Observation of nocturnal NO<sub>3</sub> during vehicular activities in the medium sized city of Calicut in coastal India

Abstract

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.