Monday, 10 February 2003
Vertical variation of nocturnal NOx chemistry in the urban environment of Phoenix
Nocturnal chemistry in the urban boundary layer is dominated by reactions of NO x and their products, such as NO 3 radicals and N 2O 5. Due to the high stability of the nocturnal boundary layer and ground emissions of NO and VOCís, unique vertical profiles of various chemical species develop throughout the night. These profiles provide rich information about the complex interaction of chemistry and vertical transport. During the Phoenix sunrise experiment in June-July, 2001, the vertical distributions of NO 2, HONO, NO 3, and several other trace gases were measured with a long-path differential optical absorption spectroscopy system in the downtown area. Strong vertical gradients of all species were observed during night, clearly showing that nocturnal chemistry in this urban environment is height dependent. The disappearance of the gradients in the morning shows the onset of vertical mixing during the transition of stable boundary layer to a well-mixed layer. During very stable nights, when NO and VOC emissions were trapped at the ground, mixing ratios of NO 3 up to 200 ppt were found at 110-140 m above the ground. Using this information together with the measurements of NO 2, vertical profiles of N 2O 5 can be derived. In addition, the vertical distributions of HONO and NO 2 allow the investigation of heterogeneous HONO formation processes at the surface and the impact of relative humidity on this process. Clear gradients of HCHO were also observed during most nights with high vertical stabilities, suggesting important direct sources of HCHO. The observations will be compared to results from a 1D chemical box model.