Nitrous acid (HONO) emissions under real-world driving conditions from vehicles in a UK road tunnel.

Measurements of atmospheric boundary layer nitrous acid (HONO) and nitrogen oxides (NO_x) were performed in summer 2016 inside a city centre road tunnel in Birmingham, United Kingdom. HONO and NO_x mixing ratios were strongly correlated with traffic density, with peak levels observed during the early evening rush hour as a result of traffic congestion in the tunnel. A daytime ΔHONO / ΔNO_x ratio of 0.85 % (0.72–1.01 %, 95 % CI) was calculated using reduced major axis regression as the overall fleet-average (comprising 59 % diesel-fuelled vehicles). A comparison with previous tunnel studies and analysis on composition of the fleet suggest that goods-vehicles have a large impact on the overall HONO vehicle emissions; however, new technologies aimed at reducing exhaust emissions, particularly for diesel vehicles, may have reduced the overall direct HONO emission in the UK. This result suggests that in order to accurately represent urban atmospheric emissions and OH radical budget, fleet-weighted HONO / NO_x ratios may better quantify HONO vehicle emissions in models, compared with use of a single emissions ratio for all vehicles. The contribution of the direct vehicular source of HONO to total ambient HONO concentrations is also investigated and results show that, in areas with high traffic density, vehicle exhaust emissions are likely to be the dominant HONO source to the boundary layer. [ABSTRACT FROM AUTHOR]