Environ. Sci. Technol. 2008, 42, 7354–7359
Automated System for Monitoring Trace C2H2 in Ambient Air by Cavity Ring-Down Spectroscopy Combined with Sample Preconcentration MANIK PRADHAN, M. S. I. AZIZ, ROBERTO GRILLI, AND ANDREW J. ORR-EWING* School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, U.K.
Received May 19, 2008. Revised manuscript received July 4, 2008. Accepted July 21, 2008.
A fully automated instrument combining a continuous wave cavity ring-down spectrometer and dual-trap sample preconcentration has been implemented for monitoring C2H2 mixing ratios in ambient air. A distributed feedback diode laser operating in the near-infrared region (λ ∼ 1534.973 nm in air) detects C2H2 in absorption via the P(17) rotational line of the (ν1 + ν3) vibrational combination band. The instrument is shown to be capable of fast, quantitative, and precise monitoring of C2H2 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 C2H2 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.
1. Introduction Acetylene (C2H2) is a nonmethane volatile organic compound (NM-VOC) that is emitted into the Earth’s atmosphere almost exclusively by human activities (1, 2). Its mixing ratios in the troposphere are typically in the range 0.15-2.5 parts per billion by volume (ppbv, 1 part in 109) in rural environments, as compared with measured levels up to ∼155 ppbv in urban areas (3, 4). It reacts with the hydroxyl radical (OH) during the daytime and contributes to the formation of excess ground-level ozone, which is a major constituent in photochemical smog (5, 6). C2H2 is, however, one of the longestlived NM-VOCs, with tropospheric lifetimes that range from days to weeks because of its stability to photochemical oxidation (7). The relatively extended atmospheric lifetime combined with the fact that it has solely anthropogenic sources has motivated quantitative measurements of C2H2 as a tracer for polluted air masses (1-3, 6). Long-term measurements should allow an assessment of changes in relative strengths of sources and sinks, and a continuous record could provide information about the wider trends in the oxidizing potential of the troposphere. * Corresponding author phone: +44 117 928 7672; fax: +44 117 925 0612; e-mail:
[email protected]. 7354
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 19, 2008
Automated gas chromatography (GC) separation coupled with either flame ionization detection (FID) or mass spectrometry (MS) has been adopted for simultaneous and continuous long-term monitoring of tropospheric C2H2 and other VOCs at mixing ratios down to as low as a few parts per trillion by volume (pptv, 1 part in 1012) (8-11). Although traditional GC-based methodologies are highly reliable when combined with air sample preconcentration techniques, there remain some intrinsic drawbacks. In particular, instruments require frequent calibration to ensure the accuracy of measurements because they do not provide directly quantitative determination of mixing ratios. The temporal resolution of the measurements is constrained by the need for comprehensive GC separation of a large number of compounds (taking ∼90 min), so short-time scale changes in mixing ratios cannot be observed. Continuous monitoring over extended periods imposes some technical and logistical difficulties if, for example, cryogens are required to cool adsorbent traps or standard, dilute gas mixtures are needed for instrument calibration. Continuous wave cavity ring-down spectroscopy (cwCRDS) is a sensitive and quantitative absorption spectroscopy method that is based on the measurement of the rate of decay of light from a high-finesse optical cavity (12-14). It has key advantages of being calibration-free (as long as the analyte sampling method introduces no losses) and providing high spatial and temporal resolution. In recent years, CRDS has been applied to atmospheric sensing of trace VOCs via overtones of C-H stretching vibrations or combination bands in the near-infrared (IR) region (15-17). This wavelength region is chosen because of the convenience of use of lowcost diode lasers and associated optical components of the types developed for use in the telecommunications industry. Although the CRDS detection provides high sensitivity and spectroscopic selectivity, the detection limits may not be sufficient in the near-IR for the direct detection of ambient tropospheric C2H2 at the trace levels expected in rural or many urban environments. The combination of cw-CRDS with air sample preconcentration on an adsorbent trap, as exploited for GC-based detectors, can, however, bring VOC levels up to the detection range of a cw-CRDS instrument, as was previously demonstrated (18, 19). Recently, we developed an approach based on combining a dual-trap sample preconcentration system with cw-CRDS for direct detection of C2H2 mixing ratios in ambient air (20). The prototype instrument performance was tested by analysis of commercial standard gas samples and intercomparisons with a calibrated GC-FID instrument, and an estimated detection limit of 35 pptv was reported. Analyses were conducted on air samples collected both inside and outside the laboratory but provided no information on temporal variation of mixing ratios. In this paper, we describe further development to achieve a fully automated, CRD-based spectrometer for selective monitoring of acetylene in air samples over extended time periods. The resultant instrument is computer-controlled through digital and serial communication protocols, operates unattended, does not require calibration, and preconcentrates ambient C2H2 prior to measurement without the need for cryogens. It employs a distributed feedback (DFB) diode laser operating in the near-IR. The spectrometer has a limit of detection of ∼8 pptv and can selectively determine C2H2 mixing ratios over a large dynamic range in the presence of other VOCs without the need for GC separation. The spectrometer therefore has considerable potential to be deployed as a field instrument for single-compound moni10.1021/es801378r CCC: $40.75
2008 American Chemical Society
Published on Web 08/19/2008
FIGURE 1. Schematic diagram of the automated preconcentration system and CRD spectrometer. OI, optical isolator; AOM, acousto-optic modulator; RDC, ring-down cavity; MFC, mass flow controller; SV, solenoid valve. toring in rural and urban environments as an alternative approach to GC-FID or GC/MS detection. We illustrate its performance by in situ measurements at 30 min intervals of C2H2 mixing ratios in indoor and outside air for extended periods during both the day and nighttime. The main features of the C2H2 mixing ratios observed during these measurements are discussed.
2. Experimental Section A schematic diagram of the experimental set up used to combine dual-trap air sample preconcentration with nearIR cw-CRDS is shown in Figure 1. Aspects of the apparatus have been described previously (20); here, we provide a brief overview of the light source and ring-down cavity (RDC), then describe the air sampling, analyte preconcentration, and automation processes. The output from a near-IR DFB diode laser (Marconi Optical Components, LD 6204) operating at wavelength λ ∼ 1.535 µm with an output power of
