Evaluation of the Oxidation Flow Reactor for particulate matter emission limit certification

As secondary aerosols become larger fractions of ambient PM_2.5, emission factors and source profiles need to reflect the photochemical aging of multipollutant emissions that account for changes in transport between source and receptor. Oxidation Flow Reactors (OFR) intend to simulate gas/particle transformation and volatilization with time-scales of hours to days in order to estimate mass gains or losses due to atmospheric aging. The commercially-available Potential Aerosol Mass (PAM)-OFR (Aerodyne Research, LLC, Billerica, MA, USA) is evaluated for its feasibility in establishing an emission limit certification protocol for China and for determining changes to source marker abundances used in source apportionment model applications. The OFR without ultraviolet radiation is reasonably inert to the passage of reactive gases, such as sulfur dioxide (SO₂), and particles >~100 nm. The largest uncertainty in estimating aging times is the commonly used assumption that the average atmospheric hydroxyl concentration (OH_atm) is 1.5 × 10⁶ molecules/cm³; measurements show that OH_atm can vary by an order of magnitude in different environments. OH exposures (OH_exp) estimated within the OFR are more precise than this assumption. Plug flow is a reasonable estimate of residence time within the OFR, as it does not differ much from the weighted-mean of the residence time distribution. A survey of previous studies shows that only a few examine the PAM-OFR performance, and that mass is both gained and lost with aging, depending on the combustion fuels, burning conditions, and aging times. At present, the OFR appears to be more practical for source profile aging than for emission certification, and further experiments are needed prior to application for multipollutant effects on particulate emission rates.