Automated System for Monitoring Trace C_2H_2 in Ambient Air by Cavity Ring-Down Spectroscopy Combined with Sample Preconcentration

摘要

A fully automated instrument combining a continuous wave cavity ring-down spectrometer and dual-trap sample preconcentration has been implemented for monitoring C_2H_2 mixing ratios in ambient air. A distributed feedback diode laser operating in the near-infrared region (λ ～ 1534.973 nm in air) detects C_2H_2 in absorption via the P(17) rotational line of the (ν_1 + ν_3) vibrational combination band. The instrument is shown to be capable of fast, quantitative, and precise monitoring of C_2H_2 mixing ratios, with a detection limit of ～8 pptv (parts per trillion by volume). It thus has potential to be deployed for analysis of air samples in many rural and urban environments. In situ measurements were carried out at 30 min intervals over periods of up to 15 h on several days for indoor and outdoor air samples. For indoor air monitored on a Sunday, the C_2H_2 mixing ratio was stable at 1.45 ± 0.04 ppbv (parts per billion by volume). On weekdays, both indoor and outside air analyses showed peaks in the range 2-4 ppbv in the early morning and late afternoon that coincided with periods of busy road traffic.