A European aerosol phenomenology – 3: Physical and chemical characteristics of particulate matter from 60 rural, urban, and kerbside sites across Europe

Abstract

This paper synthesizes data on aerosol (particulate matter, PM) physical and chemical characteristics, which were obtained over the past decade in aerosol research and monitoring activities at more than 60 natural background, rural, near-city, urban, and kerbside sites across Europe. The data include simultaneously measured PM₁₀ and/or PM_2.5 mass on the one hand, and aerosol particle number concentrations or PM chemistry on the other hand. The aerosol data presented in our previous works (Van Dingenen et al., 2004, Putaud et al., 2004) were updated and merged to those collected in the framework of the EU supported European Cooperation in the field of Scientific and Technical action COST633 (Particulate matter: Properties related to health effects). A number of conclusions from our previous studies were confirmed. There is no single ratio between PM_2.5 and PM₁₀ mass concentrations valid for all sites, although fairly constant ratios ranging from 0.5 to 0.9 are observed at most individual sites. There is no general correlation between PM mass and particle number concentrations, although particle number concentrations increase with PM_2.5 levels at most sites. The main constituents of both PM₁₀ and PM_2.5 are generally organic matter, sulfate and nitrate. Mineral dust can also be a major constituent of PM₁₀ at kerbside sites and in Southern Europe. There is a clear decreasing gradient in SO₄²⁻ and NO₃⁻ contribution to PM₁₀ when moving from rural to urban to kerbside sites. In contrast, the total carbon/PM₁₀ ratio increases from rural to kerbside sites. Some new conclusions were also drawn from this work: the ratio between ultrafine particle and total particle number concentration decreases with PM_2.5 concentration at all sites but one, and significant gradients in PM chemistry are observed when moving from Northwestern, to Southern to Central Europe. Compiling an even larger number of data sets would have further increased the significance of our conclusions, but collecting all the aerosol data sets obtained also through research projects remains a tedious task.

Introduction

Because aerosols affect human health, ecosystems (acidification, eutrophication) and visibility, emission abatement measures have been implemented over the last decades to reduce particle matter (PM₁₀ and PM_2.5) concentrations. As the legislation on particulate emission and air pollution is based on PM mass all over the world and is becoming more and more stringent (WHO, 2005), PM₁₀ and PM_2.5 concentrations are expected to further significantly decrease over the next decades. But can we assert that future PM concentration reductions will show positive effects on human health and ecosystems? Can we forecast the impact PM abatement measures on climate global and regional radiative forcing? Aerosol impacts are indeed linked to different characteristics that do not necessarily co-vary. Climate effects are related to its optical properties (scattering, absorption) throughout the atmospheric column, and cloud condensation formation potential, which in turn depends on particle size distribution and chemical composition. Impact on ecosystems depends on the deposition flux of specific species (acids, oxidized and reduced nitrogen, base cations, etc…). And we do not know yet which PM properties cause the health effects, although several epidemiological studies highlighted a link between short or long term exposure to PM mass concentration and human health (Zanobetti and Schwartz, 2005, Pope et al., 2008, Boldo et al., 2006, Pope, 2007, Miller et al., 2007). Therefore, abatement policies might be unnecessarily costly, inefficient, and even counter-productive if they address PM mass concentrations only. A thorough knowledge of the aerosol would help in designing a better legislation, using potential synergies and anticipating possible trade-offs between measures aiming at limiting air pollution and climate change. But PM₁₀ and more recently also PM_2.5 mass concentrations are still the only aerosol metrics measured systematically in national and international air pollution monitoring networks.

Our goal is to show how different (or similar) other aerosol characteristics can be among various sites located in different regions across Europe, even where equal PM mass concentrations are observed. These characteristics are generally not available from regulatory monitoring networks (see e.g. EMEP and AirBase data banks), but rather from research projects. We have already pursued this process in two previous works (Van Dingenen et al., 2004, Putaud et al., 2004). The present study revises our previous conclusions in the light of data collected from more than 30 new sites. In view of the current European directive on particulate air pollution (2008/50/EC), we stratified the data according to PM mass concentration bins to address the following questions: (1) can PM_2.5 and PM_coarse mass concentrations be inferred from PM₁₀ and PM_2.5 mass concentrations, respectively, (2) which PM constituents are responsible for PM₁₀ high mass concentrations, and (3) can sub-μm and ultrafine particle number concentrations be estimated based on PM_2.5 mass concentration measurements. Answers are modulated according to the type of sites, from natural background to kerbside sites, and their location in Northwestern, Southern or Central Europe.