Nitrous acid (HONO) and formaldehyde (HCHO) are important precursors for radicals and are believed to favor ozone formation significantly. Traffic emission data for both compounds are scarce and mostly outdated. A better knowledge of today’s HCHO and HONO emissions related to traffic is needed to refine air quality models. Here the authors report results from continuous ambient air measurements taken at a highway junction in Houston, Texas, from July 15 to October 15, 2009. The observational data were compared with emission estimates from currently available mobile emission models (MOBILE6; MOVES [Motor Vehicle Emission Simulator]). Observations indicated a molar carbon monoxide (CO) versus nitrogen oxides (NOx) ratio of 6.01 0.15(r2 ¼ 0.91), which is in agreement with other field studies. Both MOBILE6 and MOVES overestimate this emission ratio by 92% and24%, respectively. For HCHO/CO, an overall slope of 3.14 0.14 g HCHO/kg CO was observed. Whereas MOBILE6 largely underestimates this ratio by 77%, MOVES calculates somewhat higher HCHO/CO ratios (1.87) than MOBILE6, but is still significantly lower than the observed ratio. MOVES shows high HCHO/CO ratios during the early morning hours due to heavy duty diesel off-network emissions. The differences of the modeled CO/NOx and HCHO/CO ratios are largely due to higher NOx and HCHO emissions in MOVES (30% and 57%, respectively, increased from MOBILE6 for 2009), as CO emissions were about the same in both models. The observed HONO/NOx emission ratio is around 0.017 0.0009 kg HONO/kg NOx which is twice as high as in MOVES. The observed NO2/NOx emission ratio is around 0.16 0.01 kg NO2/kg NOx, which is a bit more than 50% higher than in MOVES. MOVES overestimates the CO/CO2 emission ratio by a factor of 3 compared with the observations, which is 0.0033 0.0002 kg CO/kg CO2. This as well as CO/NOx overestimation is coming from light-duty gasoline vehicles.
|Journal||Journal of the Air and Waste Management Association|
|Publication status||Published - 2013|