Combination of aerosol instrument data into reduced variables to study the consistency of vehicle exhaust particle measurements

Abstract

Several aerosol instruments are generally used to study the concentration and characteristics of exhaust particle emissions. These are based on different operational principles and usually record a different particle property. As a result, the definition of an absolute emission level is ambiguous and the consistency, i.e. the uniformity and response of the measurements to the input variables, becomes the main quality issue. Here we present four reduced variables, which are derived from the primary information provided by a condensation particle counter, a diffusion charger, an electrical low-pressure impactor and the gravimetric filter-based particulate matter measurement. The variables correspond to a total vs. solid particle number concentration, a mean diameter, an apparent density and a mass-specific surface. The variables were first compared on a relative scale, in order to examine the uniformity of the underlying measurements. Then they were compared on an absolute scale with particle properties found in the literature to confirm the validity of the measurement level of each instrument. Based on the values of the variables, it is verified that different aerosol instruments and methods may produce consistent measurements when attention is given to the sampling protocol and conditions. However, measurement particularities intrinsic to some methods, such as the adsorption of gaseous species on the gravimetric filter and the assumption of unit density for the calculation of the number concentration by the electrical low-pressure impactor, may lead to deviations from this general rule. Overall, our comparisons demonstrated that such reduced variables may serve both as a means to recognise abnormal measurement occasions and to discriminate the effects of fuel, driving condition and vehicle technology on particle emissions.