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 fleet, although significant uncertainty remains.

Main Abstract: Two tunable infrared laser differential 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)×10⁻⁵ N₂O/CO₂. A van mounted TILDAS sampling system making on-road N₂O measurements in mixed traffic in June, 1998 in Manchester, New Hampshire yielded a mean N₂O/CO₂ ratio of (12.8±0.3)×10⁻⁵, 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) finding lower values.