Long-range transport episodes of fine particles in southern Finland during 1999–2007

Abstract

The frequency, strength and sources of long-range transport (LRT) episodes of fine particles (PM_2.5) were studied in southern Finland using air quality monitoring results, backward air mass trajectories, remote sensing of fire hot spots, transport and dispersion modelling of smoke and chemical analysis of particle samples (black carbon, monosaccharide anhydrides, oxalate, succinate, malonate, SO₄²⁻, NO₃⁻, K⁺ and NH₄⁺). At an urban background site in Helsinki, the daily WHO guideline value (24-h PM_2.5 mean 25 μg m⁻³) was exceeded during 1–7 LRT episodes per year in 1999–2007. The 24-h mean maximum concentrations varied between 25 and 49 μg m⁻³ during the episodes, which was 3–6 times higher than the local mean concentration (8.7 μg m⁻³) in 1999–2007. The highest particle concentrations (max. 1-h mean 163 μg m⁻³) and the longest episodes (max. 9 days) were mainly caused by the emissions from open biomass burning, especially during springs and late-summers in 2002 and 2006. During the period 2001–2007, the satellite remote sensing of active fire hot spots and transport and dispersion modelling of smoke indicated that approximately half of the episodes were caused partly by the emissions from wildfires and/or agricultural waste burning in fields in Eastern Europe, especially in Russia, Belarus and Ukraine. Other episodes were mainly caused by the LRT of ordinary anthropogenic pollutants, e.g. from energy production, traffic, industry and wood combustion. During those ‘other episodes’, air masses also arrived from Eastern Europe, including Poland. The highest concentrations of biomass-burning tracers, such as monosaccharide anhydrides (levoglucosan + mannosan + galactosan) and K⁺, were observed during open biomass-burning episodes, but quite high values were also measured during some winter episodes due to wood combustion emissions. Our results indicate that open biomass burning in Eastern Europe causes high fine particle concentration peaks in large areas of Europe almost every year.

Introduction

The composition and mass concentrations of fine particles (PM_2.5) are substantially affected by long-range transport (LRT), especially in areas with low local emissions (EMEP, 2006, WHO, 2006). LRT can cause high PM_2.5 concentration peaks when air masses arrive during suitable meteorological conditions (no rain and weak mixing of air masses) from regions with high emissions of particles and/or their precursor gases. During LRT episodes, particles can cause detrimental health effects far from their emission sources, including rural background areas (WHO, 2006).

In Finland, anthropogenic emissions of fine particles and their precursor gases are low compared with the more polluted regions of Europe (EMEP, 2006). In general, atmospheric PM_2.5 concentrations are also low in Finland. For instance, the annual mean PM_2.5 concentrations are ∼10–12 μg m⁻³ at urban traffic sites of Helsinki and only ∼7–8 μg m⁻³ at a rural background site (Luukki) near Helsinki (Laakso et al., 2003, Myllynen et al., 2007). The new directive on ambient air quality and cleaner air for Europe (Directive, 2008/50/EC) sets annual limit value for PM_2.5 (25 μg m⁻³ to be met before year 2015) but the value is very high compared with the Finnish concentration levels. However, the daily particle concentrations in Finland vary substantially due to LRT. Even in the urban areas of Helsinki, 50–75% of the PM_2.5 mass originates from LRT (Karppinen et al., 2004, Vallius et al., 2003). During the strongest LRT pollution episodes, the 24-h mean particle mass concentrations multiply compared with the annual mean levels, and the WHO guideline value (25 μg m⁻³) for 24-h PM_2.5 mean concentrations is exceeded over large areas of southern Finland (Niemi et al., 2004, Niemi et al., 2005, Niemi, 2007, Saarikoski et al., 2007, Anttila et al., 2008).

The emission sources of fine particles during LRT episodes have been studied in detail during few LRT episodes in Finland. On 19–20 February 1998, an LRT episode peak was caused by sea salt from the Atlantic Ocean and by anthropogenic emissions from Western Europe (Tervahattu et al., 2002). During the years 2002, 2004 and 2006, emissions from wildfires and agricultural waste burning in fields together with other anthropogenic emissions in Eastern Europe caused several LRT episodes of fine particles in large areas over Finland (Niemi et al., 2004, Niemi et al., 2005, Niemi et al., 2006b, Saarikoski et al., 2007, Saarnio et al., 2006, Anttila et al., 2008). Although the above-mentioned episodes have been studied in detail, the long-term comprehensive view on LRT episodes has been missing so far in Finland.

In this work, the frequency, duration and strength of LRT episodes of fine particles in southern Finland were investigated in 1999–2007, using long-term air quality monitoring data. The potential source areas of particles and the presence of open biomass burning as a particle source were estimated during episodes in 2001–2007, based on backward air mass trajectories, satellite remote sensing of active fire hot spots and transport modelling of smoke. Several chemical components were analysed for source characterisation during the episodes in 2001–2005. To our knowledge, this is the first study that illustrates the long-term perspective on LRT fire smoke episodes in northern Europe.