Physical and Chemical Characteristics and Oxidative Potential of Fresh and Photochemically Aged Diesel Exhaust Particles

Abstract

Air pollution, particularly from particulate matter (PM), has been an important global environmental issue with its effect on climate and air quality. One of the major contributors to PM pollution is the emission from diesel engines which was found to induce harmful effects on the respiratory and cardiovascular system. Diesel exhaust is composed of a complex mixture of different chemical species that are present in the gas, liquid, and particle phase. Once these are emitted in the atmosphere, they undergo oxidation - a process initiated by the atmospheric oxidants primarily hydroxyl radicals (OH·). Atmospheric oxidation governs the chemistry happening in the atmosphere and along this process is the change in the properties of the oxidized species. The different processes involved in atmospheric oxidation are being studied with the use of big environmental chambers and the recently developed oxidation flow reactors. In this study, the Potential Aerosol Mass Oxidation Flow Reactor (PAM OFR) was used to study the change in the physical and chemical properties and oxidative potential of diesel exhaust particles (DEP). The PAM reactor was characterized and the OH exposure produced inside the reactor was calibrated by measuring the decay of sulfur dioxide (SO2) as it reacts with OH radicals. OH exposure was inversely affected by the initial concentration of SO¬2 (with corresponding external OH reactivity). Diesel exhaust particles (DEP) were first diluted in a dilution chamber before passing through the PAM reactor, where aging was done by adjusting the voltage of the lamps inside the reactor (186 nm: 1.6 V and 254 nm: 1.6-7 V). The fresh and aged DEP were characterized based on particle number size distribution, mass concentration, chemical properties (organic, elemental, and water-soluble organic carbon, nitrate, and sulfate), and oxidative potential through dithiothreitol (DTT) assay. The geometric mean diameter and mass concentration increased after aging suggesting condensational growth and possible effects of heterogeneous oxidation. This was supported by the increase in the organic and water-soluble carbon fractions after aging which can be a result of oxidation of low-volatility VOCs in the exhaust condensing on existing primary particles. The ionic components of DEP were found to not have a significant change after aging. On the other hand, fresh DEP showed reasonable values on the assessment of its oxidative potential (OP) which are comparable to other studies. Aging the sample at an equivalent atmospheric age of 19.4 hours showed higher results for the OP activity. This was the same for the mass-normalized and volume-normalized values of OP indicating the higher intrinsic capacity of aged DEP to produce reactive oxygen species (ROS) and with the same amount of exposure, aged DEP would result in higher ROS production compared to fresh DEP. The contribution of the water-insoluble components of DEP was also found to be significant in the reaction of the DTT assay.