Remote sensing of angular scattering effect of aerosols in a North American megacity.

The angle-dependent scattering effect of aerosols in the atmosphere not only influences climate through radiative forcing effects but also impacts trace gas remote sensing by modifying the path of radiation through the atmosphere. 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. Here, we report multi-year measurements from a mountain-top remote sensing instrument: the California Laboratory for Atmospheric Remote Sensing Fourier Transform Spectrometer (CLARS-FTS), which overlooks the Los Angeles megacity. The observational geometries of CLARS-FTS provide a wide range of scattering angles, from about 20° (forward) to about 140° (backward), which is larger than the range provided by any existing aerosol remote sensing instrument. We then quantify the aerosol angular scattering effect using the O 2 ratio, which is the ratio of retrieved O 2 Slant Column Density (SCD) to geometric O 2 SCD. The O 2 ratio quantifies the light path modification due to aerosol scattering, with a value of 1 representing an aerosol-free scenario. The lower the O 2 ratio value than 1, the stronger the aerosol loading. CLARS-FTS measurements are highly sensitive to the angular scattering effect of aerosols in the Los Angeles (LA) urban atmosphere, due to the long light path going through the boundary layer and the wide range of observational angles. The differences in aerosol scattering between different surface reflection points targeted by CLARS-FTS can be explained by differences in their angular scattering geometries. The correlation between measurements at different targets can be used to quantify the strength of the angular dependence of the aerosol phase function. Applying the correlation technique to CLARS-FTS measurements, we find that, from 2011 to 2018, there is no significant trend in the aerosol phase function in the LA megacity. Overall, this study provides a practical observing strategy for quantifying the angular dependence of aerosol scattering in urban atmospheres that could potentially contribute towards improved greenhouse gas remote sensing in megacities. • A mountain-top observatory for monitoring aerosols in megacities is introduced. • The observatory makes measurements at a wide range of scattering angles. • Aerosol scattering is quantified based on retrieved oxygen slant column. • The aerosol scattering pattern can be explained by variations in scattering angle. • No significant change in aerosol phase function in LA from 2011 to 2018. [ABSTRACT FROM AUTHOR]