Frank Helleis

Temperature dependent CH3OCl formation in the reaction between CH3O2 and ClO

The reaction CH3O2 + ClO → CH3OCl + O2 (2b) has been investigated at temperatures of 295, 255, 230 and 215 K. The temperature dependent branching ratio for CH3OCl formation (α2b) is given by α2b = (T/300)−1.65 −0.223exp(411/T). The result at 200 K (ca 0.23) can be combined with our previous determination of the overall rate constant for the reaction between CH3O2 + ClO [Helleis et al, 1993] to calculate a channel specific rate constant for reaction 2b of ca (4.1±1) × 10−13 cm³ molecule−1 s−1 at the temperatures prevalent in the lower polar stratosphere at springtime.

CH3OCI: UV/visible absorption cross sections, J values and atmospheric significance

The UV/visible absorption spectrum of CH3OCl has been measured at wavelengths between 200 and 460 nm and used to calculate photodissociation lifetimes in the stratosphere. At zenith angles between 80° and 90° and at a pressure of 100 mbar the lifetime with respect to photodissociation is of the order of 4 hours. The possible fate of CH3OCl in the stratosphere is discussed and analogies are drawn with HOCl.

Design and field application of an automated cartridge sampler for VOC concentration and flux measurements

One of the major limitations in advancing the understanding of tropospheric ozone and aerosol generation and developing strategies for their control is the technical ability to accurately measure volatile organic compounds (VOCs). This paper describes the design of a constant flow VOC sampler. The versatile sampler can be used for fully automated concentration and flux measurements of VOCs. The sampler incorporates a microprocessor control unit and provides highly accurate mass flow control and significant ease of operation. Sampling sequences can be programmed directly or by remote control through a PC. All important operational parameters necessary for a complete sampling audit trail are logged. Compact weatherproof housings and low power consumption allow operation at remote sites and locations which are sensitive to disturbances or have restricted access. Inner wetted surfaces of the sampler are constructed from non-contaminating materials that do not sorb or emit VOC, and thus permit the collection of representative samples even in environments with very low VOC concentrations. The cartridge magazine provides a maximum of 20 sequential cartridge samples, which allows for long-term air quality assessments. In the dual channel mode, two samples can be collected simultaneously through two independent sample loops, providing ten sequential sample pairs. This design allows the parallel collection of (a) quality assurance backup samples, (b) samples on two different types of cartridges/sorbents to allow a variety of analyses, or (c) differential samples for flux measurements using enclosure, aerodynamic profile, or relaxed eddy accumulation (REA) methods. Field applications including airplane profile measurements above a tropical rainforest area, as well as gradient and REA measurements over a mid-latitude mixed forest stand are described, and demonstrate the validity and flexibility of the system. In particular, the application of the VOC sampler as an integrated part of a REA system is emphasized.

Automated in situ analysis of volatile sulfur gases using a Sulfur Gas Analyser (SUGAR) based on cryogenic trapping and gas-chromatographic separation

Based on the technique of cryogenic trapping, gas-chromatographic separation, and flame-photometric detection, we designed an instrument for the analysis of reduced sulfur compounds. The fully automated Sulfur Gas Analyser (SUGAR) consists of a dual sampling system, an electrically cooled cryotrap, and a newly devised small gas chromatograph, which is integrated mechanically and in terms of system control. The system incorporates microprocessor control and provides significant ease of operation. All operational parameters necessary for a complete sampling audit trail are logged on non-volatile memory. SUGAR achieves adequate sensitivity for measuring reduced sulfur gases at typical atmospheric concentrations. Applications under laboratory conditions and in the field demonstrate the versatility of the automated system. Field deployment proved the capability of continuous operation over a period of several weeks. The potential for further improvements of sensitivity and portability by use of different sulfur specific GC detectors is discussed.