Assessment of carbon monoxide (CO) adjusted non-methane hydrocarbon (NMHC) emissions of a motor fleet e A long tunnel study Wen-Tzu Liu a , Sheng-Po Chen a , Chih-Chung Chang b, * , Chang-Feng Ou-Yang a, c , Wei-Cheng Liao a , Yuan-Chang Su a , Yue-Chuen Wu d , Chieh-Heng Wang e , Jia-Lin Wang a, * a Department of Chemistry National Central University, Chungli, Taiwan b Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan c Department of Atmospheric Sciences, National Central University, Chungli, Taiwan d Environmental Analysis Laboratory, EPA, Executive Yuan, Taiwan e Center for Environmental Studies, National Central University, Chungli, Taiwan highlights  Speciated NMHCs and CO were measured inside a long tunnel.  Using CO as a reference to cancel out the factor of traffic volume.  CO-normalized NMHCs were sensitive to the driving conditions e slow vs. fast.  Alkenes and aromatics are predominant in the OFP CO-normalized .  Our NMHC/CO for the four fleet speeds fall within the range from literatures. article info Article history: Received 18 October 2013 Received in revised form 24 December 2013 Accepted 3 January 2014 Available online 21 January 2014 Keywords: Tunnel study NMHC CO NMHC/CO ratio OFP abstract Speciated hydrocarbons (NMHCs) and CO were simultaneously measured by evenly distributed samples along the entire length (12.9 km) 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 85 km h 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 12 km 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 ¼ 45 km h 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 9e22% despite their high weight percentage, whereas the contribution of the alkenes was enhanced significantly to 50e60% from their weight percent of only approximately 26%. Our total NMHC to CO ratios of 0.06e0.20 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. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Motor vehicle emissions are known to be a predominant air pollution source in metropolitan cities worldwide (Guo et al., 2007; * Corresponding authors. E-mail addresses: joechang@rcec.sinica.edu.tw (C.-C. Chang), cwang@cc.ncu. edu.tw (J.-L. Wang). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.atmosenv.2014.01.002 Atmospheric Environment 89 (2014) 403e414