Seasonal and long-term trends in atmospheric PAH concentrations: evidence and implications

Abstract

Atmospheric monitoring data for selected polynuclear aromatic hydrocarbons (PAHs) were compiled from remote, rural and urban locations in the UK, Sweden, Finland and Arctic Canada. The objective was to examine the seasonal and temporal trends, to shed light on the factors which exert a dominant influence over ambient PAH levels. Urban centres in the UK have concentrations 1–2 orders of magnitude higher than in rural Europe and up to 3 orders of magnitude higher than Arctic Canada. Interpretation of the data suggests that proximity to primary sources ‘drives’ PAH air concentrations. Seasonality, with winter (W) > summer (S), was apparent for most compounds at most sites; high molecular weight compounds (e.g. benzo[a]pyrene) showed this most clearly and consistently. Some low molecular weight compounds (e.g. phenanthrene) sometimes displayed S>W seasonality at some rural locations. Strong W>S seasonality is linked to seasonally-dependent sources which are greater in winter. This implicates inefficient combustion processes, notably the diffusive domestic burning of wood and coal. However, sometimes seasonality can also be strongly influenced by broad changes in meteorology and air mass origin (e.g. in the Canadian Arctic). The datasets examined here suggest a downward trend for many PAHs at some sites, but this is not apparent for all sites and compounds. The inherent noise in ambient air monitoring data makes it difficult to derive unambiguous evidence of underlying declines, to confirm the effectiveness of international source reduction measures.

Introduction

Polynuclear aromatic hydrocarbons (PAHs) are amongst the groups of compounds defined as ‘persistent organic pollutants (POPs)’ and subject to international atmospheric emissions controls under the 1998 United Nations Economic Commission for Europe (UNECE) protocol (UN ECE, 1998, Vestreng and Klein, 2002). PAHs are subject to long-range atmospheric transport (LRAT) and there are concerns over the carcinogenicity of some PAH compounds (UN ECE, 1998, Vestreng and Klein, 2002, IPCS, 1998). Signatories to the ‘POPs protocol’ undertake to reduce atmospheric emissions of PAHs to the levels of the reference year 1990. Some countries have adopted, or are considering, air quality standards for selected PAHs; the United Kingdom has a proposed annually averaged standard for benzo[a]pyrene of 0.25 ng/m³, for example. This value can be exceeded in both urban and rural areas (Lohmann et al., 2000).

These regulatory developments raise interesting scientific issues: (a) are the major PAH sources and national emissions inventories well enough established, now and for the 1990 reference year, to ensure compliance with the ‘POPs protocol’?; (b) what are the trends in atmospheric concentrations of PAHs over the last decade or so?; (c) how variable are PAH concentrations seasonally and spatially?; and, (d) what are the implications of this variability for sources and compliance with an annually averaged air quality standard?

Despite several years of study, there is still considerable uncertainty over several aspects of the atmospheric sources and behaviour of PAHs. For example, whilst some inventories point towards domestic burning of coal and wood as the dominant source of PAHs to the atmosphere, others implicate emissions from vehicles, or metal smelting/process operations (Vestreng and Klein, 2002, Wild and Jones, 1995). Without reliable information on sources, it is difficult to conceive how a country can accurately assess whether it is reducing emissions in line with its commitments to international agreements.

One useful approach to help distinguish between the dominant source categories is to examine ambient monitoring data. For example, if ambient air measurements display seasonality, this would provide clues about the dominant sources; some sources are seasonal (e.g. domestic heating; natural fire events), whilst others are not (e.g. industrial combustion, aluminium and coke production, petroleum refining). However, air concentrations are controlled by a complex array of variables, as depicted in Fig. 1. Some of these factors may also influence the seasonality in ambient air measurements, notably secondary sources of PAHs into the atmosphere (i.e. possible volatilisation from soil, water, vegetation or/and urban surfaces); atmospheric loss/removal processes, such as wet deposition, reactions with OH radicals, scavenging by vegetation; ‘dilution/advection factors’, influenced by wind speed and direction and mixed boundary layer height. Finally, temperature changes drive the gas:particle distribution and atmospheric reaction rates of PAHs.

In this paper, data from monitoring programmes were compiled and assessed, to evaluate the underlying trends and seasonality of PAH air concentrations. Data were considered for different compounds from a range of countries (UK, Sweden, Finland and Arctic Canada) and environments (urban, rural, coastal, remote). These datasets were selected because they provided time series over several years. They constitute some of the few consistent sources of measurement data available internationally. Our objective was to examine the spatial and temporal trends, to shed light on the factors which exert a dominant influence over ambient PAH levels, and to briefly consider the implications for sources and regulation.