NO_2-NO_x Ratios in Australian Road Tunnels

摘要

The precise measurement of NO_2 was historically difficult in road tunnels, hence criteria and design analysis was based on converting from NO_x concentrations via a set NO_2:NO_x ratio. The reliable direct measurement of NO_2 is now possible, however design analysis is still dominated by emissions tables and formulas specified in terms of NO_x. For converting from NO_x to NO_2 in a road tunnel environment in Australia, the ratio has typically been assumed to be ten per cent. This has generally been based on the recommendation in PIARC (2000) and on the 'average' value cited in PIARC (1995).Determining this ratio is important as changes in engine technology and in-tunnel pollution criteria along with improvements in CO and particulate emissions control, may mean that NO_2 will become the driving factor for future ventilation requirements. Correct selection of this ratio can directly impact the ventilation capacity and system design.Recent research has indicated that choosing a constant ratio of ten per cent for all tunnels may not be appropriate. The in-tunnel ratio has been estimated to be as much as 20 per cent to 30 per cent and depends on the location in the tunnel, the type of ventilation system and the traffic/fleet characteristics. This paper discusses the effect of some of these parameters on the NO_2:NO_x ratio in road tunnels.The findings indicate that the ten per cent upper limit on the NO_2:NO_x ratio in PIARC (2000) may be a conservative value for design purposes for longitudinally ventilated road tunnels of typical length in Australia. A value of seven per cent to eight per cent may be more appropriate in some circumstances. However, the value chosen will depend on the specific tunnel in question and the confidence in traffic conditions expected. Values above ten per cent may be encountered for short periods but usually when the NO_2 levels are low.