A tunable diode laser system for the remote sensing of on-road vehicle emissions

A tunable infrared laser differential absorption spectrometer (TILDAS) has been developed to measure airpollutant emission from on-road motor vehicles. The system uses a monostatic source–detector platform and a retroreflector. It has been shown to be able to measure the emissions from individual vehicles at highway speeds. Excellent detection sensitivity is obtained with measurement precision as small as 3 ppmv of the exhaust under ideal conditions. Long path lengths (100 m or more) can be achieved due to the coherent nature of laser radiation. NO, N2O, and NO2 emissions have been measured with this instrument. The system is also capable of measuring CO, NH3, H2CO, CH3OH, and other small molecules in vehicle exhaust. PACS: 42.68.Kh; 82.80.Ch; 42.55.Px Public demand for improved air quality drives the need to reduce air-pollutant emissions. Motor vehicles are thought to be the single largest source of many air pollutants in urban areas [1]. However, the exact amounts of air pollutants emitted by motor vehicles are highly uncertain. Many recent studies have found that even the best emission inventory models cannot consistently predict real-world emissions better than within a factor of 2 [1–7]. For these reasons considerable effort has recently been expended to develop instrumentation and measurement techniques which better specify CO, VOC, and NOx motor vehicle fleet emissions under actual operating conditions [8, 9]. Tunnel studies [2–5] can be used to obtain average fleet emission factors, while ambient air measurements in the vicinity of large roadways [6, 7] can assess the pollutant emission ratios (CO/NOx and VOC/NOx). However, neither of these methods can provide information about vehicle-to-vehicle variations in emissions. Although it is possible to fit individual vehicles with on-board instrumentation capable of pollutant monitoring, such instrumentation is necessarily limited to a small number of test vehicles and cannot measure the emissions of non-cooperating vehicles. Remote sensing instruments using open path spectroscopy can monitor individual vehicle emissions by scaling the pollutant column density to the exhaust carbon dioxide column density measured immediately behind the vehicle. These instruments have the potential to provide the best possible information on the actual in-use emission profiles. By simultaneously capturing a video image of the vehicle’s license plate, the distribution of emissions versus vehicle age, vehicle type, or manufacturer can be determined. Remote sensing can also be used as a way to identify high emitters and direct them to inspection and maintenance programs. These actions can lead to more cost-effective improvements in air quality [10, 11]. Non-dispersive infrared (NDIR) instruments to measure CO emissions have been pioneered by the University of Denver [12] and also demonstrated by General Motors [13] and Hughes Santa Barbara Research Center (SBRC) [14]. These three groups have also developed NDIR instruments which measure a portion of the VOC emissions [14–16]. Successful remote sensing measurements of exhaust NOx emissions from on-road vehicles has been more difficult. Field-proven instruments, based on either NDIR or ultraviolet absorption spectroscopy [14, 17] claim a precision (1σ) of about 300 ppm NO in the exhaust of the vehicle. Since most vehicles are thought to emit less than 300 ppm NO, a more precise instrument than the commercially available ones is needed. Such an instrument would also be advantageous for more precise identification of high-NO emitters. A new ultravioletbased remote sensor claims a standard deviation of about 20 ppm NO in the exhaust under parking-lot conditions [18], but on-road data have not yet been presented. A gas-filter correlation (GFCR) instrument which claims a potential sensitivity of 10 ppm NO is in the early stages of development [19]. In this paper we describe our tunable infrared laser differential absorption spectroscopy (TILDAS) remote sensing instrument. The TILDAS technique is also known as tunable diode laser (TDL) spectroscopy and tunable diode laser absorption spectroscopy (TDLAS). This instrument can be used to monitor the overwhelming majority of interesting atmospheric species, including CO, NO2, N2O, NH3, CH2O, and other small-molecule hydrocarbons. Due to its high spectral resolution, the TILDAS instrument is inherently very sensitive and interference free. In addition, the range of the instrument is excellent, allowing measurements of vehicle emissions across several traffic lanes. The primary purpose of its