Observation of nocturnal NO₃ during vehicular activities in the medium sized city of Calicut in coastal India

Abstract. Nitrate radical (NO₃) is the most important nocturnal oxidant in urban atmosphere. The city of Calicut (Kozhikode) is a medium sized urban location in India. One of the bus terminals at Palayam part of the city [11.2495° N, 75.7842° E] is adjacent to the vegetable sorting facilities cum market, both of which have intense activities by buses and trucks from about 3 AM till about 8 PM on all working days. We report preliminary measurements observing NO₃ on five nights during a weeklong measurement campaign in the autumn of 2018. Measurements were made between 10 PM and 6 AM, and focus was when diesel vehicles were found idling along about half a km stretch during 3–6 AM while the loading/unloading of vegetables at sorting facility happens. Incoherent Broadband Cavity Enhanced Absorption Spectroscopic technique in open-path configuration (OP-IBBCEAS) was employed for measurements. The instrument was installed 8.5 m above the ground level over the east wing of Palayam bus terminal building adjacent to the vegetable market. The 285 cm long optical resonator was arranged on a custom-made aluminium profile platform 1 m high. The stability of the instrument for the entire period of measurement was excellent, and high NO₃ mixing ratios with levels exceeding several hundred pptv were observed during early morning hours when heavy vehicles were idling. The highest NO₃ mixing ratio observed was (497 ± 140) pptv during one of the nights. The fit uncertainty, which was considered as the uncertainty in retrieved concentration, was found to increase with increased aerosol loading. The uncertainty for a spectral averaging interval of 10 min was recorded as ~20 pptv and ~100 pptv during the lowest and the highest aerosol loading events respectively.