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Nitrous oxide (N2O) is an important anthropogenic greenhouse gas emitted into the atmosphere that can contribute to ozone destruction. Considering its environmental importance, the real-time monitoring and molecule-specific detection of atmospheric N2O with high sensitivity have received much attention in the 21stcentury. In this study, a widely tunable continuous wave (cw) external-cavity quantum cascade laser (EC-QCL)-based cavity ring-down spectroscopy (CRDS) in the mid-infrared region has been used to measure the mixing ratios of N2O in ambient air. The detection of atmospheric N2O was made using a rotationally resolved R(8e) absorption line of N2O centred at 1887.666 cm−1. Several atmospheric air samples were collected at various locations in Kolkata on seven consecutive days in different periods of the day. In situmeasurements were carried out by the EC-QCL-based high-resolution cw-CRDS method. The laser-based CRDS sensor allowed us to perform direct, quantitative and selective measurements of atmospheric N2O mixing ratios at the levels of parts per billion by volume (ppbv). A significant change in N2O levels was observed in different sub-areas depending on the source of local pollution. We also observed a marked difference in N2O levels between morning and afternoon sessions of the day in a particular sub-area. The CRDS sensor for the detection of N2O allows a minimum detectable absorption coefficient of αmin= 4.8 × 10−9cm−1and an estimated detection limit of 4.5 ppbv at atmospheric pressure is also reported.