The measurement of volatile particles is a topic of debate, because they depend on many parameters. Small changes can result in orders of magnitude difference in the number concentration. The next section will summarize all these parameters and conclude with a proposal of a sampling system.
In the laboratory, the engine is connected to a dynamometer or the vehicle is placed on a chassis dynamometer. A prescribed “cycle” is followed where the engine speed and torque vary or, respectively, the vehicle speed profile. The whole exhaust gas is diluted in a dilution tunnel following constant volume sampling (CVS). The instruments sample diluted exhaust gas to determine the pollutants concentrations. For the determination of particulate matter (PM) mass emissions, a small portion of the diluted flow is extracted and passes through a filter. The filter mass change over the complete cycle determines the PM mass emissions. The temperature of the filter, depending on the regulatory context is maintained between 20 and 52 °C (EU light-duty) or 47 ± 5 °C (EU heavy-duty, USA). The actual dilution ratio varies over the test cycle since total diluted flow is kept constant: the dilution is high when the exhaust flow is low (idle and low speeds) and, vice versa, low when the exhaust flow is high (high speeds and loads). This is the opposite to what would be experienced with atmospheric dilution. Actually, both mixing and dilution ratio evolution until measurement differs between laboratory and atmospheric conditions. Furthermore, the dilution ratio evolution will be different for different vehicles, different CVS flow rates and different facilities. The influence of the dilution ratio on the PM mass has been discussed in the atmospheric science community
[1], but also in the automotive emissions community
[2][3][4]. It has been clearly shown that dilution ratio, temperature, and concentration of semi-volatile species all have significant influence on the partitioning of those species between the gas and particulate phases
[5]. For example, decreasing the concentration of organics from 1 mg/m
3 to 20 μg/m
3 at 25 °C can cause approximately half of the condensed organics to evaporate
[6].
PM increases at small dilution ratios (<5:1), as long as diluted exhaust temperature reaches ambient levels, due to the relatively high concentration of semi-volatile species that preferably partition to the particulate phase
[1]. Compared to atmospheric dilution, the CVS exposes the exhaust at low dilution ratios for prolonged residence time before measurement, resulting in relatively high PM mass. Increasing the dilution reduces the PM mass as partial pressures of semi-volatile species drop, while diluted exhaust temperature is not significantly affected. This phenomenon is mostly visible with organic species. Inorganics such as sulfuric acid do not evaporate at typical ambient conditions once they have condensed on particles, because of their low volatility
[6].
Even though the particulate mass of inorganics is rather insensitive to dilution ratio, this may not be the same for particle number. Actually, several studies have shown that the dilution air temperature, the dilution ratio, the relative humidity, and the residence time at such conditions affect nucleation mode particles
[7][8][9][10][11]. The nucleation mode number concentration and mean size depend on the sampling conditions (dilution temperature, dilution ratio, relative humidity etc.)
[9]. For the same dilution ratio, the lower the temperature the higher the nucleation mode
[7][9][12]. Τhe trend was not so clear for the dilution ratio. Some studies found the maximum nucleation at the minimum dilution ratio they tested, e.g., 4:1
[10][13][14][15] or 12:1
[16][17][18][7][19][20]. However, another study found higher nucleation at 23:1
[9] than at dilution 9:1. A modeling study found the maximum nucleation at dilution ratio 15:1 due to the interaction of volatile precursors concentration, available soot surface and temperature
[21]. Another exception was a study with humid air that found a higher nucleation mode with higher dilution ratios, probably due to the contribution of the humidity to the sulfuric acid growth
[11]. Simplified calculations also estimate maximum nucleation at dilutions between 15–30:1
[22]. The residence (aging) time is also an important parameter because it results in bigger sizes due to particle growth from condensation of organics and agglomeration. Theoretical and experimental studies showed that the nucleation mode number concentration maximized at approximately 0.2 s after initial dilution, whereas particle diameter stabilized approximately 1 s later
[23][24][25]. Other parameters might also be equally or even more important for the potential for nucleation mode formation, e.g., turbulence and the relative time constants for mixing and cooling
[25][26][27], cooling of the tailpipe
[28], presence of solid particles
[18], length of sampling line, and storage/release of volatiles
[22][29]. In general, the dilution corrected nucleation mode number concentration and mean size remain constant with dilutions >30:1
[13][20] or 50–60:1
[30]. Very high dilutions (range 200:1 to 1000:1) during chasing experiments did not reveal any changes of the size distributions
[31]. Adding a secondary dilution to the primary dilution does not seem to change the characteristics of the nucleation mode
[32].
The nucleation mode growth during the first seconds is very rapid (15 nm/s
[23][24]). Similar rates are expected in the wake of the vehicle. Note the big difference compared to the growth of the nucleation mode in the atmosphere (diluted) (5 nm/h
[33][34]). Such laboratory studies were conducted with high concentrations of sulfur and hydrocarbons, but newer studies with typical concentrations for modern vehicles are needed. Studies found that the organic species condensed onto the soot particles may contribute to 20–40% of the particle volume or mass for accumulation mode particles and up to 80% for nucleation mode particles
[35]. Studies with diesel vehicles equipped with oxidation catalysts demonstrated that the condensed material contributed only to a few nm to the diameter of the accumulation mode
[36]. The size changes of the accumulation mode due to condensation or agglomeration are typically small.