Evolution under dark conditions of particles from old and modern diesel vehicles in a new environmental chamber characterized with fresh exhaust emissions

Abstract

Atmospheric particles have several impacts on health and the environment, especially in urban areas. Parts of those particles are not fresh and have undergone atmospheric chemical and physical processes. Due to a lack of representativeness of experimental conditions and experimental artifacts such as particle wall losses in chambers, there are uncertainties on the effects of physical processes (condensation, nucleation and coagulation) and their role in particle evolution from modern vehicles. This study develops a new method to correct wall losses, accounting for size dependence and experiment-to-experiment variations. It is applied to the evolution of fresh diesel exhaust particles to characterize the physical processes which they undergo. The correction method is based on the black carbon decay and a size-dependent coefficient to correct particle distributions. Six diesel passenger cars, Euro 3 to Euro 6, were driven on a chassis dynamometer with Artemis Urban cold start and Artemis Motorway cycles. Exhaust was injected in an 8 m3 chamber with Teflon walls. The physical evolution of particles was characterized during 6 to 10 h. Increase in particle mass is observed even without photochemical reactions due to the presence of intermediate-volatility organic compounds and semi-volatile organic compounds. These compounds were quantified at emission and induce a particle mass increase up to 17 % h−1, mainly for the older vehicles (Euro 3 and Euro 4). Condensation is 4 times faster when the available particle surface is multiplied by 6.5. If initial particle number concentration is below [8–9] × 104 cm−3, a nucleation mode seems to be present but not measured by a scanning mobility particle sizer (SMPS). The growth of nucleation-mode particles results in an increase in measured [PN]. Above this threshold, particle number concentration decreases due to coagulation, up to −27 % h−1. Under those conditions, the chamber and experimental setup are well suited to characterizing and quantifying the process of coagulation.