Pollution maps of grass contamination by platinum group elements and polycyclic aromatic hydrocarbons from road traffic

Abstract

The increase in deposition of air pollutants such as the platinum group elements (PGEs) Pt, Pd and Rh, and polycyclic aromatic hydrocarbons (PAHs) from vehicles on plants has raised concerns about the risk for the quality of food and, in turn, for human health. Here, we report the first pollution maps of PGEs and PAHs at the agricultural scale. PGEs have recently emerged in ecosystems due to their massive use as catalysts in the automobile industry. PAHs are combustion by-products known for their carcinogenic properties. PAHs and PGEs emitted by vehicles can enter the food chain through deposition on plants growing near roads. Despite many investigations of PAHs and PGEs, knowledge on their spatial distribution near roads is very scarce. Here, we assessed their localization on the scale of an agricultural field close to a road with 7200 vehicles per day. While classical studies usually involve direct measurements of plants growing in the field, we used an alternative experimental approach to allow an unambiguous assessment of the pollution impact. Ryegrass (Lolium perenne) was first cultivated in pots in a closed greenhouse to isolate the plants from the outdoor atmosphere. Then, ryegrass pots were transferred to the agricultural field. The spatial distribution of PGEs and PAHs was assessed by placing ryegrass pots 0 m, 10 m, 25 m and 50 m from the road. This method thus allowed the measurement of PGE and PAH deposit from vehicle emissions during a known timeframe. After a one-month exposure, the PGE and PAH concentrations in ryegrass samples were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) and high-pressure liquid chromatography (HPLC), respectively. Our results demonstrate a significant effect of pollution by the outdoor atmosphere for Pd and PAHs. Indeed, the concentrations increased by a factor of 1.5 for Pd and by a factor of 7 for PAHs after one month’s exposure in the isolated pasture. The results also demonstrate clearly the impact of road traffic. Indeed, the highest PGE and PAH concentrations were detected in ryegrass exposed at the road site. Pd and PAH concentrations decreased significantly up to 39% and 60%, respectively, from 0 to 50 m away from the road. The pollution maps show that the maximum depositions occur precisely along the road plan. The highest pollution levels of grasses of 23 ng Pd/g grass and 260 ng PAHs/g grass dry weight were recorded between 0 and 10 m on both sides of the road. Pollution maps should thus help to assess more precisely the impact of road traffic on food quality.