A New Aerosol Flow System for Photochemical and Thermal Studies of Tropospheric Aerosols

Abstract

For studying the formation and photochemical/thermal reactions of aerosols relevant to the troposphere, a unique, high-volume, slow-flow, stainless steel aerosol flow system equipped with UV lamps has been constructed and characterized experimentally. The total flow system length is 8.5 m and includes a 1.2 m section used for mixing, a 6.1 m reaction section and a 1.2 m transition cone at the end. The 45.7 cm diameter results in a smaller surface to volume ratio than is found in many other flow systems and thus reduces the potential contribution from wall reactions. The latter are also reduced by frequent cleaning of the flow tube walls which is made feasible by the ease of disassembly. The flow tube is equipped with ultraviolet lamps for photolysis. This flow system allows continuous sampling under stable conditions, thus increasing the amount of sample available for analysis and permitting a wide variety of analytical techniques to be applied simultaneously. The residence time is of the order of an hour, and sampling ports located along the length of the flow tube allow for time-resolved measurements of aerosol and gas-phase products. The system was characterized using both an “inert” gas (CO ₂ ) and particles (atomized NaNO ₃ ). Instruments interfaced directly to this flow system include a NO _x analyzer, an ozone analyzer, relative humidity and temperature probes, a scanning mobility particle sizer spectrometer, an aerodynamic particle sizer spectrometer, a gas chromatograph-mass spectrometer, an integrating nephelometer, and a Fourier transform infrared spectrophotometer equipped with a long path (64 m) cell. Particles collected with impactors and filters at the various sampling ports can be analyzed subsequently by a variety of techniques. Formation of secondary organic aerosol from α-pinene reactions (NO _x photooxidation and ozonolysis) are used to demonstrate the capabilities of this new system.

We are grateful to the U.S. Department of Energy (Grant # DE-FG02-05ER64000) for support of this work. Additional support was provided by the AirUCI Environmental Molecular Science Institute funded by the National Science Foundation (Grant # CHE-0431312). E.A.B. acknowledges financial support from a National Science Foundation graduate student fellowship. This research was in part in collaboration with the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. Department of Energy's Office of Biological and Environmental Research at Pacific Northwest National Laboratory (PNNL) and supported by the U.S. Department of Energy Office of Basic Energy Sciences, Chemical Sciences Division. PNNL is operated by the U.S. Department of Energy by Battelle Memorial Institute under contract # DE-AC06-76RL0 1830. We appreciate helpful discussions with Prasad Pokkunuri. We are grateful for the invaluable technical expertise and advice from George Schade, Reliable Sheet Metal Works, Fullerton, CA; Lee Moritz, Ron Hulme, and Richard Busby of the UCI Physical Sciences Machine Shop; and Jörg Meyer of the UCI Chemistry Department Glassblowing Shop. Finally, we thank J. N. Pitts Jr. for helpful comments on the article.

[Supplementary materials are available for this article. Go to the publisher's online edition of Aerosol Science and Technology to view the free supplementary files.]

Notes

^a Subsequently analyzed using LC-UV and LC-MS.

^b 20 L is sampled from the flow system into the long path cell to give a static sample.

^c Subsequently analyzed using GC-MS, ESI-MS, API-MS, LC-MS, and LC-UV.

^d Subsequently analyzed using FTIR.