21Vehicular emission (VE) is one of the important anthropogenic sources for ground-level volatile 22 organic compounds (VOCs) in both urban and suburban areas. A first comprehensive campaign was 23 conducted at an urban roadside in Xi'an, China in summer, 2016. A total of 57 VOCs, as known as critical 24 surface ozone (O 3 ) precursors, and other trace gases were measured simultaneously during the sampling 25 period. Iso-pentane, a tracer of gasoline evaporation, was the most abundant VOC in the roadside samples, 26 followed by isobutane and benzene, attributed to the largest composition (~70%) of gasoline-fueled 27 vehicles on the road. The molar ratio of toluene/benzene (T/B) in our study (0.36) is far lower than the 28 range reported in other cities, indicating the stronger contributions from diesel emissions. The results of 29 source apportionment achieved with positive matrix factorization (PMF) receptor model were highly 30 consistent with the vehicles compositions, strongly evidenced that the precise characterization of the VE 31 sources from those marker species. The degrees of individual compound contributed to O 3 production 32 were weighed by ozone formation potential (OFP). Propylene (20%), 1-butene (11%) and 33 iso-pentane(10%) were the top three contributors at the roadside. The information of this study 34 complements the VOCs database regarding to the VE sources in Northwestern China. 35 36 Capsule: Illustrate the current composition and the major emission sources of VOCs at the roadside 38 environment in northwest China, and discuss their impacts on ozone formation.

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