Highly Instrument Reactor for Atmospheric chemistry (HIRAC)

A detailed description of the design, setup and initial testing of the HIRAC chamber can be found here with a brief description of some of the important features provided below.

The Design

HIRAC is a stainless steel cylinder with internal dimensions of 2.0 m long and 1.2 m diameter giving a total volume of ~2.25 m³. HIRAC was built with numerous mounting and access ports to allow a range of instrumentation, pumps, release valves, circulating fans and pressure and temperature measurement devices to be coupled to the chamber. Due to its strong stainless steel construction experiments can be carried out over a range of pressures of any desired bath gas up to 1 atm with the lower limit for the chamber pressure being determined by the specifications of the instrumentation required for a particular experiment. HIRAC can be pumped from atmospheric pressure to <0.003 mbar in ~60 mins using a rotary pump backed roots blower. This allows partial cleaning of the chamber between experimental runs. Thorough mixing of reaction mixtures is achieved by four low-vibration circulation fans mounted in pairs at each end of the chamber. Mixing times inside the chamber have been measured and are ≤ 70 s.

Temperature Control

HIRAC will undergo the installation of a temperature control system in the autumn of 2009 which will enable temperatures within the chamber of between ~-70°C and +70°C to be obtained. The chamber was built with stainless steel tubes welded to the skin to allow circulation of a thermofluid evenly across the outer surface area. The thermofluid will be heated/cooled by two large commercial systems. The chamber will also be insulated with two layers of 20 mm thick expanded neoprene. Control of the temperature of the chamber will allow us to perform in depth mechanistic studies of atmospherically relevant reactions as well as to study reactions occuring in other planetary atmospheres due to our unique ability to simulataneously control the pressure of the chamber over a wide range.

Radiation Sources

Light for photochemical studies can be provided by several different types of lamps which have different spectral profiles and intensities in the UV region and are selected depending on the absorption spectrum of the photolabile species. The lamps, which are of two different lengths, are housed in eight quartz tubes mounted radially inside the reactive volume parallel to the chamber's principle axis. So far three different types of lamps have be used for experiments in HIRAC; Philips TLK40W/05 actinic UV blacklamps, GE G55T8 / OH 7G UVC lamps and Philips TL 40W/12 RS SLV UV-B medical therapy lamps. The output of the lamps is strongly temperature dependent outside of a narrow temperature range (~35-39°C) and so fans have been fitted at the ends of the quartz lamp housing tubes to force lab air through the tubes removing hot gases and regulating the temperature.

The homogeneity of the radiation profile within HIRAC is of particular importance where a homogeneous radiation field greatly simplifies the analysis and modelling of results of photolytic studies. Additionally the use of the FAGE instrument to measure OH and HO₂ radicals (typically produced from photolytic sources) in HIRAC means that a homogenous radiation profile is necessary to avoid radical gradients within the chamber. Through a combination of experimental, analytical and ray tracing results it has been shown that J_NO2 is within ± 15% of the average for ~75% of HIRAC's volume. Additionally, measurements of OH across the chamber diameter showed that there is no significant radical gradient.