combustion.
For all three PAH groups (2-3 ringed PAHs, 4-ringed PAHs, and 5-6 ringed PAHs), the concentrations underwent a linear decrease with increasing distance from SH73 in the Waimakariri Valley, an exponential decrease with increasing distance from SH73 in the Hawdon Valley, and no overall change in concentration in the Taramakau Valley. The differences in patterns are likely a results of several factors: the directional orientation of the valleys with respect to global- and local-scale wind patterns, the geomorphological characteristics of each valley, the side of the Southern Alps on which the valleys are positioned (the Taramakau Valley lies on the western slope and the Waimakiriri and Hawdon Valleys lie on the eastern slope) and the resulting differences in vegetation unique to the two coasts of the South Island of New Zealand. By investigating the distribution of PAHs in three different valleys in a national park, this study was able to demonstrate that meteorology and landscape play critical roles in the atmospheric transport of PAHs.