Gas-phase photolytic production of hydroxyl radicals in an ultraviolet purifier for air and surfaces.

We have measured the concentration of hydroxyl radicals (OH) produced in the gas phase by a commercially available purifier for air and surfaces, using the time rate of decay of n-heptane added to an environmental chamber. The hydroxyl generator, an Odorox® BOSS™ model, produces the OH through 185-nm photolysis of ambient water vapor. The steady-state concentration of OH produced in the 120 m3 chamber is, with 2σ error bars, (3.25 ± 0.80) × 106 cm-3. The properties of the hydroxyl generator, in particular the output of the ultraviolet lamps and the air throughput, together with an estimation of the water concentration, were used to predict the amount of OH produced by the device, with no fitted parameters. To relate this calculation to a steady-state concentration, we must estimate the OH loss rate within the chamber owing to reaction with the n-heptane and the 7 ppb of background hydrocarbons that are present. The result is a predicted steady-state concentration in excellent agreement with the measured value. This shows we understand well the processes occurring in the gas phase during operation of this hydroxyl radical purifier. IMPLICATIONS: Hydroxyl radical air purifiers are used for cleaning both gaseous contaminants, such as volatile organic compounds (VOCs) or hazardous gases, and biological pathogens, both airborne and on surfaces. This is the first chemical kinetic study of such a purifier that creates gas-phase OH by ultraviolet light photolysis of H2O. It shows that the amount of hydroxyls produced agrees well with nonparameterized calculations using the purifier lamp output and device airflow. These results can be used for designing appropriate remediation strategies.

Production of the NO photofragment in the desorption of RDX and HMX from surfaces.

A promising scheme for the remote detection of nitrate-based explosives, which have low vapor pressure, involves two lasers: the first to desorb, vaporize, and photofragment the explosive molecule and the second to create laser-induced fluorescence in the NO fragment. It is desirable to use for the first a powerful 532 nm frequency-doubled Nd:YAG laser. In this study, we investigate the degree of photofragmentation into NO resulting from the irradiation of the explosives RDX and HMX coated on a variety of surfaces. The desorption step is followed by femtosecond laser ionization and time-of-flight mass spectrometry to reveal the fragments produced in the first step. We find that modest laser power of 532 nm desorbs the explosive and produces adequate amounts of NO.

Membrane introduction/laser photoionization time-of-flight mass spectrometry.

Two-photon resonance enhanced multiphoton ionization (REMPI) has been shown to be a unique ionization method for mass spectrometry, exhibiting both high sensitivity and chemical selectivity. Because REMPI is a gas-phase method, its applications have been limited either to direct analysis of vapor phase samples, or in conjunction with an initial laser desorption or other vaporization step. We describe here for the first time a combination of membrane introduction mass spectrometry (MIMS) and REMPI with time-of-flight mass spectrometry (TOF-MS), which allows for the direct analysis of trace amounts of organic compounds in water samples. The objective of our research was the detection of very low levels of aromatic contaminants, particularly benzene, toluene, and xylene (BTX), in aqueous solutions without interference due to the water. We have measured limits of detection (LOD) for selected aromatics in water below 1 part-per-trillion with an averaging time of less than 10 s using a continuous sample flow.

Rotational energy transfer in NO (A 2Sigma+,v'=0) by N2 and O2 at room temperature.

State-to-state rotational energy transfer (RET) rate coefficients for NO (A 2Sigma+, v'=0, J=5.5, 11.5, 17.5) were measured for N2 and O2 at room temperature using a pump-probe method. The NO A 2Sigma+ state is prepared by 226 nm light and the RET is monitored by fluorescence from the D 2Sigma+ v'=0 state, following excitation by a time-delayed laser at approximately 1.1 microm. Additionally, total collisional removal and final state distributions were measured exciting in the Q1+P21 band head, to simulate an NO laser-induced fluorescence atmospheric monitoring scheme. Time-resolved modeling is used to understand relaxation mechanisms and predict relaxation times in ambient air. H2O at atmospherically relevant concentrations does not affect the degree of RET in ambient air.