## 2 CATS Aerosol-Typing Algorithm Description
Aerosol optical properties from the Optical Properties of Aerosols and Clouds (OPAC) database [38] are used for seasalt, sulfate, and carbonaceous species, while an observationally derived set of aerosol optical properties are used for dust [28; 39]. In addition to meteorological observations to constrain the dynamical state of the system, MERRA-2's aerosol optical thickness  
Figure 1: Schematic of the CATS aerosol-typing algorithm for both science modes. (AOT) is assimilated to constrain simulated aerosol loadings using observations from both Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua and Terra sensors [40, 41], the Advanced Very High Resolution Radiometer (AVHRR) [42], the Multiangle Imaging SpectroRadiometer (MISR) [43], and the Aerosol Robotic Network (AERONET) ground network of sunphotometers [44]. Following a modified methodology described in [PERSON] et al. (2015) [28], MERRA-2 aerosols are mapped to CATS aerosol types to distinguish smoke (carbonaceous dominated) from polluted continental (sulfate dominated-sulfate mixed with carbonaceous aerosols). The use of the MERRA-2-based aerosol typing is conservatively invoked only when MERRA-2 simulates a smoke or polluted continental layer at the exact altitude range observed by CATS with a minimum AOT at 532 nm of 0.05, consistent with the CATS daytime minimal detection limits for an aerosol layer at 532 nm in Mode 7.1. Following [PERSON] et al. (2015) [28], MERRA-2 aerosols are mapped to the CATS smoke aerosol type when the fraction of the total carbonaceous to the total aerosol extinction within the layer is at least 0.75. MERRA-2 aerosols are mapped to the CATS polluted continental type when the fractions of the total sulfate to the total extinction exceeds 0.75 or when the fraction of the total sulfate and carbonaceous extinctions to the total aerosol extinction is at least 0.25 individually and their combined extinction to the total aerosol extinction is at least 0.75. If aerosols simulated in MERRA-2 fail to meet these criteria, a default IGBP surface type and elevation criteria are used to distinguish smoke from polluted continental. An example of how CATS observables and MERRA-2 aerosols are used to classify the aerosol type on 6 August 2016 is provided in Figure 2. On this day, CATS flew over Africa and classified depolarizing aerosols as dust north of 10\({}^{\circ}\) N and non-depolarizing aerosols as smoke followed by polluted continental to the south, comparing favorably to the aerosol types observed by Suomi/NPP Deep Blue [45] (Figure S1). The MERRA-2 aerosol information was invoked to inform the transition in the aerosol type from smoke to polluted continental around 14\({}^{\circ}\) S.  
An example showing the use of the spectral particulate depolarization ratio to discern between polluted and clean marine layers on 8 March 2015 is shown in Figure 3. Here, CATS observes a layer of relatively clean marine aerosols to the south and west of the Philippines with an average spectral particulate depolarization ratio of 1.36 (left side of Figure 3) and more polluted layers to the north and east with average spectral particulate depolarization ratios of 1.65 (right side of Figure 3). Analyzing the back trajectories, the northeastern polluted region originated near southeast China and the cleaner southwestern region over the remote Pacific Ocean (Figures S2 and S3). In Figure 4, we present the seasonal variation in the CATS most frequently classified aerosol type below 4 km for day and night granules for 2015-2017. A key feature to note is the increased transport of Saharan dust into the Caribbean during JJA [46, 47, 48], observed by CATS. Additionally, we find the increased detection and transport of smoke associated with seasonal biomass burning over northwestern North America [49, 50], the Amazon [51, 52], and over central and southern Africa [53, 54] in JJA and SON.