Non-negligible HONO emissions from vehicles: A Systematic Review

Gaseous nitrous acid (HONO) is an important precursor of tropospheric hydroxyl radicals (OH) and influences urban atmospheric oxidation capacity. Vehicle exhaust is recognized as a non-negligible near-surface HONO source, but reported emissions vary substantially among measurement approaches, vehicle types, and operating conditions. This systematic review summarizes recent advances in vehicular HONO measurements, formation pathways, emission characteristics, and environmental implications. The literature search identified 15 eligible studies from CNKI and Web of Science, and extracted records were synthesized using descriptive comparisons and exploratory random-effects analyses. Current measurement techniques have progressed from fleet-scale observations to high-resolution online measurements, although interference control and uncertainty reduction remain challenges. Vehicular HONO formation may involve direct combustion emissions, exhaust after-treatment processes, and secondary NO 2 -to-HONO conversion on exhaust-related surfaces. Reported emission differences among gasoline, diesel, cold-start, warm-start, tunnel, roadside, and field observations reflect the combined effects of vehicle technology, operating conditions, dilution, and atmospheric processing. Quantitative synthesis revealed extremely high heterogeneity (I 2 = 99.99%), indicating that pooled estimates should be interpreted as literature-level summaries rather than stable fleet-average emission factors. Diesel cases showed higher HONO/NO x ratios in some conditions, highlighting uncertainties associated with fixed-ratio emission inventories. Future studies should improve dynamic emission models, measurement strategies, and assessments of vehicle-related HONO impacts on urban air chemistry.