Measurements and analysis of nitrogen oxides and ozone in the yard and on the roof of a street-canyon in Suzhou.

The concentrations of air pollutants such as nitrogen oxides and ozone characterised by very fast chemical reactions can
significantly vary within urban street-canyon due to the short distances between sources and receptor. With the primary
objective to analyse this issue, NO, NO2, NOx, O3, BTX, and wind flow field were continuously measured for 1 week at two
heights (a street-level yard and a 25-m-high rooftop) in an urban canyon in Suzhou (China). The yard ozone
concentrations were found to be up to six times lower than on the roof. Different frequency distributions (FD), dynamical
and chemical processes of the pollutant variations from yard to roof are discussed to explain the findings. The predominant
factors for the dissimilar pollutant vertical diffusion at the two measurement locations were associated to dissimilar fluiddynamic
and heterogeneous removal effects that likely induced dissimilar ozone chemical processes relative to NOx and
BTX precursors.