MEASUREMENT OF TOTAL NONMETHANE ORGANIC-CARBON (C-Y) - DEVELOPMENT AND APPLICATION AT CHEBOGUE POINT, NOVA-SCOTIA, DURING THE 1993 NORTH-ATLANTIC REGIONAL EXPERIMENT CAMPAIGN

A technique was developed for the measurement of total nonmethane orga nic carbon (C-y) in the gas phase based on cryogenic collection, prese paration of CO, CH4, and CO2, conversion of all carbon to CH4, and mea surement by flame ionization detector. Collection and recovery efficie ncies were found to be essentially quantitative for C-2-C-7 hydrocarbo ns and better than 95 and 93% for methanol and formaldehyde, respectiv ely. CO2 prevented the measurement of the C-2 hydrocarbons in atmosphe ric samples, and system response to ambient water vapor limited the pr ecision of the measurement at very low C-y. The measurement system had an estimated uncertainty of 10% at ambient mixing ratios of 100 ppb C , and a detection limit of between 5 and 7 ppb C. C-y was measured in Boulder, Colorado, during June of 1993 and at Chebogue Point, Nova Sco tia, during the North Atlantic Regional Experiment intensive campaign, August to September 1993. C-y values ranged from several hundred ppb C to 11 ppb C in Boulder, and from 177 to below 5 ppb C at Chebogue Po int, with medians of 55 and 11 ppbC, respectively. C-y was found to co rrelate with both CO and O-3 at higher levels of those two pollutants. The sum of C-y and the C-2 hydrocarbons was virtually always equal to or greater than the sum of the hydrocarbons. Several exceptions to th is occurred when high HC values were observed in cannister samples but not C-y measurements. These instances were attributed to short-term p erturbations from local sources. There was a net difference between to tal nonmethane organic carbon and the sum of hydrocarbons and carbonyl s of 3.5 +/- 7.8 ppb C overall. This difference appears to be broadly correlated with anthropogenic pollution. There does not appear to be a significantly large reservoir of reactive carbon unaccounted for by h ydrocarbon or carbonyl measurements.