Hydrocarbon Concentrations at Harvard Forest EMS Tower 1992-2001

Air was drawn continuously from two inlets (24 and 29 m). Samples for analysis were extracted from the inlet lines, and passed through glass cold traps at -20 C and Ascarite II (Thomas Scientific) traps to remove H2O, O3, and CO2. Samples were cryogenically preconcentrated on dual traps (40 ml min-1 of air for 10 minutes onto fused silica lined 1/16" OD stainless steel tube, Silcosteel), and injected into a gas chromatograph with dual Flame Ionization Detectors (Hewlett Packard 5890 series II). Chromatographic separation was accomplished using 30 meter PLOT GS-Alumina Megabore capillary columns (J+W Scientific). Every fifth pair of samples was taken from the same altitude (29 m) by switching a valve near the inlet of the 24 m sampling line in order to determine the NULL for the observed concentration gradient. The measurement system could operate continuously and unattended for more than two weeks, although data were normally downloaded at six day intervals. Concentrations for most hydrocarbons were determined using relative response referenced to an internal neohexane standard (Scott-Marrin, NIST traceable * 2%) added to every sample near the sample inlet by dynamic dilution. Response factors for isoprene were determined from dynamic dilution of isoprene standards (Scott-Marrin, NIST traceable * 2%) added to ambient air samples near the sample inlet periodically from May to November 1995. The accuracy of the system was estimated to be better than * 18% for hexane and for hydrocarbons eluting before hexane, based on the cumulative uncertainty of the neohexane standard, measurements of standard addition flows, the integrity of individual compounds in the sampling and analysis process, and relative response factors. Accuracy of the isoprene measurements was estimated to be better than * 8% (more accurate than other species because the uncertainty of the relative response factor was eliminated). Measurement precision was approximately 3% at 1 ppbv, 5% at 0.5 ppbv, 10% at 0.2 ppbv, and 20% for concentrations less than 0.1 ppbv, as determined by the variance between measurements taken from the same level every fifth injection. The detection limit for these compounds was approximately 0.01 ppbv. The analytical system was checked for contamination daily by running zero-air blanks. In addition, the Teflon sampling tubes were checked for contamination and memory effects by introducing zero-air at the sample inlets on top of the tower. No contamination or memory effects were detected for isoprene. Concentration gradients of CO2 were measured simultaneously with the hydrocarbon gradients using a differential infrared gas analyzer (LICOR 6251), with air from 29 meters passed through the reference cell and air from 24 meters through the sample cell. Water vapor was removed from the air samples using nafion dryers, and the samples were assumed to be at a common temperature before analyzing for CO2. The NULL gradient was measured after every sampling period by filling both cells with air from 29 meters. Instrument gain was determined by standard addition of CO2 selectively to both the sample and reference air. The standard deviations of the zero gradient measurements were determined by comparing the NULL gradient measured every fifth sampling period (when hydrocarbon NULL gradients were determined) to the zero measurement directly following that period. The standard deviation in the zero measurements for CO2 (0.18 ppm) was much greater than 20 % of the mean midday gradients (-0.9 ppm CO2). Flux determinations were not attempted when observed gradients were very small, i.e. within 1 standard deviation of zero. The CO2 fluxes and gradients, and all the NMHC measurements are reported as mole fractions relative to dry air at a common temperature, avoiding the need for density corrections due to gradients in temperature or H2O.