Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 117

DOAS can also be used for emission studies. For example, a DOAS consisting of
a emitter/receiver unit and three retroreflectors was used for a motorway pollutant
emission study. One path was oriented across the motorway (120 m path length)
in about 10 m altitude above street level, another path was set up in parallel to the
motorway, and the third path was operated perpendicularly away from the motor-
way. Comparison of the concentrations found along these three paths allows for an
estimation of the pollution emission from the motorway (Schäfer et al. 2006b ).

4.4 Vertical Flux Profiles

Fluxes of momentum, energy, and substances are important links between the dif-
ferent compartments of the Earth system. Fluxes from the atmosphere to the Earth's
surface and vice versa must pass through the ABL and have considerable influences
on the state of the ABL. Fluxes usually consist of a mean and a turbulent part. The
mean part happens with mean atmospheric motions while the turbulent fluxes are
due to the covariance between turbulent fluctuations of the wind and the respected
transported quantity without any mean motion. We will focus here on vertical fluxes
through the ABL. Due to the proximity of the Earth's surface mean vertical motions
in the ABL are usually small, so that turbulent fluxes are the dominant fluxes in
the ABL. The direct measurement of turbulent fluxes requires the simultaneous
observation of vertical wind fluctuations and fluctuations of the respective trans-
ported quantity with high temporal resolution of at least 10 Hz. Such observations
are already difficult with in situ instrumentation, the more this is problematic with
remote-sensing instrumentation. Nevertheless, quite considerable efforts have been
made in the past decades to detect vertical profiles of turbulent fluxes in the ABL
(Table 4.3 ).
An overview of the measurement of turbulent momentum and heat fluxes with
wind profiler RADAR, SODAR, and RASS has been given by Engelbart et al.
( 2007 ). Tables 4.4 and 4.5 have been updated from this review. Although they
see impressive advances in the development of methods for remote determination
of turbulent fluxes in the ABL, no presently known method can be used already
for routine measurements. Therefore, the following subsections on remote flux
measurements can only serve as references to the current efforts to work on this
issue.

Table 4.3 Overview on methods to derive vertical profiles of turbulent fluxes using ground-based
remote sensing (see rightmost column for section number)

Variable Detection method Section

Momentum K- and X-band RADAR, Doppler wind LIDAR 4.4.1

Heat RASS, scintillometry 4.4.2

Humidity DIAL plus Doppler wind LIDAR or wind profiler 4.4.3