We hereby present the variability of fine particles (PM1 ≤ 1 micron) that has not yet been investigated under the influence of COVID-19-induced lockdown in three major cities of different climatic zones in India. We unfold the baseline level of PM1, a level to which the population is chronically exposed and extremely critical for epidemiologists for fixing health markers. This has been achieved using the saturation point methodology under fair weather conditions. The baseline level of PM1 for Delhi and Mumbai is found to be 13 µg m− 3 and 9 µg m− 3, respectively. The processes leading to a significantly higher decline in the level of PM1 as compared to coarser particles (PM2.5) are discussed. The varying magnitude of the declining trends in PM1 is found to be linked to the intensity of residential emissions, which vary from one city to another and was exempted during lockdown. Our findings are critical in understanding the dominant role played by different sources of PM1 in framing effective health risk management policies.
