Nighttime measurements of ambient N 2O 5, NO 2, NO and O 3 in a sub-urban area, Toyokawa, Japan

Abstract

We report on the development of a highly sensitive instrument for measuring ambient N 2O 5 molecules and its application to nighttime measurements of N 2O 5 in a sub-urban area, Toyokawa, Japan, during February 2006. The mixing ratio of N 2O 5 was quantified by cavity ring-down spectroscopy detection of NO 3 at 662 nm, in which the NO 3 radicals were produced by thermal decomposition of the N 2O 5 molecules. The minimum detection limit (1 σ) for N 2O 5 was estimated to be 2.2 pptv in a 100 s averaging time. The mixing ratios of ambient NO 2, NO and O 3 were simultaneously measured along with N 2O 5. The ambient N 2O 5 mixing ratios range from below the detection limit (2.2 pptv) to 20 pptv during nighttime. By applying the steady-state approximation on the data of 27–28 February when the surface wind field was characterized by northerly wind associated with winter monsoon surges over Japan, the heterogeneous loss rate of N 2O 5 was estimated. Based on the mixing ratios and the loss rate of N 2O 5, the nocturnal loss rate of NO x was evaluated as 0.5 ppbv per night for that night. In addition, the temporal variation of N 2O 5 concentration during several hours after the local sunset on that night was calculated using the time-dependent box model. By comparing the results with the observed data, significant contributions of gas-phase loss processes of NO 3 were inferred.