title: Radical precursors and related species from traffic as observed and modeled at an urban highway junction.
abstract: 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 Traffic emission data for both compounds are scarce and mostly outdated.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 (NO(x)) ratio of 6.01 +/- 0.15 (r2 = 0.91), which is in agreement with other field studies. Both MOBILE6 and MOVES overestimate this emission ratio by 92% and 24%, 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/NO(x) and HCHO/CO ratios are largely due to higher NO(x) 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/NO(x) emission ratio is around 0.017 +/- 0.0009 kg HONO/kg NO(x) which is twice as high as in MOVES. The observed NO2/NO(x) emission ratio is around 0.16 +/- 0.01 kg NO2/kg NO(x), 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/NO(x) overestimation is coming from light-duty gasoline vehicles.
authors.affiliation: Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204, USA. brappenglueck@uh.edu
authors.forename: Bernhard
authors.initials: B
authors.lastname: Rappenglück
authors.affiliation: Houston-Galveston Area Council, Houston, TX, USA.
authors.forename: Graciela
authors.initials: G
authors.lastname: Lubertino
authors.affiliation: Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204, USA.
authors.forename: Sergio
authors.initials: S
authors.lastname: Alvarez
authors.affiliation: Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204, USA.
authors.forename: Julia
authors.initials: J
authors.lastname: Golovko
authors.affiliation: Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204, USA.
authors.forename: Beata
authors.initials: B
authors.lastname: Czader
authors.affiliation: Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204, USA.
authors.forename: Luis
authors.initials: L
authors.lastname: Ackermann
mesh_terms: D000393:Air Pollutants / Q000032:analysis*; D005557:Formaldehyde / Q000032:analysis*; D008962:Models, Theoretical; D009608:Nitrous Acid / Q000032:analysis*; D001335:Vehicle Emissions / Q000032:analysis*
chemical_list: D000393:Air Pollutants; D001335:Vehicle Emissions; D005557:Formaldehyde; D009608:Nitrous Acid