Five-year PM Chemical Composition and Organic Aerosol (OA) Sources in a European Megacity, London

Improving air quality in urban areas is essential since 97% of European cities exceed the annual PM_2.5 value of the WHO guidelines (10 μg/m³). Therefore, in order to mitigate adverse health outcomes of air pollutants, it is extremely important to understand the long-term chemical compositions and organic aerosol (OA) sources in the largest European city, London. North Kensington (50.52° N, 0.21° W) is an urban background monitoring station located in a residential area of west London. A Quadrupole Aerosol Chemical Speciation Monitors, Q-ACSM (Aerodyne Research Inc., MA, USA), was deployed to continuously monitor submicron non-refractory particulate matter (NR-PM₁) from March 2013 until May 2018. The knowledge of OA sources is crucial not only because OA is the main constituent of particulate matter (PM), but also because different sources of OA are known to have different toxicities. Positive matrix factorization (PMF) on the OA matrix of ACSM data remains the most common technique to conduct source apportionment (SA) analyses. By following the standardized protocol developed by Chen et al. (2022), we have retrieved high-quality SA results in these 5 years using the most advanced SA techniques (i.e., rolling PMF (Parworth et al., 2015), the ME-2 solver (Paatero, 1999), and bootstrap resampling (Erfon, 1979)). Additionally, back trajectory analysis has helped identify the geographical origin of OA sources. A variety of auxiliary measurements have been conducted on the same site, which has been used to validate our SA results. Overall, this study provided a rare opportunity to investigate the long-term trends of PM composition and OA sources in a European megacity and provided valuable information for policymakers to understand the effects of mitigation strategies and design more efficient policies.