Determinants of black carbon, particle mass and number concentrations in London transport microenvironments

We investigated the determinants of personal exposure concentrations of commuters’ to black carbon (BC), ultrafine particle number concentrations (PNC), and particulate matter (PM₁, PM_2.5 and PM₁₀) in different travel modes. We quantified the contribution of key factors that explain the variation of the previous pollutants in four commuting routes in London, each covered by four transport modes (car, bus, walk and underground). Models were performed for each pollutant, separately to assess the effect of meteorology (wind speed) or ambient concentrations (with either high spatial or temporal resolution). Concentration variations were mainly explained by wind speed or ambient concentrations and to a lesser extent by route and period of the day. In multivariate models with wind speed, the wind speed was the common significant predictor for all the pollutants in the above-ground modes (i.e., car, bus, walk); and the only predictor variable for the PM fractions. Wind speed had the strongest effect on PM during the bus trips, with an increase in 1 m s⁻¹ leading to a decrease in 2.25, 2.90 and 4.98 μg m⁻³ of PM₁, PM_2.5 and PM₁₀, respectively. PM_2.5 and PM₁₀ concentrations in car trips were better explained by ambient concentrations with high temporal resolution although from a single monitoring station. On the other hand, ambient concentrations with high spatial coverage but lower temporal resolution predicted better the concentrations in bus trips, due to bus routes passing through streets with a high variability of traffic intensity. In the underground models, wind speed was not significant and line and type of windows on the train explained 42% of the variation of PNC and 90% of all PM fractions. Trains in the district line with openable windows had an increase in concentrations of 1 684 cm⁻³ for PNC and 40.69 μg m⁻³ for PM_2.5 compared with trains that had non-openable windows. The results from this work can be used to target efforts to reduce personal exposures of London commuters.