In air environments, acetic acid gas may lead to irritations, generate pungent odor, and cause objects to corrode. This paper examines an online parallel-plate wet denuder system to monitor gaseous acid at ppbv concentration levels. Denuder surfaces which comprised frosted glass plates coated with a thin layer of TiO2 nanoparticles were covered by a thin film made of ultrapure water or 0.5 mM H2O2 to absorb the test acetic gases that were generated from a small thermostatic evaporation tube. Denuder effluent was then injected automatically into an ion chromatography for real-time detection. The method was validated by comparing measured concentrations with those obtained simultaneously by using a well established impinger technique. Test results showed that the current technique is an efficient collector for gaseous acetic acid, achieving the collection efficiency between 93.8% to 96.4% by using ultrapure water film. It can monitor 0.9–698.0 ppbv gaseous acetic acid accurately in real-time with response time of 10 minutes, a detection limit of 1.2 ppbv and a time resolution of either 10 minutes or 20 minutes, depending on the concentration of the eluent used in the ion chromatography. The technique was applied successfully to measure the exposure concentration of acetic acid gas leaking from a fume hood in a laboratory.