## 2 CATS Aerosol-Typing Algorithm Description  
For the past two decades, backscatter lidars such as the ground-based MPLNET, airborne Cloud Physics Lidar (CPL), and spaceborne CALIOP have demonstrated the ability to determine aerosol types using backscatter and depolarization measurements along with ancillary information. The CATS instrument was built on the heritage of these missions both in instrument design and in its algorithms. As described in [PERSON] et al. (2016) [11], CATS operated in two science modes. From 10 February 2015 to 21 March 2015, CATS operated in Mode 7.1 which provided 532 and 1064 nm total attenuated backscatter and volume depolarization ratio measurements from two fields of view separated by 7 km on the ground. Beginning 25 March 2015 and through the remainder of operations, CATS operated in Mode 7.2, providing total attenuated backscatter and volume depolarization ratio measurements at 1064 nm from one 0.5\({}^{\circ}\) forward field of view. CATS Level 1B (L1B) products such as the total attenuated backscatter, volume depolarization ratio, and associated uncertainties are reported at a 60 m vertical and 350 m horizontal resolution. A detailed description of the CATS cloud-aerosol discrimination (CAD) algorithm and feature confidence scoring is provided in [PERSON] et al. (2021) [23]. Additionally, as described in [PERSON] et al. (2021) [23], CATS Level-2 (L2) products such as the feature detection and classification are reported at a 60 m vertical and 5 km horizontal resolution. For daytime CATS granules, a coarser horizontal (60 km) averaging of the L1B products is regularly used to increase the signal-to-noise ratio (SNR) for improved feature detection in sceneswith high solar background contributions; however, the products are still reported at a 5 km horizontal resolution. The CATS aerosol-typing algorithm leverages the V3 CALIOP aerosol-typing algorithm [24]. However, because CATS operated in Mode 7.2 for most of its lifetime, the primary observable inputs to the CATS algorithm are at 1064 nm, as opposed to 532 nm used by CALIOP. The observed inputs to the CATS operational aerosol-typing algorithm include feature-integrated total attenuated backscatter (sr\({}^{-1}\)) at 1064 nm (\(\gamma^{\prime}{}_{1064}\)), feature-integrated volume depolarization ratio (defined as the ratio between the perpendicular total backscatter to the parallel total backscatter) at 1064 nm (\(\delta^{\prime}{}_{1064}\)), and the feature-integrated spectral volume depolarization ratio (\(\delta^{\prime}{}_{1064}\)/\(\delta^{\prime}{}_{532}\)), though the latter input is available only in Mode 7.1. In addition to the observed quantities, the CATS aerosol-typing algorithm uses ancillary information, such as the surface type from the International Geosphere-Biosphere Programme (IGBP) [25], feature thickness and elevation, and aerosol typing provided by MERRA-2, as described later in this section. The Mode 7.1 operational aerosol-typing algorithm classifies eight aerosol types: desert dust, dust mixture, smoke, polluted continental, clean/background, marine, marine mixture, and upper troposphere-lower stratosphere (UTLS) aerosol (Figure 1). Based on the CPL observations of Saharan dust from the NASA Tropical Composition, Cloud, and Climate Coupling (TC\({}^{4}\)) [26] and Hurricane and Severe Storm Sentinel (HS3) [27] field campaigns, desert dust features are identified when \(\delta^{\prime}{}_{1064}\) is greater than 0.25. A dust mixture, which is meant to characterize desert dust mixed with other aerosol types such as smoke and marine aerosol types, is classified when \(\delta^{\prime}{}_{1064}\) is greater than 0.15 and less than or equal to 0.25. For spherical aerosols (low depolarization ratios) and owing to similar minimal detectable backscatter (MDB) between CATS at 1064 nm and CALIOP at 532 nm [11], a clean/background type is assigned to layers with \(\gamma^{\prime}{}_{1064}\) less than 0.0005 sr\({}^{-1}\), as in the CALIOP V3 aerosol-typing algorithm.