SEASONAL-VARIATIONS OF NONMETHANE HYDROCARBONS IN RURAL NEW-ENGLAND - CONSTRAINTS ON OH CONCENTRATIONS IN NORTHERN MIDLATITUDES

Concentrations of C-2-C-6 hydrocarbons are reported at Harvard Forest in Central Massachusetts (42.54 degrees N, 72.18 degrees W; elevation, 340 m), measured at 45-min intervals from August 1992 through July 1994. The seasonal cycle of background concentrations for anthropogenic alkanes (ethane, propane, butane, isobutane, pentane, and hexane) and acetylene could be cleanly extracted from the similar to 900 measurements each month. Background concentrations exhibited seasonal maxima in winter and minima in summer. The phase of the seasonal cycle for each species reflected its rate of reaction with OH: The faster the reaction, the earlier the peak in winter, and the more rapid the decline in spring. Winter:summer ratios for hydrocarbons more reactive than propane were similar, implying a summer:winter ratio for OK of 9 +/- 2 at northern midlatitudes. The annual cycle for ethane provides a measure of the mass-weighted annual mean OH concentration at northern midlatitudes, 7(+/- 3.5) x 10(5) cm(-3), as simulated by using a three-dimensional chemical tracer model.