Abstract
Abstract Semi-volatile polycyclic aromatic hydrocarbons (PAHs) partition between the vapour and condensed phases, both of which undergo oxidation. However, the relative contributions of oxidation in the vapour and condensed phases remain controversial. This is particularly important in respect of benzo(a)pyrene, which is responsible for a substantial proportion of the carcinogenic risk associated with airborne PAHs and is used as a marker for the PAH mixture in air quality standards. There is substantial evidence from field measurements that B(a)P is less reactive in the atmosphere than low molecular weight PAHs, but many uncertainties remain regarding the rate and determinants of its atmospheric decay. Using novel methodology, changes in the ratios of PAHs during atmospheric transport are used to infer the concentrations of OH and NO3 radicals responsible for the gas phase oxidation of low molecular weight PAHs. These are then used to examine the reactivity of B(a)P during atmospheric transport, and it is concluded that the loss of B(a)P can plausibly be described by the vapour phase reaction with the OH radical. These results offer an alternative mechanism to some others involving heterogeneous chemistry proposed in the literature.
Highlights
Polycyclic aromatic compounds (PAHs) are a group of semi-volatile compounds which undergo atmospheric transport, but the main mechanisms by which they are oxidised are a matter of controversy, despite considerable research
These are used to examine the reactivity of B(a)P during atmospheric transport, and it is concluded that the loss of B(a)P can plausibly be described by the vapour phase reaction with the OH radical
The WHO Unit Risk Factor for the polycyclic aromatic hydrocarbons (PAHs) mixture is set in terms of a concentration of B(a)P, and the WHO Review of the Health Aspects of Air Pollution (REVIHAAP, 2013) highlighted the relatively high level of risk associated with exposure to 1 ng m−3 of B(a)P and implicitly recommended consideration of a stricter guideline
Summary
Polycyclic aromatic compounds (PAHs) are a group of semi-volatile compounds which undergo atmospheric transport, but the main mechanisms by which they are oxidised are a matter of controversy, despite considerable research. We use atmospheric field measurements to make inferences as to the mechanism of oxidation of the semi-volatile PAH congener, benzo(a)pyrene, generating results which may be of relevance to the atmospheric oxidation of semi-volatile compounds more generally. Acknowledging the known carcinogenicity of PAHs, the European Union has set a target level of 1 ng m−3 of benzo(a)pyrene (B(a) P) taken as a representative of the entire mixture (European Union, 2004). The UK Air Quality Objective for PAHs is somewhat stricter, i.e. 0.25 ng m−3 of B(a)P taken as a representative of the mixture (EPAQS, 1995). The WHO Unit Risk Factor for the PAH mixture is set in terms of a concentration of B(a)P, and the WHO Review of the Health Aspects of Air Pollution (REVIHAAP, 2013) highlighted the relatively high level of risk associated with exposure to 1 ng m−3 of B(a)P and implicitly recommended consideration of a stricter guideline. A modelling study of the continental US concluded that the incremental lifetime cancer risk from B(a)P exposure exceeds 1 × 10−5 in many areas (Zhang et al, 2016)
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