Vertical transport and mixing of

reactive trace gases in the boundary layer affect the chemistry of urban

pollutants. In the stable Nocturnal Boundary Layer (NBL), strong gradients of

ozone, nitrogen oxides and other species have been observed, making the

chemistry at night highly altitude dependent. Less pronounced gradients are

expected during the day. The vertical dependence of urban air chemistry

complicates the interpretation of ground observations of boundary layer

composition, as well as the quantification of urban pollution. The study of

this interaction both during the night and the day is therefore crucial to

improve our understanding of urban air quality and to further develop urban airshed

models.

Here we present data from the TRAMP

experiment held in August and September 2006 in Houston, TX. A long-path

Differential Optical Absorption Spectrometer (LP-DOAS) was set up at 70m

altitude on the roof of Moody tower on the University of Houston campus. Three retroreflector

arrays were mounted ~5km north – northwest in downtown Houston at altitudes of

20m, 130m, 300m. Concentrations of O₃, NO₂, SO₂,

HCHO, HONO, and NO₃ were retrieved in the altitude intervals between

20-70m, 70 – 130m, and 130 – 300m. Pronounced vertical gradients of all species

were observed during most nights. The upper interval was often probing the

residual layer, while the lower intervals were heavily influenced by ground

emissions. On a few occasions vertical gradients were also observed during the

day. A comparison with in-situ data showed that during the night the top of

Moody tower (70m agl) fluctuated between representing the lowest and the middle

LP-DOAS interval, showing the difficulty in interpreting nighttime data without

knowledge on the NBL properties. The impact of vertical mixing on urban air

chemistry is discussed based on various meteorological observations, including

those by a scanning aerosol LIDAR, and a 1D chemical transport model.

Particular attention is given to the chemistry of HONO, and its impact on

daytime OH levels, as well as the nocturnal NO₃ radical chemistry,

and its impact on the NO_x budget in Houston.