Propagation of atmospheric aerosol and the area of representativeness of its measurements in the Baltic Sea region

Area of representativeness (AR) of atmospheric aerosol measurements and horizontal dimensions of a cloud of particles are estimated by time series of aerosol particle size number spectrum measured simultaneously at two locations. The distances between sampling points are varied, 7, 30, 50, 100 or 375 km. The spectrum in form of 12 fraction concentrations is measured every 10 or 15 min during five 2–4 week measurement campaigns in Estonia. During one measurement campaign one measurement point was in Estonia (Tahkuse) and the other in Finland (Hyytiälä). Two electrical aerosol spectrometers (EAS) designed at University of Tartu, having diameter range 10 nm–10 μm, were used. At Hyytiälä, the particle size spectrum (d=3–500nm) was measured by two Hauke type differential mobility analysers (DMA). The transport of the submicron fraction (d=60nm–1μm) of atmospheric aerosol is investigated during the time intervals when an air mass moves from the one measurement point towards the other or crosswise to this direction. Sample cross-correlation functions (SCCF) are calculated between prewhitened time series. The time lag corresponding to the mode of these functions are found to be 3–5 h for upwind–downwind case and zero for crosswind case. This time lag should indicate the mean time of pollution transport for upwind–downwind case. A certain minimal distance between sampling points should exist, when statistically significant correlation between the time series cannot be found. Considering this circumstance, the half-width of the AR is estimated. It is found to be more than 100 km but less than 375 km when the sampling points are located on the same line with the wind direction; and more than 50 km but less than 100 km when the sampling points are located on a straight line crosswise to the wind direction. The pollutant concentrations in the limits of AR are estimated by regression analysis. To support these results, the dispersion calculations by HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectories) model are used.