The Potential Aerosol Mass (PAM) is an Oxidation Flow Reactor (OFR) that provides a highly oxidizing environment that simulates atmospheric oxidation processes on timescales ranging from a day to several days in a few minutes in the laboratory / field. The reactor was designed by Prof. William Brune's group at Penn State. In addition to being used as a source of secondary organic aerosol (SOA) particles, the PAM reactor is also used to simulate atmospheric processing of soot and other model primary organic aerosols. The reactor is a Pyrex or metal cylinder 46 cm L x 22 cm W, providing an internal volume of 13.3 liters. UV lamps (λ = 254 nm & 185 nm) are located inside the chamber:
Figure 1. Pyrex PAM reactor, 4 UV lamp configuration (Photo: Andrew Lambe).
There are two modes of running the reactor:
In the OFR254 mode only 254 nm photons are available, due to the use of quartz sleeves around the lights. 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 with a mercury lamp (λ = 185 nm) outside the PAM reactor. Since the amount of O3 introduced is very important, the terminology OFR254-30 when e.g. 30 ppm O3 are introduced has been proposed (Peng et al., 2015).
In the OFR185 mode both 254 and 185 nm photons are available due to the use of teflon sleeves around the lights. 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. Corresponding OH exposures range from about 1 to 10 days of atmospheric oxidation.
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 atmospherical 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