How COVID-19 related policies reshaped organic aerosol source contributions in central London

Abstract. Particulate matter (PM) poses both health and climate risks. Understanding pollution sources is therefore crucial for effective mitigation. Positive Matrix Factorization (PMF) of Aerosol Chemical Speciation Monitor (ACSM) data is a powerful tool to quantify organic aerosol (OA) sources. A year-long study of ACSM data from London's Marylebone Road monitoring station during the COVID-19 pandemic provides insights into the impact of lockdown and the Eat Out To Help Out (EOTHO) scheme, which offered support to the hospitality during the pandemic, on PM composition and OA sources. Five OA sources were identified including hydrocarbon-like OA (HOA, traffic-related, 11 % to OA), cooking OA (COA, 20 %), biomass burning OA (BBOA, 12 %), more-oxidized oxygenated OA (MO-OOA, 38 %), and less-oxidized oxygenated OA (LO-OOA, 21 %). Lockdown significantly reduced HOA (-52 %), COA (-67 %), and BBOA (-41 %) compared to their pre-COVID levels, while EOTHO increased COA (+38 %) significantly compared to the post-lockdown period. However, MO-OOA and LO-OOA were less affected, as these primarily originated from long-range transport. This research has highlighted the importance of commercial cooking as a significant source of OA (20 %) and PM₁ (9 %) in urban areas. The co-emission of BBOA with COA observed in Central London demonstrates a similar diurnal cycle and response to the EOTHO policy, indicating that cooking activities might be currently underestimated and contribute to urban BBOA. Therefore, more effort is required to quantify this source and develop targeted abatement policies to mitigate emissions as currently limited regulation is in force.