Multicomponent remote sensing of vehicle exhaust by dispersive absorption spectroscopy. 2. Direct on-road ammonia measurements

Remote sensing was employed for the first time to measure NH3 levels in the exhaust of on-road light duty motor vehicles. The sensor also measured the concentration of CO2, CO, hydrocarbons, and NO, among other pollutants, in the emitted exhaust. Field measurements were conducted at a Los Angeles freeway on-ramp; vehicles traveled at cruise speeds between 20 and 25 m s(-1) (45-55 mi h(-1)). Mean fleet NH3 levels of 44.7 +/- 4.1 ppm were observed. These emissions exhibited a highly skewed distribution: 50.1% of the emitted NH3 was contributed by 10% of the sampled fleet. The pollutant distribution among high NH3 emitters is analyzed to identify the conditions that lead to three-way catalyst malfunction and, hence, NH3 formation. In contradiction with previous reports, we found that high NH3 emissions could not be attributed to vehicles running under rich air-fuel conditions. We estimate a mean fleet NH3 mass emission rate of 667 57 mg L-1 (E-r = 94 +/- 8 mg km(-1)). These findings could have significant implications on air quality in the South Coast Air Basin (SoCAB) of California, since they support the hypothesis that emissions from motor vehicles constitute a dominant regional source of NH3, between 20 and 27% of total daily emissions. As NH3 is the predominant atmospheric base, tropospheric levels play a key role in the buffering capacity of the atmosphere and, hence, the formation of fine aerosol. Our results could explain the ubiquitous distribution of ammonium fine particles observed during fall stagnation conditions in the SoCAB.