Cross road and mobile tunable infrared laser measurements of nitrous oxide emissions from motor vehicles

Context Abstract: Nitrous oxide (NO2) is a potent greenhouse gas whose atmospheric budget is poorly constrained. One known atmospheric source is the formation of N2O on three-way motor vehicle catalytic converters followed by emission with the exhaust. Previous estimates of the magnitude of this N2O source have varied widely. Two methods employing tunable infrared lasers to measure N2O/CO2 ratios from a large number of on-road motor vehicles have been developed. Both methods add support to lower estimates of N2O emissions from the US motor vehicle ̄eet, although signi®cant uncertainty remains. Main Abstract: Two tunable infrared laser di€erential absorption spectroscopy (TILDAS) techniques have been used to measure the N2O emission levels of on-road motor vehicle exhausts. Cross road, open path laser measurements were used to assess N2O emissions from 1361 California catalyst equipped vehicles in November, 1996 yielding an emission ratio of …8:8 2:8† 10ÿ5 N2O/CO2. A van mounted TILDAS sampling system making on-road N2O measurements in mixed trac in June, 1998 in Manchester, New Hampshire yielded a mean N2O/CO2 ratio of …12:8 0:3† 10ÿ5, based on correlated N2O and CO2 concentration peaks attributed to motor vehicle exhaust plumes. The correlation of N2O emissions with vehicle type, model year and NO emissions are presented for the California data set. It is found that the N2O emission distribution is highly skewed, with more than 50% of the emissions being contributed by 10% of the vehicles. Comparison of our results with those from four European tunnel studies reveals a wide range of derived N2O emission indices, with the most recent studies (including this study) ®nding lower values. Ó 2000 Elsevier Science Ltd. All rights reserved.