Venting of
heat and
carbon dioxide from urban canyons at night
Salmond, J.A.,
Oke, TR.,
Grimmond, C.S.B.,
Roberts, S.,
Offerle, B.
JOURNAL OF
APPLIED
METEOROLOGY 44 (8): 1180-1194 AUG 2005
Abstract: Turbulent
fluxes of carbon dioxide and sensible heat were observed in the surface
layer
of the weakly convective nocturnal boundary layer over the center of
the city
of Marseille, France, during the Experience sur Sites pour Contraindre
les
Modeles de Pollution Atmospherique et de Transport d'Emission
(ESCOMPTE) field
experiment in the summer of 2001. The data reveal intermittent events
or bursts
in the time series of carbon dioxide (CO2) concentration and air
temperature
that are superimposed upon the background values. These features relate
to
intermittent structures in the fluxes of CO2 and sensible heat. In
Marseille,
CO2 is primarily emitted into the atmosphere at street level from
vehicle
exhausts. In a similar way, nocturnal sensible heat fluxes are most
likely to
originate in the deep street canyons that are warmer than adjacent roof
surfaces. Wavelet analysis is used to examine the hypothesis that CO2
concentrations can be used as a tracer to identify characteristics of
the
venting of pollutants and heat from street canyons into the above-roof
nocturnal urban boundary layer. Wavelet analysis is shown to be
effective in
the identification and analysis of significant events and coherent
structures
within the turbulent time series. Late in the evening, there is a
strong
correlation between the burst structures observed in the air
temperature and
CO2 time series. Evidence suggests that the localized increases of
temperature
and CO2 observed above roof level in the urban boundary layer (UBL) are
related
to intermittent venting of sensible heat from the warmer urban canopy
layer
(UCL). However, later in the night, local advection of CO2 in the UBL,
combined
with reduced traffic emissions in the UCL, limit the value of CO2 as a
tracer
of convective plumes in the UBL.