[en] Long-term trend and seasonal variation of the mean free tropospheric volume mixing ratios of carbon monoxide (CO) and hydrogen cyanide (HCN) have been derived from analysis of a time series of solar absorption spectra recorded from the US National Solar Observatory on Kitt Peak (31.9 degrees N, 111.6 degrees W, 2.09 km altitude) spanning almost three decades. The results of a fit to the CO 258 daily averages from May 1977 to April 2005 as a function of time with a model that assumes a sinusoidal seasonal cycle and a linear long-term trend with time yield a mean volume mixing ratio of 102 +/- 3) parts per billion (10(-9) per unit volume) below 10km altitude, I sigma. The CO measurements show a seasonal cycle with a maximum in March and a minimum in September with an amplitude of (22.3 +/- 1.5)% relative to the mean. The best-fit corresponds to a long-term CO trend of (0.15 +/- 0.14)% yr(-1), 1 sigma, relative to the mean. To quantify the possible impact of periods of intense fires, the CO measurements have been compared with the measurements of HCN, a well-documented emission product of biomass burning with a lifetime of similar to 5 months. The best fit to the full HCN time series of 208 daily averages from May 1978 to April 2005 results in a mean volume mixing ratio of (0.219 +/- 0.007) ppbv below 10 km altitude with a similar seasonal cycle, though with a lower relative amplitude than for CO. Although same-day enhancements up to a factor of 1.87 for HCN and 1.24 for CO were measured relative to values predicted by a fit to the time series that accounts for the seasonal cycles and trends of both molecules, excluding time periods of elevated fire emissions has no significant impact on the best-fit long-term free tropospheric CO and HCN trends. Our result of no long-term CO trend since the late 1970s suggests that the global average long-term decline reported from 1990 through 1995 measurements has not continued in the free troposphere. Similarly, a fit to the full time series of 208 HCN free tropospheric daily averages with the same model yields an average 2.09-10km mixing ratio of 0.219ppbv and a long-term trend of (-0.12 +/- 0.14) % yr(-1), 1 sigma, relative to the mean since 1978, also indicating no significant long-term trend above the lower mid-latitude continental US Kitt Peak station. The results for both molecules suggest the site was not significantly impacted by summer boreal fires during the time span of the measurements that in some years cause widespread pollution above northern higher latitude sites. (c) 2006 Elsevier Ltd. All rights reserved.