Development and application of an optical sensor for ethene in ambient air using near infra-red cavity ring down spectroscopy and sample preconcentration.

An automated near infra-red (IR) continuous wave cavity ring down spectrometer with sample preconcentration has been developed for the measurement of ethene (C₂H₄) in air. The spectrometer incorporated a distributed feedback diode laser operating at wavelengths λ∼ 1.6 μm and a pre-concentration system containing an adsorbent, molecular sieve 4A (MS4A). An absorption line located at 6148.58 cm⁻¹, and free from spectral overlap with other atmospheric molecules, was used for ethene detection. The spectrometer has a capacity for determination of atmospheric ethene mixing ratios at half hour time intervals, with a detection limit (2 SD above baseline noise) of 280 ppt. Both weekday and weekend measurements were performed in ambient air for periods of up to 30 hours. Average daytime mixing ratios of ethene were observed to be 2 ppbv and 1 ppbv during weekdays and weekends respectively. The mixing ratios of ethene varied from 0.6 ppbv to 1.2 ppbv in Bristol air during the weekend, with influence of meteorological conditions. The observed variations are discussed with consideration of probable sources and various meteorological parameters. A correlation is observed in the mixing ratio of ethene and nitrogen dioxide.