Measurements of surface ozone (O3), carbon monoxide (CO), nitric oxide (NO), and total reactive nitrogen (NOy) were made, in conjunction with other trace gases and fine aerosols, at Mount Waliguan (WLG, 36.28¿N, 100.90¿E, 3816 m above sea level) in the late spring and summer of 2003 in order to better understand the source(s) of ozone and other chemically active gases over the remote highlands of western China. The average mixing ratio (plus or minus standard deviation) was 58 (¿9) ppbv for O3, 155 (¿41) ppbv for CO, and 3.83 (¿1.46) ppbv for NOy in the spring phase, compared to a summer average value of 54 (¿11) ppbv for O3, 125 (¿36) ppbv for CO, and 3.60 (¿1.13) ppbv for NOy. The daytime (0800--1759 local time) average NO mixing ratios were 72 (¿79) pptv and 47 (¿32) pptv in the spring and summer, respectively. The ozone mixing ratios exhibited a minimum in late morning, while CO (and NOy in spring) showed enhanced concentrations at night. The latter is in contrast to the diurnal behaviors observed in many remote mountain sites. Analysis of 10-day back trajectories using output from Fifth-Generation National Center for Atmospheric Research/Penn State University Mesoscale Model (MM5) simulations shows that air masses from the remote western regions contained the lowest level of CO (121--129 ppbv) but had the highest O3 (60 ppbv), compared to the other three air mass groups that were impacted by anthropogenic emissions in eastern/southern China and in the Indian subcontinent. Ozone correlated negatively with CO (and water vapor content), particularly during summer in air originating in the west, suggesting that the high-ozone events were mostly derived from the downward transport of the upper tropospheric air and not from anthropogenic pollution. An examination of in situ chemical measurements (CO-NOy correlation, ethyne/propane, and benzene/propane) as well as Measurements of Pollution in the Troposphere (MOPITT) and Moderate-Resolution Imaging Spectroradiometer (MODIS) remote-sensing data revealed some impacts from forest fires in central Asia in the late spring of 2003 on the background concentrations of trace gases over western China. While the O3 and CO levels at WLG are comparable to those at remote continental sites in Europe and North America, the NOy concentrations were substantially higher at WLG. The possible reasons for the abnormally high NOy levels are discussed. While more studies are needed to pin down these sources/causes, including a possible contribution from long-range transport, we believe that microbial processes in soils and animal wastes associated with animal grazing were an important cause of the elevated NOy. The observed daytime NO concentrations imply a net photochemical production of O3 at WLG, suggesting a positive contribution of photochemistry to the ozone budget.