Development and application of diode laser based optical sensors for continuous monitoring of trace atmospheric constituents

• Main Author: Aziz, Md. Shafiqul Islam
• Published: University of Bristol 2011
• Subjects: 628.530284
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.570714

An improved version of an automated near infra-red continuous wave cavity ring down spectrometer with sample preconcentration has been developed, for the detection of trace gases in the atmosphere and isotopologue ratio analysis. Three different lasers operating at wavelengths in the range 1.5-1.6 urn have been used for the experiments in conjunction with three different pre-concentration systems. The instrument has been used for three separate studies: continuous measurement of ethene (C2H4) in Bristol air; detection of nitrous oxide (N20) in ambient air; and acetylene isotopologue ratio (13CH4/12CH4) analysis in cigarette smoke samples. Mixing ratios of ethene have been observed to vary from 0.6 ppbv to 1.2 ppbv in Bristol air with a strong influence of the prevailing wind direction. Winds from the west and the north were observed to decrease and increase the ethene mixing ratio in Bristol air respectively. The observed variations have been discussed with consideration of probable sources and sinks and different meteorological parameters. Nitrous oxide in Bristol air has been observed to have a mixing ratio of ~320 ppb; consistent with global abundance of this long-lived greenhouse gas. Acetylene isotopologue ratios have been measured in smoke samples from two different brands of cigarette chosen as a test source of C2H2 from combustion of plant material. The average s\3e value of the five consecutive determinations for each cigarette brand was found to be -11.86 ± 0.55 %0 (Marlboro) and -4.98± 0.39 %0 (Lambert & Butler). The observed isotopologue ratios match closely with similar measurement of different samples derived from biomass burning conducted in different laboratories and analysed using isotopologue ratio mass spectrometry (IRMS). An improved version of a mid infra-red cavity enhanced absorption spectroscopy (CEAS) apparatus has been developed for isotopologue ratio analysis in gas samples. A difference frequency generation (DFG) based laser system was used as source of mid-IR radiation and a lock-in amplifier was used to demodulate the signal from sources of noise. The spectrometer has been used successfully to conduct methane isotopologue ratio analysis (13CH4/12CH4) in a 5% methane mixture in argon and carbon dioxide (COL) buffer gas. An average isotopologue ratio value of -47.3 ± 2.3 %0 has been observed which matches closely with two separate measurements on this standard sample, one using IRMS and another using a commercial near IR cavity ring down spectrometer.