Measurements of HNO3 vapor and particulate NO3- were made in 1977 and in 1978 over western North America and the Pacific Ocean, both in the boundary layer and in the free troposphere. The boundary layer HNO3 ranged from <0.01 to 0.86 ppbv, with a median of 0.06--0.08 in remote areas. Free tropospheric HNO3, was more uniform, ranging from <0.03 to 0.54 ppbv with a median of 0.12 ppbv. The lack of an average gradient from 70¿N to 55¿S at 5--6 km suggests that a dispersed natural NOx source is responsible for this free tropospheric HNO3, rather than an anthropogenic one, since most urban NOx is emitted in the northern hemisphere. This and the large variability of boundary layer HNO3 with proximity to populated areas suggest that much anthropogenic NOx is removed fairly near its source. Particulate nitrate varied from <0.04 to 0.8 ppbm (median 0.11) in the free troposphere and from <0.04 to 3.5 ppbm (median 0.15) in the boundary layer. The molar ratio of vapor to particulate nitrate is generally much larger than 1 in the free troposphere.