Contribution of natural sources to air pollution levels in the EU - a technical basis for the development of guidance for the Member States


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Exceedences of air quality limit values represent breaches of Community law which can have significant legal consequences for the Member States. For some existing limit values, such as those in directive 1999/30/EC (1st Daughter Directive), an exceedence which is caused by particular natural sources can be ignored for the purposes of ensuring compliance with Community law. For example, Article 2.15 of the first daughter directive defines 'natural events' as volcanic eruptions, seismic activities, geothermal activities, wild-land fires, high wind events or the atmospheric resuspension or transport of natural particles from dry regions.

A new air quality directive proposal is currently being negotiated in the Council and the European Parliament and this is likely to extend this principle to natural (non-anthropogenic) sources of pollution generally so long as the 'natural contribution' can be quantified and documented. This could include sea-spray and biogenic organic aerosol amongst other materials. The aim of this report is to document that information and methodologies which are available to permit Member States to determine and document natural sources of air pollution. This information will later be incorporated into specific guidance to be developed by the
European Commission in the context of implementing the new legislation once adopted by the Council and the European Parliament.

In this report different types of natural sources contributing to PM levels in Europe are identified according to experts’ judgment and based on literature studies. The methods currently implemented by research groups of the Member States for the identification and quantification of natural sources are also described.

The content of this report is based on discussions with experts from various Member States, on the results of a questionnaire which has been circulated to experts of all EU Member States, on available literature studies and on the outcomes of a workshop on “Contribution of natural sources to PM levels in Europe” organized by the JRC in Ispra in October 2006; the workshop aimed at identifying the natural sources that may contribute to the different PM levels in Europe and the available methods to quantify this contribution.

In relation to the issue of compliance with Air Quality limit values the most important PM contributions by natural sources in Europe may be identified as wind-blown, long-range transported mineral dust and sea salt. However, a multitude of natural sources may affect PM levels at lower, but still appreciable extents:
- Primary biological aerosol particles (PBAPs), which include individual units as pollens or
spores, as well as fragmented material as plant debris.
- Biogenic non-sea-salt sulfur aerosol
-Volcanic activities, limited to restricted areas in Europe and only occasionally causing exceedences of PM limit values.

PM of other natural sources that originate outside of the EU that are transported over long distances into Europe should also be considered. Androgenic pollution events outside the EU which contribute to elevated air pollution levels in the Member States could also justify such an exceedence for the purposes of compliance with EC law, however, this is a distinct issue outside the remit of this report and which would need to be notified to the Commission on an adhoc basis.

As extensively discussed at the workshop, other sources may be natural in origin but strongly influenced by human actions. Among these are:

- Secondary organic aerosol (SOA) formed by oxidation of biogenic volatile organic compounds (VOC) in particular during summer time in vegetated areas. The most important interaction of biogenic VOC is with compounds that may be both anthropogenic and natural (NOx and O3), the associated complex chemical pathways still being an area of active research.
- Biomass burning and forest fires, generally of anthropogenic origin, may be in many cases controlled with appropriate actions
- Re-suspended particles may be of natural origin (crustal dust) and re-suspended by human activities, or anthropogenic.

For the purposes of compliance with air quality limit values, experts agreed that only contributions to PM from natural sources and that may not be influenced by human action can be deducted from PM levels according to the indications given in the directives. All contributions derived from interactions between natural and anthropogenic actions should no be considered. A number of methods for analyzing natural contributions to total PM are implemented by Member States; the apportionment may be done using different methodologies: 1) using routine measurement methods available in the existing Air Quality monitoring networks, 2) using information obtained in parallel from background stations and 3) using advanced tools implemented by research groups, including experimental analysis, satellite imaging and modeling. Many of these procedures however have the disadvantage to require continuous sampling and analysis for a number of components and may be very expensive because of the
advanced instrumentation that they require; consequently they cannot be applied routinely in EU air quality monitoring networks.

A simple method for the identification of natural contributions to PM consists of the combined monitoring of PM1 and PM10 concentrations. This methodology is based on the observation that mineral dust and sea salt, the most important natural source contributions to PM, are mainly in the coarse fraction (>90%). However, a change in the legal metrics of PM from the actual PM10 and PM2.5 to a future PM1 appears not feasible in the near future. Some procedures are implemented by few Member State on routine basis. Meteorology, satellite imagery and modeling tools are widely used to detect natural PM episodes. For those days exceeding the limit values, levels of natural PM contributions measured at the regional background monitoring sites are subtracted from the levels measured at urban agglomerations in order to determine the natural or anthropogenic origin of exceedences. This procedure is applied by a few Member States for African dust outbreaks.

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