Aerosol size distributions measured in urban, rural and high-alpine air with an electrical low pressure impactor (ELPI)

An electrical low pressure impactor (ELPI) was used to study atmospheric aerosol particle number, surface, and mass concentrations and size distributions over a diameter range of 7 nm–10 μm at urban, rural and high-alpine locations along an alpine altitude transect across Southern Germany. The measurements were performed in the city of Munich and at the global atmosphere watch (GAW) stations Hohenpeisenberg and Zugspitze in the years 2001–2004. To minimize particle bounce effects and enable chemical analysis of the collected particles without disturbance by grease on the impaction substrates, the sample flow was conditioned to about 75% relative humidity. The performance of the ELPI instrument was evaluated by comparison with well-established aerosol measurement techniques including condensation particle counters, scanning and differential mobility particle sizers, filter sampling, and gravimetric determination of particulate mass. In general, particle number concentrations, size distributions, and PM2.5 concentrations determined with the ELPI were in good agreement with alternative techniques (rank correlation coefficients ρ = 0.70–0.95). The ELPI filter stage data for the particle diameter range of 7–30 nm, however, appeared to be strongly biased towards high values. Long-term measurements at the rural site (Hohenpeisenberg) revealed distinct seasonal patterns with the highest number concentrations in summer (median daily average: 3100 cm−3) and the highest mass concentrations in spring and fall (median daily average PM2.5 and PM10: 21–25 and 27–35 μg m−3, respectively). In spring and fall we also observed pronounced maxima of particle surface and mass concentration in the coarse mode (peak at ∼3 μm), which are most likely due to primary biological material. Relatively clean air (PM10 = 10 μg m−3).