## 1 Introduction  
Road transport is a significant source of air pollution in the European Union (EU) and vehicle emissions regulations try to limit the emission levels [1]. For instance, carbon dioxide (CO\({}_{2}\)) is a greenhouse gas regulated by the EU with emission limits for vehicles [2]. Exceedance of these limits by the vehicle manufacturers are subject to fines. The current regulations are traditionally based on measurements of diluted samples from bags filled from a dilution tunnel where the whole exhaust gas is diluted during roller bench tests. However,direct measurements of the raw exhaust from the tailpipe are permitted in the EU and worldwide in the case of engine emission tests (e.g., heavy duty diesel engines and small utility gasoline engines) [3]. Moreover, direct tailpipe measurements are often conducted for research purposes [4]. Recently, on-road measurements with Portable Emissions Measurement Systems (PEMS) were introduced both in the heavy-duty [5; 6] and light-duty regulations [7]. The majority of the commercial systems use the same principle as the laboratory grade equipment in order to measure carbon monoxide (CO) and CO\({}_{2}\): Non-Dispersive Infrared detection (NDIR) after drying the exhaust gas to reduce water (H\({}_{2}\)O) spectral interference. However, the PEMS specifications in EU permit other principles as well, as long as the equivalency is proven [7]. Recently the wet-basis measurement with heated NDIR was introduced. Such measurements need to take into account the water interference effects (spectral overlap and molecular interaction) on CO and CO\({}_{2}\). Roller-bench and on-road tests for research purposes are also conducted with Fourier-Transform Infrared Spectroscopy (FTIR) [8] because several chemical components can be simultaneously detected [9; 10; 11]. The measurement in this case is wet-based. The dry-wet conversion of the gas concentrations is done with a multiplicative correction factor (\(K_{d\text{-}w}\)) given in several national and international regulations [6; 7; 12]. They are based on established combustion equations to derive air-to-fuel ratios (derived in the past for the performance of carburetors and fuel injection systems) [13; 14]. The different expressions of this correction factor in the various regulations are very similar to each other, but not identical. The correction factors are based on the gaseous dry-based concentrations of combustion products (mainly CO\({}_{2}\) and CO) and the fuel properties assuming that neither condensation nor evaporation of water in the pipes take place. Condensation of water vapor and several other condensable semi-volatile components in the exhaust gas can take place when the temperatures of the pipes or after-treatment devices are below the dew point temperature of the exhaust gas. The dew point of water in the gasoline exhaust is about 53 \({}^{\circ}\)C, lower for diesel, higher for CNG (about 60 \({}^{\circ}\)C) [15]. These values are for stoichiometric combustion and change for instance at lean conditions and or different winter/summer fuel formulations, especially for high ethanol content. The dew point can further increase and even exceed 100 \({}^{\circ}\)C in the presence of SO\({}_{3}\)[16]. Thus, condensation during low ambient temperatures and engine cold start is inevitable. The exhaust gas condensation has been studied to protect Exhaust Gas Recirculation (EGR) valves [16], to model condensation at aftertreatment devices [17], to recover water from military vehicles [18], to avoid condensation in the dilution tunnels for the measurement of emissions [19], or to avoid failures of NOx, oxygen, ammonia, and soot sensors utilizing ceramic sensing elements containing electrodes or electrochemical cells [20]. It has also been shown that it can result in loss of hydrophilic compounds (such as NH\({}_{3}\)) [21].