Evaluation of artifacts affecting collection of airborne PAHs.

Summary Extensive laboratory experiments have been carried out aimed at determining the aftermaths of volatilization and oxidation on atmospheric PAHs during the sampling phase. Eleven congeners were investigated, and implemented procedures allowed to assess losses distinctly associated to each of the two processes. Results show that the good practices application during the sampling phase is mandatory to assess the real PAH impact on health as well as emission sources affecting the study areas. Introduction Polycyclic aromatic hydrocarbons (PAHs) are organic compounds whose structure is formed by two or more condensed aromatic rings and are produced by every combustion process. Due to the ascertained carcinogenicity of many congeners, Directive 2008/50/EC regulates the PAH atmospheric concentrations. In particular, the concentration limit of benzo[a]pyrene (BaP) in PM10 is fixed to 1 ng m-3 (mean over a calendar year) and six other congeners must be monitored in order to assess the respective importance with regards to ambient air toxicity. The PAH determination in PM is affected by biases up from the collection step. In fact, being semi-volatile they can evaporate from the filter, stripped by the air flow. Moreover, PAHs can be decomposed by atmospheric oxidants such as ozone. These phenomena can result into significant losses that undermine the representativeness of collected PAHs with regards to air composition. In the framework of the implementing collection devices and procedures able to minimize the PAH sampling artifacts, important laboratory experiments have been carried out in order to evaluate distinctly the effects of volatilization and oxidation on each of PAH compounds. Methodology and Results Twin pairs of samples were obtained by using a Hydra dual sampler modified for the purpose. Filters for the particulate collection were simultaneously exposed over 48 hours at a flow rate equal to 2.3 m3/h, during the winter season. After sampling one filter of each pair was analyzed, the latter was treated to investigate oxidative degradation or volatilization suffered by PAHs. To determine oxidative degradation (ageing), filters were tested with an air flow spiked with known concentrations of ozone; in this case, the losses due to volatilization were minimized through using flow rates one order of magnitude lower than those used in in-field tests and the time of exposure was reduced to one hour. Three ozone levels were chosen, i.e. 0.4 0.8 and 1.2 ppm*h. To assess volatilization from filters, the second filter was exposed to another sampling cycle (48h, 2.3 m3/h) at 19±2 °C. An absolute filter was located upstream of the PM-loaded filter in order to allow zero air crossing the sample and, by consequence, putting in the evidence the neat effects of PAH stripping and evaporation. Volatilization generated losses from negligible (e.g., indeno[1,2,3-cd] pyrene) up to 55% (benzo[a]anthracene), while the benzo[a]pyrene losses associated to oxidative degradation could exceed 50%. Conclusions Concerning the oxidative degradation, especially the BaP losses can lead to underestimate the environmental toxicity associated to PAHs, while differences between the congeners in the volatilization or in the reactivity to ozone can originate differences between the real distribution pattern of PAHs in the atmosphere and those in the collected samples. This phenomenon must be adequately taken in account whereas the statistical multivariate approach is applied to determine the emission sources affecting any study area.