Nocturnal chemistry in urban areas can considerably influence the composition of the boundary layer (BL) and therefore determine the conditions at sunrise. Since the trace gas levels in the morning BL influence the photochemistry of the day it is crucial to understand the nocturnal processes. Studying the chemical composition of the BL is difficult because of the interplay of homogeneous and heterogeneous chemical processes and weak turbulent mixing. This interaction together with direct emissions of NO and hydrocarbons at the ground can lead to strong vertical gradients of all compounds. This is particularly significant for the chemistry of the main oxidants ozone and NO₃ which are responsible for the removal of NO _x and various VOC’s. It also plays an important role in the formation of daytime OH precursors such as nitrous acid. Here we present data from a number of recent field studies (SOS 1999 Nashville, TN; TEXAQS 2000, Houston, TX; Phoenix 2001, AZ; etc.) showing the vertical distribution of various trace gases measured by differential optical absorption spectroscopy (DOAS). The measurements show vertical gradients of ozone, NO ₂, NO ₃, and HONO during many nights. A comparison of the results of the individual campaigns and the results from a one-dimensional chemical box model allows us to compose a picture of the chemistry in the nocturnal boundary layer. Common features are elevated HONO levels at the ground, and high NO ₃ levels aloft. Our measurements also allow us to determine the significance of processes such as NO _x and VOC removal, as well as the investigation of several heterogeneous reactions. The implications of our findings for air quality monitoring and the photochemistry of the following day will also be discussed.