Diverging long-term trends in ambient urban particle mass and number concentrations associated with emission changes caused by the German unification

Air quality in urban areas has considerably changed over time. Recent changes of particle emissions in Eastern Germany during the last 10 years provide a quick motion picture of a development which lasted several decades in Western countries. Size distributions of fine particles (<2.5 μm aerodynamic diameter) were measured during winters 1991/1992 through 2000/2001. Within these 10 years particle size distributions shifted strongly towards smaller sizes. This was mainly due to a twofold increase of the number concentration of 0.01–0.03 μm sized particles. In parallel, fine particle mass concentration decreased by 75%. Since the mass concentration of fine particles is considered most relevant for human health today, the observed substantially decreasing trend in Eastern Germany is a desirable development. It is, however, accompanied by an increase of the number concentration of ultrafine particles in the 0.01–0.03 μm size range for which most recently associations with health effects have been found. This development associated with rapid emission changes caused by the German unification may be considered as a time-lapsed development representing partially unrecorded temporal alterations of emissions in a large entity of Western industrialized urban environments over the last five decades. Thus, the worldwide policy to reduce only the mass concentration of ambient aerosols without controlling for the number concentration may not be sufficient to assess health effects associated with urban particles.