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Vehicles contribute to atmospheric PM concentrations not just through their direct (primary) PM emissions, measured at the tailpipe but, even more significantly, through photo- oxidation and gas-to-particle processes of initially gaseous exhaust components (secondary PM). Secondary organic aerosol (SOA) formation from different combustion emission sources such as traffic emissions (on-road vehicles, aircraft, shipping), wood burning emissions as well as other sources can be studied with a Potential Aerosol Mass (PAM) Oxidation Flow Reactor (OFR). This tool provides a highly oxidizing environment for the simulation of atmospheric oxidation processes on timescales ranging from a day to several days in a few minutes in a laboratory/ field. The PAM reactor can be used as a screening tool for vehicle SOA production by carefully accounting for the effects of the large variations in emission rates. The PAM reactor is also used to simulate atmospheric processing of soot and other model primary organic aerosols.
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The reactor is an aluminium cylinder 46 cm L x 22 cm W, providing an internal volume of 13.3 litres.
UV lamps (λ = 254 nm & 185 nm) are located inside the chamber.
There are two modes of running the reactor:
In the OFR254 mode, only 254 nm photons are available, due to the use of doped fused quartz lamps that do not transmit 185 nm photons. O3 needs to be introduced in order for OH to be produced, via the reaction O3 + hv –> O2 + O(1D) followed by the reaction O(1D) + H2O –> 2OH. O3 is generated by irradiating O2 at λ = 185 nm with a separate lamp outside the PAM reactor.
In the OFR185 mode, both 254 and 185 nm photons are available due to the use of clear fused quartz lamps that transmit both 185 nm and 254 nm photons. OH and HO2 are generated directly in the reactor from H2O photolysis, and O3 is also produced due to O2 photolysis. The photolysis of O3 produces additional OH as in the OFR254 mode. In OFR185 mode there is no need to inject O3 into the reactor.
SOA is generated via gas-phase OH oxidation of volatile organic compounds (VOCs) and intermediate volatility organic compounds (IVOCs). The PAM reactor is operated under continuous flow conditions, as opposed to environmental chambers that are typically run in batch mode. The OH/HO2 and OH/O3 ratios in the PAM reactor are similar to tropospheric ratios. The amounts of OH, HO2, and O3 are 100 to 10,000 times larger than in the daytime troposphere.
There are advantages and disadvantages to using flow tubes such as the PAM reactor compared to environmental “smog” chambers.
- Wider range of oxidant exposure time (1–30 days vs. 1 day)
- Shorter experiment duration (minutes vs. hours or days)
- High OH concentrations required to simulate atmospherically aging timescales (108 – 1010 vs. 106 – 107 molec cm-3)
- High (parts-per-million) levels of O3 required for OH production
- UV emission spectrum different than troposphere (peak λ = 254 nm vs λ > 300 nm)
- Magnitude of wall effects/interactions on measurements
- Alternative UV and Vis lamps for different radical production and photolysis applications