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Abstract: Maunal Loa Observatory, located 3,400 m above sea level on the island of Hawaii in the middle of the Pacific Ocean, is a critical site for determining the background chemical reactivity of the unpolluted atmosphere and for monitoring its rate of change on a global scale. However, recent measurements of soluble nitrogen (principally HNO₃) at the observatory find mixing ratios rising from their expected background values of 0.02-0.03 parts per 10⁹ by volume (p.p.b.v.) in the winter to 0.07-0.12 p.p.b.v. in late summer with three-hour events as high as 0.25 p.p.b.v. This raises the specific question of contamination by the long-range transport of pollution and a broader question of the chemical variability of the background atmosphere. Here we show that a general circulation transport model which simulated the global spread and depostion of emissions from fossil-fuel combustion can reproduce the nitrogen measurements at the Mauna Loa Observatory. By isolating individual source regions, we show that US emissions are responsible for the late summer increase and that Asian emissions cause a smaller increase in the spring. These simulations, together with the earlier observations, indicate frequent contamination of the Mauna Loa Observatory by the long-range tranport of reactive trace gases, such as HNO₃, and suggest a highly variable background atmosphere. It is essential that we are aware of such variability in order to discern anthropogenic effects on the atmosphere.