## 3 ISS Orbit Implications for Sampling of Diurnal Variability of Aerosol Vertical Distributions
To calculate the change in the diurnal cycle over the island of Borneo (15\({}^{\circ}\)\(\times\) 15\({}^{\circ}\) box from 105\({}^{\circ}\) E west to 120\({}^{\circ}\) E and from 5\({}^{\circ}\) S to 10\({}^{\circ}\) N) (Figure 7), these extinction-weighted altitudes were binned by UTC hour, centered over the hour (e.g., from 00:30 to 01:30 for 01:00). The altitudes represented in Figure 7 include those within the two adjacent hour bins (i.e., values for 03:00 include matching observations centered over 02:00 and 04:00). The extinction-weighted altitudes were calculated as in Equation (1) for the two years of MPLNET V3 data availability (August through October 2018 and July through November 2019). The median diurnal cycle of the aerosol extinction-weighted altitude in this region is statistically resolved using CATS data, with the quality assurances previously described, and compares well with the median diurnal cycle observed by the MPLNET ground-based lidar station in Kuching, Malaysia (1.49\({}^{\circ}\) N, 110.35\({}^{\circ}\)E), and generally resides within the MPLNET interquartile region (Figure 7). Specifically, the aerosol extinction-weighted altitudes increased around 200 m from the morning (0:00 UTC, local time UTC+8) in the MPLNET record and slightly less than that as observed by CATS. These altitudes peak locally in the afternoon around 7:00 UTC (15:00 local time) before decreasing in altitude into the evening hours and are consistent with the building of the mixed layer in the daily PBL cycle. In a comparison to available extinction-weighted altitudes determined from the oxygen absorption bands obtained during daylight conditions from the sun-synchronous Earth Polychromatic Imaging Camera (EPIC) [63; 64], aerosol altitudes in this region also increase in the afternoon hours (Figure A1) (Methodology included in Appendix A). In this region, the MPLNET aerosol altitudes were generally higher than those observed by CATS--on the order of 400 m (Figure S4); however, as previously discussed, data from both lidars show a similar diurnal cycle. The diurnal cycle observed by EPIC was more pronounced with altitudes increasing around 1 km from the daily altitude minimum around solar noon to sunset (Figure A1). The discrepancies between the MPLNET and CATS aerosol altitudes may be due to differences in geographical sampling or in the years  
Figure 7: Observed diurnal cycle (difference from 0:00 UTC, local time UTC+8) in extinction-weighted altitudes for CATS and MPLNET for July, August, September, October, and November. Shaded region denotes the interquartile range (25–75% of data). sampled (2018 and 2019 for MPLNET and 2015-2017 for CATS), as the Kuching MPLNET site was not operational during the CATS lifetime. Specifically, a large portion of the 15\({}^{\circ}\)\(\times\) 15\({}^{\circ}\) CATS domain (Figure 6) was marine which is more atmospherically stable and would not exhibit as pronounced of a convective cycle as the ground site at Kuching on Borneo. Finally, discrepancies may arise due to the different measurement directions of CATS and MPLNET, as the ground-based MPLNET lidar permits measurements of aerosols below thick water clouds that would attenuate the CATS signal from space. Conversely, CATS will measure aerosols above thick water clouds where the MPLNET signal would be adversely impacted by signal attenuation. Because the CALIOP instrument provides measurements at daily equatorial crossings corresponding to roughly 1:30 a.m./p.m., it can only provide a sample of the diurnal cycle in clouds and aerosols. For many marine domains, this sampling around the night and early afternoon may correspond to the daily minimum and maximum in vertical cloud frequency; however, for terrestrial locations, this relation may not hold [12]. Indeed, when aggregating across the breadth of the CATS record with the variety of local solar times sampled, we find many regions where the CALIOP sampling hours do not correspond to the daily minimum and maximum in the CATS vertical aerosol detection frequency (Figure 8).