Quantifying road traffic influence on coarse and accumulation mode particle number concentration in a street canyon

Air pollution caused by particulate matter (PM) originating from road traffic poses a serious threat to the health of urban residents. Concentration levels depend on meteorological conditions, microclimatic parameters affecting dispersion, the influence of individual emission sources, local orographic and urban factors, as well as chemical transformation processes. The aim of the study was to determine the relationship between road traffic intensity (vehicles/hour) and particle number concentrations of accumulation mode and coarse mode particles in the 0.3-10 µm size range (hereafter referred to as PNC_0.3-10), along with an assessment of statistical associations between traffic intensity and particle number concentrations. Achieving this objective required a quantitative evaluation of the main factors influencing PNC_0.3-10 measurements, particularly the contribution of road traffic. The results indicated that meteorological conditions, specifically wind speed and mixing layer height, were the most significant variables affecting PNC_0.3-10 levels, regardless of particle size range. Therefore, it was necessary to consider how to remove the influence of these dominant factors from the PNC_0.3-10 measurements. To this end, a modified meteorological normalization method was applied, allowing for the removal of both meteorological influences and other emission sources from the PNC_0.3-10 time series. A strong statistical association was observed between traffic intensity and normalized PNC_0.3-10 (nPNC_0.3-10) for particle fractions larger than 2.5 µm (r_Spearman > 0.85, adj. R² > 0.63). In contrast, no linear relationship was found for fractions in the range of 0.3 µm to 2.5 µm (r_Spearman < 0.46, adj. R² < 0.15). These results should be interpreted as empirical statistical relationships rather than direct evidence of linear emission mechanisms.