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.

Additional Fields

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.