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 Traffic emission data for both compounds are scarce and mostly outdated.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 AE 0.15 (r 2 ¼ 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 AE 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 heavyduty 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 AE 0.0009 kg HONO/kg NO x which is twice as high as in MOVES. The observed NO 2 /NO x emission ratio is around 0.16 AE 0.01 kg NO 2 /kg NO x , which is a bit more than 50% higher than in MOVES. MOVES overestimates the CO/CO 2 emission ratio by a factor of 3 compared with the observations, which is 0.0033 AE 0.0002 kg CO/kg CO 2 . This as well as CO/NO x overestimation is coming from light-duty gasoline vehicles.Implications: Nitrous acid (HONO) and formaldehyde (HCHO) are important precursors for radicals that ultimately contribute to ozone formation. There still exist uncertainties in emission sources of HONO and HCHO and thus regional air quality modeling still tend to underestimate concentrations of free radicals in the atmosphere. This paper demonstrates that the latest U.S. Environmental Protection Agency (EPA) traffic emission model MOVES still shows significant deviations from observed emission ratios, in particular underestimation of HCHO/CO and HONO/NO x ratios. Improving the performance of MOVES may improve regional air quality modeling.

Impact of updated traffic emissions on HONO mixing ratios simulated for urban site in Houston, Texas

“…NEI provides emission rates for nitrogen oxides; during the processing with SMOKE NO x emissions for mobile sources are separated into 90 % NO, 9.2 % NO 2 , and 0.8 % HONO. However, Rappenglueck et al (2013) report much higher HONO contribution from mobile sources in Houston; based on all measurements HONO traffic emissions are 1.7 % of NO x emissions, which is about twice the previously estimated value of 0.8 % based on tunnel measurements in 2001. The HONO / NO x ratio reported by Kurtenbach et al (2001) is based on measurements performed between 6 a.m. and 2 p.m., for both weekdays and weekends where 22 200 ± 400 vehicles were passing on weekdays and 13 300 ± 1 400 cars passing on weekends.…”

“…The vehicle fleet was composed of 6.0 % heavy-duty trucks, 6.0 % commercial vans, 12 % diesel and 75 % gasoline powered passenger cars, and 1.0 % motorcycles. The ratio calculated by Rappenglueck et al (2013) is based on measurements performed during weekdays reflecting high-traffic, early morning conditions (4-8 a.m.). The measurements were performed at highway junction in Houston with very high traffic load (about 400 000 vehicles passing daily), which is much larger than that in the tunnel study.…”

Letter to the Editor on Wormhoudt, J.; Wood, E.; Knighton, W.; Kolb, C.; Herndon, S.; Olaguer, E. 2015. Vehicle emissions of radical precursors and related species observed in the 2009 SHARP campaign; J. Air Waste Manage. Assoc. 65: 699–706

“…It is not before 5:25 am CST that O 3 and PAN decrease to 5 ppb and 125 ppt, respectively, and that the NO 2 fraction of NO x decreases to about 70%. We thus believe that this data selection may have missed most of the rush hour impacts, which start around 5-6 am CST and peak around 8 am CST (see Figure 4 about the diurnal variation of VMT in Rappenglück et al, 2013). While it may be true that Wormhoudt et al considered the maximum morning peak in NO x , CO, and HONO in their data analysis represented in Figure 2, this may not have necessarily been the traffic rush hour peak, as the location of the monitor is not close to the freeway and it may have been masked by sources closer to the Moody Tower and the increasing boundary layer height after 6 am CST.…”

“…The motivation of the Rappenglück et al 2013 paper was to obtain measurements of mobile emissions in order to compare it with the mobile on-road emissions calculated by models MOBILE6 and MOVES2010 available at that time. This included all the various sources for mobile emissions represented in MOBILE6 and MOVES2010, the use of link-based information of the VMT and VMT mix [distribution of vehicle miles traveled (VMT) according to vehicle type and fuel], speed activity estimates and "off-network" emissions such as idling and starting trucks.…”

“…on May 24 sunrise was at 5:24 am CST. Rappenglück et al 2013 did not consider any data sets where global radiation was higher than 10 W m −2 . At this point it remains unclear whether the authors applied the same strict data screening as in Rappenglück et al 2013, or a different one since no detailed information related to data screening is given in the Wormhoudt et al paper.…”