Abstract

Ultrafine particles (UFP) are quickly transformed within a few 100 m distance to the source because of aerosol dynamic processes. In order to predict the transformation of particle number size distributions (PNSD) near major streets, we developed a sectional model that represents aerosol dynamics within an air parcel that is transported from a major street along a minor street into the urban background. Simplifying assumptions on the transport and transformation due to dilution, coagulation, deposition and condensation were introduced to allow fast prediction with a limited number of model parameters. Model predictions were compared to observed PNSDs from semi-mobile measurement campaigns along 200 m long transects in Berlin, Germany. The total number concentration (TNC) along the transects declined on average by 30% for the evaluated measurement runs. The model agreed well with the observed PNSDs (coefficient of determination, R2 = 0.94). The model was sensitive to the selection of a dilution parameter b (+8/-11% TNC change due to variation by factor 2) and friction velocity u∗ (up to 3% TNC change by replacing parameterized u∗ by constant values in a similar range), but almost not sensitive to the other parameters. According to the model, the dilution contributed the most to the TNC decline (approx. 73% after 200 m transport distance), followed by coagulation and deposition (13 and 14% contribution, respectively). Due to the low computational effort of the model, it may contribute to real-time forecasting of PNSDs near major streets and to determining short-term exposure to particles.

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