PAH sampling artifacts: evaluation and strategies to avoid them

Polycyclic aromatic hydrocarbons (PAHs) are organic compounds whose structure is formed by two or more condensed aromatic rings . According to the ascertained carcinogenicity of many PAH congeners, European Directive 2008/50/EC regulates the PM10 concentrations of the benzo(a)pyrene, chosen as chemical index of the atmospheric toxicity associated to PAHs, and dictates the monitoring of other 6 PAHs. It is well known that the particulate PAH determination is affected by biases already in the sampling step. In fact, PAHs are semi-volatile and the light congeners can evaporate from the collected substrate because of the pressure drop across the filter, or can be stripped by the air flow. Moreover, PAHs can be decomposed during the sampling phase in the presence of oxidants such as ozone. This phenomenon can result into significant losses of the target substances and especially of benzo(a)pyrene, which is particularly prone to oxidation (Brown and Brown 2013). Oxidative degradation and volatilization that occur during sampling undermine the global representativeness of collected particulates with regards to environmental PAHs. This aspect is very important whenever the environmental toxicity and the PAH contribution to it must be determined, and also in the case that the distribution pattern of the congeners is used to identify the emission sources affecting the air quality in study areas by means of source apportionment techniques. The reference procedures for the atmospheric PAH measurement are defined by the EN 15549:2008 (Standard Method for the Measurement of Benzo(a)pyrene in the Open Air) and Technical Specification CEN/TS 16645:2014. Both reference methods underline that PAH oxidative degradation during the sampling phase can be avoided by using an ozone denuder that selectively remove ozone from sampled air before it gets in contact with the particulate collected on the sampling filter. Nonetheless, the lack of studies aimed at testing and applying in the field the ozone denuder models developed till now prevents the use of these devices during the PAHs sampling. This study aimed at investigating both oxidative degradation and volatilization effects in charge of PAHs. The principal target was the implementation of strategies aimed at minimizing the sampling artifacts. Laboratory and in-field experiments were used for the evaluation of the individual PAH losses according to the different meteorological conditions, as well as to distinct sampling times and sampling start times. Besides, a denuder device for the ozone removal was manufactured and its efficiency was in-field tested by using a special sampling arrangement (see Figure 1) constituted by 6 sampling lines simultaneously operating. The following step was the evaluation of the differences in the atmospheric concentrations of PAHs collected with or without denuder system, according to different sampling times. The results confirm that in the summer period particulate PAHs collected according the procedure defined by the reference method EN 12341, but without the ozone scrubber, can suffer losses up to 30%. On the other hand, the losses induced by volatilization look more important than oxidative degradation for lighter congeners. The time of the sampling start can contribute to the artifacts reduction.