## 2 CATS Aerosol-Typing Algorithm Description
Utilizing the IGBP land-surface classification, marine and marine mixture aerosol layers are classified for low depolarizing layers over water that are not elevated. The CATS operational aerosol-typing algorithm modified the CALIOP V3 definition of an elevated aerosol layer to classify elevated aerosol layers as those with at least a 1 km separation between the layer base and the surface and at least 2 km in thickness. This modification is based on [PERSON] et al. (2015) [28], who found that the CALIOP V3.01 definition of an elevated aerosol layer classified transported smoke layers over the ocean as marine over the central Atlantic Ocean. Backscatter lidars, such as CATS, measure the volume depolarization ratio, which includes both molecular and particulate depolarization contributions. To help discriminate marine from marine mixture layers, the feature-integrated spectral volume depolarization ratio (\(\delta^{\prime}{}_{1064}\)/\(\delta^{\prime}{}_{532}\)) is used because polluted marine layers exhibit a greater spectral particulate depolarization ratio than clean marine layers [29]. In CATS Mode 7.1, the integrated spectral volume depolarization ratio was implemented in an attempt to identify marine boundary layers impacted by the presence of anthropogenic aerosols and, therefore, a marine mixture designation is given when the integrated spectral volume depolarization ratio is greater than 1.5 within the aerosol layer [29]. Similarly, over IGBP land surfaces, layers are classified as polluted continental when the layer exhibits low depolarization (\(\delta^{\prime}{}_{1064}\) less than or equal to 0.15), is not elevated, the integrated spectral depolarization ratio fraction is greater than 1.5 [30], or are classified as polluted continental using MERRA-2, as discussed in further detail later in this section. Smoke layers are assigned to elevated aerosol layers with low depolarization (\(\delta^{\prime}{}_{1064}\) less than 0.15), the spectral depolarization ratio fraction is less than 1.5, or classified as smoke using MERRA-2. UTLS aerosol layers are classified as layers with low depolarization (\(\delta^{\prime}{}_{1064}\) less than 0.15), with a feature base above 10 km. The CATS operational aerosol-typing algorithm does not assign a volcanic ash type within the troposphere, as layers with high depolarization and a mid-layer temperature of \(-20\)\({}^{\circ}\)C are classified as a cirrus cloud by the CAD when below the MERRA-2 tropopause height. The CATS Mode 7.2 operational aerosol-typing algorithm is very similar to the Mode 7.1 algorithm; however, the observedinputs are limited to \(\gamma^{\prime}{}_{1064}\) and \(\delta^{\prime}{}_{1064}\). Therefore, due to the lack of spectral depolarization information, a marine mixture is not distinguished from marine and therefore is not classified, and the elevation criteria and MERRA-2 information are used to distinguish polluted continental from smoke over land (Figure 1). It should be noted that a subsequent paper will focus on lidar ratios assigned to each CATS aerosol type and CATS-derived aerosol optical properties. The use of ancillary information such as the IGBP surface type and elevation criteria to distinguish the aerosol type can lead to aerosol-typing biases [28; 31], which may adversely impact retrieved aerosol optical properties such as extinction. Therefore, in both Mode 7.1 and 7.2 CATS final V3-00 operational aerosol-typing algorithms, simulated aerosol information from the NASA Goddard Earth Observing System MERRA-2 reanalysis [32; 33; 34] is utilized. MERRA-2 treats aerosols with an online version of the Goddard Chemistry, Aerosol, Radiation, Transport (GOCART) model and simulates the mass of dust, seasalt, sulfate, black carbon, and organic carbon aerosols [35; 36]. Notably, biomass-burning aerosol emissions in GEOS-5 are parameterized using the Quick Fire Emission Database (QFED), based on Fire Radiative Power (FRP) reported by the Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua and Terra sensors [37].