## 1 Introduction  
On-road gasoline and diesel vehicles are a major source of air pollutants in the urban atmosphere [1, 2]. Approximately 37% of CO emissions, 14% of Volatile Organic Compound (VOC) emissions, 39% of NOx emissions, and 3% of primary fine particle (PM\({}_{2.5}\)) emissions were contributed by on-road gasoline and diesel vehicles in the United States in 2014 [1]. These pollutants are important participants in atmospheric chemical reactions and have adverse effects on human health. NOx and VOC are important precursors to tropospheric ozone (O\({}_{3}\)) [3], which can lead to lung function damage and cause respiratory diseases. PM\({}_{2.5}\) impacts climate by scattering or absorbing solar radiation [4, 5], and exposure to PM\({}_{2.5}\) is detrimental to health [6, 7]. Roughly 50% of fine particle mass is constituted by organic species, i.e., organic aerosol (OA) or organic carbon (OC) [8, 9]. In addition to OC, PM emissions from vehicles also mainly consist of carbon soot, which is also known as elemental carbon (EC) or black carbon (BC) [10]. Gasoline and diesel vehicles have distinctive emission characteristics because of differences in engine technology. Gasoline vehicles are powered by spark-ignition (SI) engines, while diesel vehicles are powered by compression-ignition (CI) engines. The local fuel-rich conditions and the high combustion temperature of the CI engine favor the formation of PM and NO\({}_{\rm x}\)[11]. The overall fuel-to-air ratio in SI engines is higher than in CI engines, and there is not enough time in SI engine to convert all CO to CO\({}_{2}\)[11]. Gasoline vehicles emit more CO than diesel vehicles, while uncontrolled diesel vehicles emit much higher NO\({}_{\rm x}\) and PM than gasoline vehicles [12; 13; 14; 15]  
Numerous studies have been conducted to measure vehicle emissions, including dynamometer studies [13; 16; 17], on-road and near-road measurements [18; 19; 20; 21; 22], vehicle chase studies [23; 24; 25], and tunnel studies [12; 14; 26; 27; 28; 29; 30; 31; 32; 33]. On-road and near-road studies include measurements with portable emissions measurement systems (PEMS) [34] and remote sensing [35]. Dynamometer studies, in which the vehicle exhaust is sampled under a prescribed driving cycle, offer controlled sampling conditions, though questions remain whether dynamometer tests are representative of real-world driving conditions. In addition, the vehicle population sampled in a typical dynamometer campaign is tiny compared with the real-world traffic volume. For on-road, near-road, and vehicle chase studies, the vehicle exhaust is sampled under real-world driving conditions. Tunnel and near-road studies typically sample from large vehicle fleets, but these studies have to account for the dilution of emissions with background air. Tunnel studies offer real-world driving conditions and a large-volume traffic fleet. The primary advantages of tunnel studies are that the emissions are less diluted than in near-road conditions and that a large vehicle fleet can be sampled under realistic driving conditions. A major challenge of tunnel studies is to separate the gasoline and diesel vehicle emissions from a mixed fleet [33]. Also, since many tunnel studies sample from highway tunnels, only emissions from high-speed driving are characterized. Thus, tunnel measurements typically do not capture emissions at low speed or vehicle cold starts, operation modes that can have significantly different emissions rates than high-speed driving [13; 17]. Previous studies have suggested that ambient temperature can affect vehicle emissions; this in turn would drive seasonal variations in on-road vehicle emissions. [PERSON] et al. [16] tested 496 vehicles on a chassis dynamometer under summer and winter ambient temperature. They found that PM emissions from gasoline-powered vehicles increased dramatically with decreased temperature, and the temperature effects on PM emissions were most significant during the vehicle cold-start phase. [PERSON] et al. [22] and [PERSON] et al.