Assessment of carbon monoxide (CO) adjusted non-methane hydrocarbon (NMHC) emissions of a motor fleet - A long tunnel study

Speciated hydrocarbons (NMHCs) and CO were simultaneously measured by evenly distributed samples along the entire length (12.9km) of the tunnel in different traffic conditions. Four passes (two round-trips) were made at four different fleet speeds (i.e., 45, 65, 75 and 85kmh^-1). Individual NMHC and CO mixing ratios were observed to be sensitive to the fleet speed, which is compounded by driving conditions and traffic volumes. We propose using ratios of NMHCs to CO to cancel out the factor of traffic volumes to determine CO-normalized NMHC emissions, such that a less biased comparison can be made between different fleet speeds in this study and even between different studies. Moreover, to obtain robust CO-normalized NMHC emissions which are sufficiently representative of the fleet speeds, only samples collected deep inside the long-tunnels between 8 and 12km were used.Of the 61 target NMHCs, isomers of butane, pentane, ethylene, acetylene and toluene were the most abundant species regardless of the driving conditions. We observed that different driving conditions not only affected the emission strengths but also altered the compositions of the NMHCs. To simplify the data analysis, the target NMHCs were classified into four chemical groups, i.e., alkanes, alkenes, aromatics and alkynes. For the slower and more congested driving condition (fleet speed=45kmh^-1), the CO-normalized NMHC emissions were the highest, with alkanes contributing to approximately 60% of the total NMHC levels, suggesting poorer fuel utilization at slower driving conditions. In terms of the CO-normalized ozone forming potentials (OFPs), the contribution of the alkanes diminished dramatically to approximately 9-22% despite their high weight percentage, whereas the contribution of the alkenes was enhanced significantly to 50-60% from their weight percent of only approximately 26%.Our total NMHC to CO ratios of 0.06-0.20 for the four fleet speeds generally fall within the range constrained by many other tunnel or chassis dynamometer studies; however, the sensitivity to the driving conditions and the robustness with the long-tunnel data are better demonstrated in this study.