The use of tunnel concentration profile data to determine the ratio of NO₂/NO_x directly emitted from vehicles

Abstract. Recently, it is reported that primary vehicular NO₂/NO_x ratio to be 10–30% and primary vehicular NO₂ has raised much interest and concern in the control of NO₂ in urban areas. In this study, primary vehicular NO₂/NO_x ratio in Hong Kong was investigated based on intensive long tunnel (3.7–4 km in length) experiments where concentration profiles of air pollutants along the entire lengths of the tunnels were obtained. Long tunnels were selected because of the inherent low O₃ concentrations in the partially enclosed environment. In addition the concentrations of pollutants from vehicles are high. Thus, the NO₂ measured inside long tunnels would be more representative of the primary NO₂ emitted by vehicles and contribution due to atmospheric transformation would be limited. This dataset was supported by a long-term on-road air quality dataset (June 2002–August 2003). Both datasets were obtained using the Mobile Real-time Air Monitoring Platform (MAP). The primary on-road vehicular NO₂/NO_x ratio was less than 2%, detected in the mid sections of tunnels investigated, where O₃ concentration was at a minimum. In sections of the tunnels (entrance and exit) where O₃ concentrations were relatively high, the NO₂/NO_x ratio could be as high as 19%. Long-term (annual average) on-road air quality data in open air yielded NO₂/NO_x ratios up to 28%. Thus, it is apparent that directly emitted NO₂ from vehicles is not significant in atmospheric NO₂ concentration. A simple model was used to segregate the contribution of background NO₂ and transformed NO₂ measured in vehicle plumes.