Organic marker signatures in atmospheric aerosols from different environments
Casimiro Pio & Célia Alves
The atmospheric carbonaceous aerosol is formed by a complex mixture of organic compounds, normally referred as organic carbon, and a mass of carbon atoms with a graphitic-like structure. Aerosols may have adverse environmental effects. A significant portion of atmospheric particles are respirable and contain toxic compounds that deposit in the respiratory tract, affecting human health. Carbonaceous aerosols interfere on climate by absorbing (for black carbon, BC) or dispersing the solar radiation (for organic carbon, OC). Particles may also participate indirectly on climate forcing if they can serve as cloud condensation nuclei. Although organic compounds constitute 10–70% of the total dry fine particle mass in the atmosphere, their concentrations and formation mechanisms are less well understood than those of other components. This is because particulate organic matter is an aggregate of hundreds of individual compounds whose concentrations cannot be characterised by a single analytical technique; more than half of the OC mass has not yet been identified as individual compounds.
The incomplete characterisation of particulate organics coupled with the complexity of the photochemical reactions that produce secondary organic aerosol (SOA) from volatile organic emissions has prevented the characterisation of sources and respective strengths. These limitations provided an impetus for the development of sampling and analytical methodologies aiming the detailed characterisation of atmospheric aerosols from different environments. The collected aerosol has been analysed in relation to black and organic carbon content by a home-made thermal-optical equipment, extracted with solvents, fractionated into different organic classes, derivatised in the case of the more polar constituents and characterised by Gas Chromatography - Mass Spectrometry (Fig. 1). Organic chemical markers have been used to characterise particulate matter of marine, forest, semi-rural, rural and urban European sites in order to provide chemical fingerprints which are source specific and useful for identifying single or multiple contributions in samples of atmospheric aerosols.
Figure 1. Scheme for the analysis of atmospheric particulate matter.
A comprehensive aerosol data set is now available for various environmental conditions in Europe (marine/continental, rural/urban, boundary layer/free troposphere, and winter/summer). The organic material consisted primarily of aliphatic hydrocarbons, acids, alcohols, and ketones, with a predominance of molecular weights > C20, derived from vascular plant waxes. Biomarkers for vegetation sources, such as phytosterols, were also detected. Microbial lipids (< C20), vehicle exhaust constituents (e.g. phytane and pristane), meat smoke tracers (e.g. cholesterol), pyrogenic aromatic hydrocarbons and wood smoke components (e.g. levoglucosan and resinic acids), were present in the various aerosol extracts. Source assignment of these primary atmospheric particulate matter constituents revealed a predominant contribution from plant wax components at the majority of sampling sites studied. However, exceptions were detected for some sites where the load of petroleum residues was prevalent, at least, during the winter period (Fig. 2). Photochemical constituents deriving from volatile organic compounds emitted by vegetation or from anthropogenic precursors were also identified, especially in forest environments. To our knowledge many of the SOA products have been detected for the first time in the field. It has been found that while fossil-related BC predominates throughout the year at all rural or semi-rural sites, the sources of OC are for the most part biogenic and markedly different between summer and winter. In winter biomass burning primary emission is the main source (mean contribution 67–81%), with some additional contribution from fossil fuel combustion, and biogenic SOA formation is almost negligible. In contrast, in summer the latter source becomes predominant (mean 49–67% of OC), but somewhat surprisingly SOA from fossil fuel combustion add a sizable fraction (mean 15–21%) to it, making total SOA over 70% of OC.
Figure 2. Seasonal source assignment for atmospheric aerosols at selected European sites.
1. Alves C., Oliveira T., Pio C., Silvestre A., Fialho P., Barata F. and Legrand M.(2006) Characterisation of carbonaceous aerosols from the Azorean island of Terceira. Atmospheric Environment (in press).
2. Carvalho A., Pio C., Santos C. and Alves C. (2006) Particulate carbon in the atmosphere of a Finnish forest and a German anthropogenically influenced grassland. Atmospheric Research, 80: 133-150.
3. Alves C., Pio C. Carvalho A. and Santos C. (2006) Atmospheric carbonaceous aerosols over grasslands of Central Europe and a boreal forest. Chemosphere, 63: 153-164.
4. Alves C., Carvalho A. and Pio C. (2002) Mass balance of organic carbon fractions in atmospheric aerosols. Journal of Geophysical Research, 107(D21): 8345-8353.
5. Alves C., Pio C. and Duarte A. (2000) Particulate size distributed organic compounds in a forest atmosphere. Environmental Science and Technology, 34: 4287-4293.
Casimiro Pio started his academic career as a chemical engineer, obtaining his graduation and Ph.D. degrees from the University of Oporto and University of Lancaster, respectively. His research interests lie within the field of air pollution and atmospheric chemistry. Now he is Full Professor at the Department of Environment, University of Aveiro, where he is also the director of the Centre for Environmental and Marine Studies. He has a publication H index of 20. In 2006 he received the Portuguese Science Foundation prize for Scientific Excellence.
Célia Alves got her Ph.D. in 2001 from the Department of Environment and Planning, University of Aveiro, working on the organic composition of atmospheric aerosols. Presently, she is assistant researcher in the Centre for Environmental and Marine Studies. She was the single author of one book and published over 20 SCI papers with about 200 citations. She has also been a regular reviewer of leader journals in Environmental Sciences, such as Atmospheric Environment, Environmental Science and Technology, Chemosphere and Science of the Total Environment.