Revealing Hidden Evaporative Emissions: A Streamlined Method for Distinguishing Canister and Non-Canister Sources.

As tailpipe standards tighten, evaporative emissions have become a major VOC source. Current whole-vehicle regulations overlook the distinct contributions of canister bleed versus non-canister emissions, hindering accurate assessment and control. We developed a synchronized method coupling mini- and variable-temperature SHED to separately measure both sources during diurnal tests. Testing China 6 vehicles under the standard certification protocol, non-canister emissions averaged 170.3 ± 49.1 mg/day, while canister emissions averaged only 3.2 ± 1.2 mg/day, a ratio of ∼59:1. Across test conditions, non-canister emissions exceeded canister emissions by ∼19-fold on average. Canister emissions were highly condition-dependent, reaching 18 times the baseline under off-cycle conditions, with VOC composition shifting toward more reactive species (alkenes, aromatics) and ozone formation potential rising to nearly 600% of certification levels. In contrast, non-canister emissions remained stable in both composition and magnitude. These findings support a shift from mass-based whole-vehicle limits to differentiated, species-resolved strategies. Our streamlined method enables separate yet simultaneous testing of both source types, offering a practical approach for next-stage evaporative standards and progress toward near-zero emissions.