  The angle-dependent scattering effect of aerosols in the atmosphere can be
used to infer their compositions, which in turn is important to understand
their impacts of human health and Earth climate. The aerosol phase function,
which characterizes the angular signature of scattering, has been continuously
monitored from ground-based and space-borne observations. However, the range of
scattering angles these instruments can sample is very limited. There is a
dearth of research on the remote sensing of aerosol angular scattering effect
at a city scale that analyzes diurnal variability and includes a wide range of
scattering angles. Here, we quantify the aerosol angular scattering effect
using measurements from a mountain-top remote sensing instrument: the
California Laboratory for Atmospheric Remote Sensing Fourier Transform
Spectrometer (CLARS-FTS). CLARS-FTS is located on top of the Mt. Wilson (1.67km
above sea level) overlooking the Los Angeles (LA) megacity and receives
reflected sunlight from targeted surface reflection points. The observational
geometries of CLARS-FTS provide a wide range of scattering angles, from about
20 degrees (forward) to about 140 degrees (backward). The O2 ratio, which is
the ratio of retrieved O2 Slant Column Density (SCD) to geometric O2 SCD,
quantifies the aerosol transmission with a value of 1.0 represent aerosol-free
and with a value closer to 0.0 represents stronger aerosol loadings. The
aerosol transmission quantified by the O2 ratio from CLARS measurements
provides an effective indicator of the aerosol scattering effect.
