The origin of particles within the atmosphere has long been subject to debate. The problem is even more critical in urban areas, due to their potential harmful impact on public health. Recently, isotope tools have proved their great added value for unambiguously deciphering the origin of particles (both PM2.5 and PM10), which could lead to a better design of management plans reducing aerosol levels in the atmosphere. A review of several isotope systems shows that they may certainly help to distinguish between natural versus anthropogenic, or mineral versus organic origins of these particles.
Natural or anthropogenic, mineral or organic? These are some of the main questions to be solved regarding aerosols in urban atmospheres. Their presence and the health impacts have been well documented for a long time as there is evidence that particulate matter in the air has a direct influence on mortality levels and on the number of hospital admissions due to respiratory diseases. Aerosols vary in size and exhibit a wide range of chemical compositions. The origin of these particles may be either i) primary (direct emission into the atmosphere through processes of combustion or natural alteration) or ii) secondary (post-emission formation in the atmosphere, e.g. nitrate in aerosols which results mainly from the degradation and conversion of gaseous nitrous oxides).
The air in many big cities contains high concentrations of fine particles. The concentration of PM10 particles can be as high as 300 µg.m-3. Fine atmospheric particles have a damaging effect on public health and have recently become a cause of major concern. So far, little stable-isotope work has been conducted on atmospheric particles in urban environments and even fewer studies have used nitrogen isotopes, although preliminary results showed they could be a potential tracer of their origin.
The particles having aerodynamic diameters less than or equal to 10 µm often display a bimodal distribution with fine particles (diameters <2.5 µm) and coarse particles (diameters in the range 2.5-10 µm). These two size fractions generally have different sources (e.g. origins), properties (chemical and physical), spatial distributions and deposition processes. PM2.5 results primarily from combustion processes and gas-to-particle conversions, while particles in the coarse size fraction (PM10) may be linked to mechanical processes (e.g. wind-blown dust). The sources of particles can be subdivided into three groups: (a) sea salt aerosols, (b) terrestrial aerosols (soil dust, biological emissions), and (c) anthropogenic sources (industry, agriculture, burning of vegetation and fossil fuels, fertilizers). Therefore particles typically consist of a mixture of inorganic and organic chemicals, including carbon, sulfates, nitrates, metals, acids, and semi-volatile compounds. Terrestrial aerosols may originate from soil erosion close to the collector (local source), from distant areas within the continent (external source) or even from other continents (exogenic source). Likewise, aerosols derived from anthropogenic sources may originate in the vicinity of the collector (local source) or at large distances (distant source). Thus the relative abundance of particles is a function of the importance of the production processes and the distance from sources and coarser material should have a shorter lifetime. PM10 may remain airborne from periods of time ranging from a few hours to several weeks.
Our group, so far, has focused on the following isotope systematics: C, N, S, Sr, Pb, Zn, Cd, Cr and Hg.