INSTITUTE OF PHYSICS PUBLISHING MEASUREMENT SCIENCE AND TECHNOLOGY Meas. Sci. Technol. 15 (2004) 2295–2302 PII: S0957-0233(04)78648-8 On-road measurement of automotive particle emissions by ultraviolet Lidar and transmissometer: theory P W Barber, H Moosm ¨ uller, R E Keislar, H D Kuhns, C Mazzoleni and J G Watson Division of Atmospheric Sciences, Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512-1095, USA E-mail: hans.moosmuller@dri.edu Received 24 March 2004, in final form 8 July 2004 Published 8 October 2004 Online at stacks.iop.org/MST/15/2295 doi:10.1088/0957-0233/15/11/015 Abstract An on-road vehicle emissions remote sensing system (VERSS) utilizing an ultraviolet laser (operating at a wavelength of 266 nm) has been developed to quantify the particulate mass in vehicle exhaust. The system simultaneously measures the backscatter and the transmission of the laser light. Obtaining the particulate matter mass concentration from the measurements depends upon a prior laboratory calibration as well as knowledge of the physical, chemical and optical characteristics of the exhaust particles. Keywords: particulates, Lidar, scattering, transmissometer, extinction, exhaust 1. Introduction On-road remote sensing is an efficient method for determining the tailpipe emissions from passing vehicles [1]. While routine inspection and maintenance programmes, commonly referred to as ‘smog tests’, measure emissions from single vehicles every year or two for limited operational conditions, remote sensing offers the opportunity to individually sample tens of thousands of vehicles in a matter of days in a real-world setting [2]. Furthermore, the measurements can be repeated at a specific location at different times, separated by days to years, to assess temporal changes in the vehicle emissions for a specific area [3]. In all these cases, a snapshot of the exhaust emission rates of the on-road vehicle fleet at the time of measurement is obtained. These measurements can be used to define the contribution of total vehicle emissions to the air pollution in a specific area and to identify groups of vehicles, even specific vehicles and operating conditions, that result in very high emissions. These data provide important information for the design of mitigation strategies. Until recently, vehicle exhaust remote sensing systems have only measured exhaust gases—commercial systems are available for nitrogen oxide (NO), hydrocarbons (HC), carbon monoxide (CO) and carbon dioxide (CO 2 ). The most widely used systems direct ultraviolet and infrared beams across a single lane of traffic to a mirror that returns the beam to a detector collocated with the beam source [4]. Filters at the detector isolate the absorption bands of the different gases of interest and a reference band that is unaffected by the gases present. An ambient measurement prior to vehicle passage provides the background level. Fleet-average on-road data for the three exhaust gases, NO, HC and CO, have been shown to correlate well with data from inspection and maintenance programmes [5]. Although there are many ways to present the measured data, one approach is to calculate a ‘fuel-based emission factor’ for each exhaust gas in units of g of pollutant emitted per kg of fuel consumed. The emitted carbon, mostly in the form of CO 2 , CO and HC, is proportional to the fuel consumed [6]. Therefore, fuel-based emission factors for NO, HC and CO can be obtained by taking the ratio of the measurement for each particular gas to the total carbon measurement [7]. With new concerns about visibility [8, 9] and the impact of vehicle emissions on the overall environment and human health [10], there is also interest in measuring particulate matter (PM) in vehicle exhaust. A remote measurement of PM mass would permit the determination of a fuel-based emission factor for PM as PM mass emitted per fuel mass 0957-0233/04/112295+08$30.00 © 2004 IOP Publishing Ltd Printed in the UK 2295