TERPENES IN A CONIFER FOREST - DETECTION AT SUB-PARTS PER BILLION LEVEL WITHOUT SAMPLE PRECONCENTRATION

In a natural atmosphere free from anthropogenic emissions, terpenes constitute a major part of the biogenic hydrocarbon family. The study of their involvement in chemical reactions with various oxidants in the atmosphere requires continuous and accurate checking of their diurnal concentration variations. In past studies, the ever present sample preconcentration step with large sample volumes is expected to be associated with the problems of sample loss or chemical modifications in addition to their limitations on the measurement frequency. To improve upon these drawbacks, a commercially available microprocessor-based portable gas chromatograph with an ultra-sensitive detector was successfully tested and used for in situ measurements. With a direct sample volume of a few hundred microlitres of ambient air and suitable selection of the column packing material, quick separation of the various components was achieved. Excellent operational features and on-line calibration with pure samples allowed accurate and instant measurements during field studies. The photoionization (PI) detector allowed high S/N ratio even at sub-parts per billion level concentration of the hydrocarbons. Data collected over a one-year period under varying meteorological conditions from a conifer forest situated at Gulmarg (34-degrees 03'N, 74-degrees 24'E, 2650 m MSL) indicates the suitability of this system for remote field studies and also throws light on the lifecycle of these hydrocarbons in the natural atmosphere. The different hydrocarbons measured in this evaluation study includes isoprene, alpha-pinene and beta-pinene.