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We present the first real-time composition of submicron particulate matter (PM1) in Old Delhi using high resolution aerosol mass spectrometry (HR-AMS). Old Delhi is one of the most polluted locations in the world, and PM1 concentrations reached ~ 600 µg m−3 during the most polluted period, the post-monsoon, where PM1 increased by 178 % over the pre-monsoon period. Using positive matrix factorisation (PMF) to perform source apportionment analysis, two burning-related factors contribute the most (35 %) to the post-monsoon increase. The first PMF factor, semi-volatility biomass burning organic aerosol (SVBBOA), shows a high correlation with earth observation fire counts in surrounding states which links its origin to crop residue burning. The second is a solid-fuel OA (SFOA) factor with links to local open burning due to its high composition of polyaromatic hydrocarbons (PAH) and novel AMS measured marker species for polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). Two traffic factors were resolved, one hydrocarbon-like OA (HOA) factor and another nitrogen-rich HOA (NHOA) factor. The N compounds within NHOA were mainly nitrile species which have not previously been identified within AMS measurements. Their PAH composition suggests that NHOA is linked to diesel, and HOA to compressed natural gas and gasoline. These factors combined make the largest relative contribution to primary PM1 mass during the pre-monsoon and monsoon periods, while contributing the second highest in the post-monsoon. A cooking OA (COA) factor shows strong links to the secondary factor, semi-volatility oxygenated OA (SVOOA). Correlations with co-located volatile organic compound (VOC) measurements and AMS measured organic nitrogen oxides (OrgNO) suggest SVOOA is formed from aged COA. It is also found that a significant increase in chloride concentrations (488 %) from pre-monsoon to post-monsoon correlates well with SVBBOA and SFOA suggesting that crop residue burning and open waste burning are responsible. A reduction in traffic emissions would effectively reduce concentrations across most of the year. In order to reduce the post-monsoon peak, sources such as funeral pyres, solid waste burning and crop residue burning should be considered when developing new air quality policy.