Here we present the first HONO vertical profiles in the atmospheric boundary layer (BL) and the lower free troposphere (FT) over a forested region in northern Michigan and the neighboring Great Lakes, measured from a small aircraft in summer of 2007. The HONO mixing ratios ranged from 4 to 17 pptv in the FT and from 8 to 74 pptv in the BL. The HONO distribution pattern was strongly influenced by the air column stability, i.e., strong negative HONO gradients existed in stable BL in the morning hours, whereas the HONO distribution was relatively uniform in the unstable and well-mixed BL in the afternoons. The ground surface was a major source of HONO in the lower BL. The presence of substantial daytime HONO in the FT (\~{}8 pptv) and in the upper BL (25 pptv) suggests that a significant in situ source of HONO exists in the air column.