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Measurement of PM2.5 is carried out simultaneously outdoors as well as indoors using a Compact and Useful PM2.5 Instrument (CUPI) from Nov 2018 to Oct 2019. A high-time resolution data (~ 2 min interval) was recorded at Dwarka (West Delhi), New Delhi, to investigate the difference between outdoor and indoor ambient particulate matter. Year-long observations provide an opportunity to decipher that indoor PM2.5 concentration increases significantly from mid-November 2018 to mid-December 2018. During this high concentration occurrence of PM2.5, outdoor concentration was higher in the range of 100–200 µg/m3 than indoor. Indoor pollution was less by 30–50% in comparison to the outdoor environment. Analyses reveal that during a high pollution period, intermittently for a short period, outdoor PM2.5 was less, which coincides with clear weather days suggesting that indoor pollution did not disperse due to a closed environment. Diurnal variation of PM2.5 clearly shows high concentration stays until 12:00 noon in both indoor and outdoor environments, which reduces in the afternoon and again picks up in the evening. Diurnal variation of solar radiation showed a consistent increase from 9 am to 2 pm, while relative humidity declined considerably from 10 am to 4 pm (local time); these two factors correspond to less concentration in both indoor and outdoor environments. However, there is no significant difference between outdoor and indoor concentration during summer and monsoon season.