Estimation of Primary NO 2 /NOx Emission Ratio from Road Vehicles Using Ambient Monitoring Data

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Studies in Atmospheric Science Volume 1, 2014

www.seipub.org/sias

Estimation of Primary NO2/NOx Emission Ratio from Road Vehicles Using Ambient Monitoring Data Yasuyuki Itano*1, Makiko Yamagami2, Toshimasa Ohara3 Osaka City Institute of Public Health and Environmental Sciences 8-34 Tojo-cho, Tennoji-ku, Osaka 543-0026, Japan 2Nagoya City Institute for Environmental Sciences, 5-16-8 Toyoda, Minami-ku, Nagoya, Aichi 457-0841, Japan 3National Institute for Environmental Studies, 16-1 Onogawa, Tsukuba, Ibaraki 305-8506, Japan 1

*1

y-itano@city.osaka.lg.jp

Abstract We developed a new method to estimate the primary emission ratio of nitrogen dioxide (NO2) to total oxides of nitrogen (NOx: NO + NO2) from road vehicles. A pair of simultaneous monitoring data for NO2, NOx, and O3 concentrations at two neighboring roadside sites was used for estimation. The primary NO2/NOx emission ratio was estimated by adjusting the α value so that Ox′ ([Ox′] = [O3] + [NO2] – α × [NOx]) concentrations at the two sites would show the best consistency. The present method does not need to select a suitable background site, as is necessary in some previous methods. Furthermore, the present method seemed to estimate the ratio with a smaller error range than preceding works. The monthly estimated primary NO2/NOx emission ratio for Osaka City, Japan from May 2006–April 2007 ranged 0.10–0.15. These values showed good agreement with those estimated from different datasets from another pairs of monitoring sites in Osaka City. An increasing trend of the ratio was observed in Osaka City for 2003–2010. Keywords Automobile Exhaust; Oxidant; Ptential Ozone; Urban Air Pollution; Vehicular Emissions

Introduction It is well recognized that atmospheric nitrogen dioxide (NO2) adversely affects human health. Furthermore, it plays a key role in photochemical air pollution, especially as the direct precursor of tropospheric ozone (O3). The World Health Organization (2006) has set a guideline value for atmospheric NO2 (40 μg/m3 for annual mean, 200 μg/m3 for 1-hour mean) for worldwide use. However, NO2 concentrations remain at high levels in large cities of the world (Baldasano et al., 2003). In Japan, air pollution from NO2 is a crucial concern,

especially at sites that are strongly affected by vehicular exhaust. In 1992, the Law Concerning Special Measure for Total Emission Reduction of Nitrogen Oxides from Automobiles in Specific Areas was issued, under which the use of some older vehicles in specific areas has been prohibited, and introduction of low-emission vehicles has been promoted. Consequently, average NO2 concentrations have recently shown a decreasing trend. However, the rate of the reduction is slower than that observed in total oxides of nitrogen (NOx: NO + NO2) concentration (Itano et al., 2007). Carslaw and Beevers (2005) showed similar trend in London, UK, where NO2 concentrations showed no clear trend during 1997–2003 though NOx was reduced significantly, and pointed out the increase in the primary NO2/NOx emission ratio from vehicles as the possible factor. To settle NO2 pollution problems in urban roadside environments, the primary NO2/NOx emission from vehicles is an important issue (Carslaw and Beevers, 2004a). Preceding works demonstrated that the primary NO2/NOx emission ratio varies among different engine types, exhaust treatment systems, cruising speeds, and accelerations (Lennerm et al., 1983; Soltic and Weilenmann, 2003; Alvarez et al., 2008; Kobayashi et al., 2008). The primary NO2/NOx emission ratios from the vehicular fleet under actual driving conditions were estimated from the proportion of increment in oxidant (Ox, [Ox] = [O3] + [NO2]) concentrations versus to that in NOx concentrations at a roadside site (Kimura and Shiihashi, 1988; Clapp and Jenkin, 2001; Minoura and Ito, 2010). Itano et al. (2007) roughly estimated the ratio by plotting hourly NO2/NOx ratios versus NOx concentrations at a roadside, which converged at

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