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.
Keywords: LP-LIF; emission standard; light-duty diesel truck; missing OH reactivity; total OH reactivity.