Evaluation of a photoacoustic detector for water vapor measurements under simulated tropospheric/lower stratospheric conditions.

Although water vapor is one of the most important and certainly the most variable minor constituent of the atmosphere, accurate measurements of p(H20) with high time resolution are difficult, particularly in the cold upper troposphere/lower stratosphere. This work demonstrates that a diode laser-based photoacoustic (PA) water vapor detector is a viable alternative to current water vapor sensors for airborne measurements. The PA system was compared with a high-quality frost point hygrometer (FPH) and with a Lyman-alpha hygrometer in the pressure range of 1000-100 hPa at frost point temperatures between 202 and 216 K. These conditions were simulated in a large environmental chamberfor 14 h. Simultaneous measurements with the three instruments agreed within 6%. Nitric acid vapor interferes with the FPH measurements at low frost point temperatures but does not affect the other instruments. The sensitivity of the PA system is already sufficient for measurements in the upper troposphere, and straightforward improvements can extend its useful range above the tropopause. Rugged construction, extreme simplicity, small size, and potential for long-term automatic operation make the PA system potentially suitable for airborne measurements.

Metastable oxygen molecules in the troposphere.

The sources and steady-state concentration of singlet oxygen in the atmosphere are assessed in view of potential effects on the biosphere. Collision-induced absorption of sunlight by molecular oxygen in 1 atm of air produces O2 (a1 delta g) at a rate P = 1.6 x 10(9) cm-3 s-1 in bright sunlight. Less than 10% are added to this purely natural source by the photolysis of ozone, and by anthropogenic sensitizers (SO2, NO2, volatile aromatics). Collisional quenching of O2 (a1 delta g) by ground state oxygen establishes a steady-state concentration of ca. 1.7 x 10(8) cm-3. Reactions of singlet oxygen with other atmospheric pollutants are entirely negligible when compared with the concurrent reactions of ambient OH and O3. Potential effects of atmospheric singlet oxygen on the biosphere are limited by the deposition rate F less than or equal to 0.051 P, which depends on the production rate P of O2 (a1 delta g) in the air layer immediately above the flat surface.

Fluorescence determination of low formaldehyde concentrations in air by dye laser excitation.

This paper describes a fluorescence technique for the detection of sub-ppm-concentrations of formaldehyde in air at atmospheric pressure. The aldehyde is excited by a frequency doubled tunable dye laser covering the 320-345-nm wavelength range. The fluorescence intensity, measured at right angles through a 400-nm cutoff filter, is proportional to the formaldehyde concentration in air. Interferences by NO(2) and SO(2) are negligible. The detection limit of the system is 5 pphm at present, in good agreement with an estimate of the fluorescence yield in 1 atm of air. A considerable increase of the sensitivity is possible, and the use of this technique for measuring formaldehyde in ambient air in the ppb concentration range is projected.