Mobile monitoring reveals the importance of non-vehicular particulate matter sources in London†

This study uses mobile monitoring to gain a better understanding of particulate matter (PM) sources in two areas of Central and Outer London, UK. We find that, unlike emissions of nitrogen oxides (NO + NO₂ = NO_x), which are elevated in Central London due to the high number of diesel vehicles and congestion, fine particulate matter (PM_2.5) emissions are well-controlled. This finding provides evidence for the effectiveness of vehicle particulate filters, supporting the view that their widespread adoption has mitigated PM_2.5 emissions, even in the highly dieselized area of Central London. However, mobile monitoring also reveals infrequent elevated PM_2.5 concentrations caused by malfunctioning vehicles. These events were confirmed through simultaneous measurements of PM_2.5 and sulfur dioxide (SO₂), the latter being a strong tracer of engine lubricant combustion. A single event from a gasoline car, representing just 0.15% of the driving distance in Outer London, was responsible for 7.4% of the ΔPM_2.5 concentration above background levels, highlighting the ongoing importance of addressing high-emission vehicles. In a novel application of mobile monitoring, we demonstrate the ability to identify and quantify non-vehicular sources of PM. Among the sources unambiguously identified are construction activities, which result in elevated concentrations of coarse particulate matter (PM_coarse = PM₁₀ − PM_2.5). The mobile measurements clearly highlight the spatial extent of the influence of such sources, which would otherwise be difficult to determine. Furthermore, these sources are shown to be weather-dependent, with PM_coarse concentrations reduced by 62.1% during wet conditions compared to dry ones.