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. Air pollution in urban environments has been shown to have a negative impacton air quality and human health, particularly in megacities. Over recentdecades, Delhi, India, has suffered high atmospheric pollution, withsignificant particulate matter (PM) concentrations as a result ofanthropogenic activities. Organic aerosols (OAs) are composed of thousands ofdifferent chemical species and are one of the main constituents of submicronparticles. However, quantitative knowledge of OA composition, their sourcesand their processes in urban environments is still limited. This is importantparticularly in India, as Delhi is a massive, inhomogeneous conurbation,where we would expect the apportionment and concentrations to varydepending on where in Delhi the measurements/source apportionment isperformed, indicating the need for multisite measurements. This studypresents the first multisite analysis carried out in India over differentseasons, with a focus on identifying OA sources. The measurements were takenduring 2018 at two sites in Delhi, India. One site was located at the IndiaMeteorological Department, New Delhi (ND). The other site was located at theIndira Gandhi Delhi Technical University for Women, Old Delhi (OD).Non-refractory submicron aerosol (NR-PM1) concentrations (ammonium,nitrate, sulfate, chloride and organic aerosols) of four aerosol massspectrometers were analysed. Collocated measurements of volatile organic compounds, black carbon,NOx and CO were performed. Positive matrix factorisation (PMF) analysiswas performed to separate the organic fraction, identifying a number ofconventional factors: hydrocarbon-like OAs (HOAs) related to trafficemissions, biomass burning OAs (BBOAs), cooking OAs (COAs) and secondary OAs(SOAs). A composition-based estimate of PM1 is defined by combining black carbon (BC) andNR-PM1 (C-PM1= BC + NR-PM1). No significant difference wasobserved in C-PM1 concentrations between sites, OD (142 ± 117 µg m−3) compared to ND (123 ± 71 µg m3),from post-monsoon measurements. A wider variability was observed betweenseasons, where pre-monsoon and monsoon showed C-PM1 concentrations lowerthan 60 µg m−3. A seasonal variation in C-PM1 compositionwas observed; SO42- showed a high contribution over pre-monsoonand monsoon seasons, while NO3- and Cl− had a highercontribution in winter and post-monsoon. The main primary aerosol source wasfrom traffic, which is consistent with the PMF analysis and Aethalometermodel analysis. Thus, in order to reduce PM1 concentrations in Delhithrough local emission controls, traffic emission control offers thegreatest opportunity. PMF–aerosol mass spectrometer (AMS) mass spectra will help to improve futureaerosol source apportionment studies. The information generated in thisstudy increases our understanding of PM1 composition and OA sources inDelhi, India. Furthermore, the scientific findings provide significantinformation to strengthen legislation that aims to improve air quality inIndia.

PM&lt;sub&gt;1&lt;/sub&gt; composition and source apportionment at two sites in Delhi, India, across multiple seasons

PM<sub>1</sub> composition and source apportionment at two sites in Delhi, India, across multiple seasons