# Aerosol Data Fields

## Aerosol Extinction

There are two primary aerosol fields that may be of use when looking at extinction:

extinction

Aerosol extinction at 750 nm in units of km-1. Only converged values in the retrieval range and above any cloud layer are reported.

extinction_cloudy

The same as extinction, except that it has not been cloud cleared.

The associated error in the retrieved extinction is given by:

extinction_error

Standard uncertainty of the aerosol extinction at 750 nm in units of km-1.

The vertical resolution is computed for every retrieved profile from the FWHM of the averaging kernel and is given by:

vertical_resolution

FWHM of the averaging kernel, with units of km.

Although not recommended for general use, the field _rtm_internal_extinction may be useful in investigating possible biases due to retrieval saturation or other similar effects. This field includes altitudes affected by clouds and those that have not fully converged due to high aerosol loading or other reasons.

_rtm_internal_extinction

The full aerosol extinction profile as used in the SASKTRAN radiative transfer model.

## Clouds

Substantial work has gone into developing a cloud flagging algorithm for this dataset to help improve the aerosol product. While care has been taken to limit false positives, it should be noted that the OSIRIS spectrum has limited cloud/aerosol discrimination information and there is no singular threshold that will perfectly discriminate between all cases. As such, care should be taken when using the extinction product, as there are likely to be cases where thin clouds remain and where thick aerosol layers have been filtered.

Clouds are separated into two categories:

cloud_top_altitude

The altitude, in km, of the top of a cloud detected in this profile, NaN otherwise

psc_altitude

The altitude, in km, of a polar stratospheric cloud detected in this profile, NaN otherwise

Note

Sensitivity to clouds varies considerably with altitude, scattering angle and aerosol loading and this should be considered if using the cloud altitude for scientific purposes. Additionally, while the extinction value is reported in _rtm_internal_extinction when clouds are flagged, it is unlikely to be a reliable quantitative measure, as assumptions made in the retrieval are not consistent with cloud microphysics.

## Meteorology

A few meteorological fields are included in the data to aid in analysis.

temperature

Temperature taken from MERRA 2 and interpolated to the current geometrical altitude grid.

pressure

Pressure in hPa taken from MERRA 2 and interpolated to the current geometrical altitude grid.

tropopause_altitude

Tropopause in km based on WMO lapse-rate tropopause definition; the first parameter for the lowest tropopause.Calculated from MERRA 2 temperature profiles.

## Geometry

For ease of use, each OSIRIS scan is assigned to a specific geographic location and time. This is not strictly true, as a scan takes approximately 90 seconds to complete and spans a few degrees latitude over this time. The geometric data fields are defined at the 30 km point of each scan.

latitude

latitude in degrees north

longitude

longitude in degrees east

time

days since 1900-01-01 00:00:00

local_solar_time

Local solar time in hours

In addition to the geographic location, three solar angles are provided that define the measurement geometry of the line of sight with respect to sun. Solar scattering angles are particularly important for aerosol retrievals as biases in the assumed particle size are dependent upon this parameter.

ssa

Solar scattering angle. The angle between the line of sight and the solar beam.

sza

Solar zenith angle. The angle between solar beam and the vertical direction at the tangent point.

saa

Solar azimuth angle. The angle between the horizontal projection of the solar beam and the line of sight.

albedo

The retrieved albedo of the surface.

## Retrieval

These fields provide information about the retrieved aerosol extinction profiles. It should be noted that values outside of the bounds given by retrieval_lowerbound and normalization_altitude are truncated from extinction and extinction_cloudy. Profiles with poor convergence are also filtered from these products.

retrieval_lowerbound

Lowest altitude in km that is valid for the aerosol retrieval.

normalization_altitude

Lowest normalization altitude in km used in the aerosol retrieval, which sets the upper bound of the retrieved aerosol.

convergence_ratio

Convergence ratio of the aerosol retrieval.

chi_sq

The $${\chi}$$2 value of the aerosol retrieval.