Upgrading Emission Standards Inadvertently Increased OH Reactivity from Light-Duty Diesel Truck Exhaust in China: Evidence from Direct LP-LIF Measurement

Abstract

Vehicular exhaust is an important source of reactive gases responsible for the formation of ozone and secondary organic aerosols (SOAs) in the atmosphere. Although significant efforts have been made to characterize the chemical compounds associated with vehicular exhaust, there is still a wealth of compounds that are unable to be detected, posing uncertainties in estimating their contribution to atmospheric reactivity. In this study, by improving laser-induced fluorescence techniques, we achieved the first-ever direct measurement of the total OH reactivity (TOR) from light-duty diesel truck (LDDT) exhaust with different emission standards. We found that the TOR from the LDDT exhaust was 80−130 times the TOR from the gasoline exhaust measured in Japan. Unexpectedly, we discovered increased TOR emissions along with upgrading emission standards, possibly as a collective result of high combustion temperature in the engine and the oxidation catalysts in the exhaust after-treatment that favor production of highly oxidized organics in the stricter emission standard. Most of these oxidized organics are unable to be speciated by routine measurements, resulting in the missing OH reactivity increasing rapidly from 1.91% for China III to 42.0% for China V LDDT. Upgrading the emission standard failed to reduce the TOR from LDDT exhaust, which may inadvertently promote the contribution of LDDT to the formation of ozone and SOA pollution in China.

A Review of the Direct Measurement of Total OH Reactivity: Ambient Air and Vehicular Emission

“…Despite the widely adopted approaches for k OH measurement in ambient air, it is not reasonable to directly apply the k OH measurement instrument for ambient air to vehicular sources. Prior studies measured the k OH values for exhaust emission based on the chassis dynamometer [8,20,21,27]. As shown in Figure 1, the schematic diagram of the measurement system is presented, including the detection and sampling systems.…”

“…As shown in Figure 1, the schematic diagram of the measurement system is presented, including the detection and sampling systems. In the detection system, CO, NO, NO 2, and VOC species are measured along with the total OH reactivity generally, and the LP-LIF system is usually used for the k OH measurement of vehicles in previous studies [8,20,21]. In terms of the principle for the LP-LIF approach, high OH concentrations are generated artificially in the flow tube.…”

“…For the sampling system, the exhaust gas from the vehicular sources, which has a high temperature, high relative humidity, and high pollutant concentrations, needs to be first diluted in the dilution system. Nevertheless, the NO concentration is still sufficiently high to generate the interference, leading to no OH radicals being produced in high NO conditions [20,21]. Thus, the diluted gas is mixed with excessive O 3 before being pumped into the flow tube to circumvent NO interference [20,21].…”

“…Nevertheless, the NO concentration is still sufficiently high to generate the interference, leading to no OH radicals being produced in high NO conditions [20,21]. Thus, the diluted gas is mixed with excessive O 3 before being pumped into the flow tube to circumvent NO interference [20,21]. The appropriate O 3 concentration should be calculated to ensure the complete oxidation of NO with no oxidation of VOCs.…”

“…To fully characterize the reactive VOC species of vehicular sources, Nakashima et al (2010) evaluated the k OH of the exhaust emissions for the first time [8]. Thereafter, more k OH measurements for vehicular sources occurred over the past decade [19][20][21]. The direct measurement of k OH for vehicular sources can help us to identify whether the measurement of reactive species originating from vehicles is complete and, thus, it could further improve the accuracy of the evaluation for secondary pollutant formation from vehicular sources.…”