Modeling studies of the urban atmosphere show that the composition of the boundary layer is often altitude dependent, in particular in the presence of weak vertical mixing and high surface emissions of NO. Despite our qualitative understanding of the altitude dependence of atmospheric composition and chemistry, little is known about the trace gas vertical profiles in the altitude interval between 50 and 500 m, a range that is not probed by surface measurements and is often inaccessible by aircraft.

In this paper, we will present the LP-DOAS measurements of O₃, NO₂, SO₂, HCHO, HONO and NO₃ in three altitude intervals (30-70m, 70 – 130m, and 130 – 300m). These measurements were performed during the 2009 SHARP experiment in Houston, TX from April 15 to May 29, 2009. The observations were horizontally averaged over 4 -5km between the University of Houston and downtown Houston. Vertical gradients were often observed during the night, with lower ozone and higher NO₂ and HONO mixing ratios near the surface. During many days gradients of NO₂ and ozone were also found, albeit smaller than at night. Daytime HONO at levels of 50 – 100ppt were observed. In many cases HONO mixing ratios were higher near the surface. We will also compare our 2009 observations with those made at the same location in 2006 as well as with results from a 1D chemical transport model.