## 3 ISS Orbit Implications for Sampling of Diurnal Variability of Aerosol Vertical Distributions  
Investigating the diurnal variation in clouds and aerosols from space necessitates measurements and observations throughout the daily cycle. Smoke aerosols from biomass burning have been noted to exhibit a diurnal cycle in the Maritime Continent of Southeast Asia, e.g., [17; 57]. In this region, convection can be a major driver of the observed vertical aerosol altitude diurnal cycle. MPLNET is a global network of ground-based lidars and has operated since 2000 [22] providing key vertical measurements of the aerosol and cloud diurnal cycle. Many of these MPLNET sites, including the site in Kuching, Malaysia, featured in our analysis (Figure 6), are co-located with AERONET and other meteorological data collection sites. For comparison to the aerosol diurnal cycle observed by CATS, we use the quality screened MPLNET Version 3 L1.5 aerosol product, a near real-time product [58; 59; 60]. At the time of writing, fully reanalyzed MPLNET Version 3 L2 products are still being reprocessed and Kuching data were not available in the older MPLNET Version 2 archive. The differences between the Version 3 L1.5 and L2 products are generally small as both are quality assured, meaning the L1.5 is sufficient for this comparison to CATS. Due to the differences between the daytime and nighttime sensitivities with lidars such as CATS, aerosol layers are more frequently detected at night than during the day because the minimum detectable backscatter is lower at night. This allows for more tenuous layers to be detected at night. However, there is not a discernable relationship between altitude and the layer optical depth in the CATS-detected aerosol layers in the Maritime Continent, indicating it is expected that there would be little to no diurnal bias in the extinction-weighted altitudes. To quantify the diurnal cycle of aerosols for the regions we analyzed, we employed quality checks on the CATS L2 profile data. Aerosol typing in CATS provides a feature type score associated with the confidence in the feature being aerosol or cloud [23], similar to CALIOP [61]. As the region is plagued by clouds, we excluded UTLS (upper tropospheric/lower stratospheric) aerosols and aerosols in the profile with feature scores of less than 50% confidence to concentrate the analysis on higher-confidence aerosols rather than mistyped clouds. For determining the diurnal cycle of aerosol altitudes, we calculated extinction-weighted altitudes (\(z_{\mathrm{a}}\)) [62; 63; 64] for profiles containing any quality-checked aerosols (Equation (1)):  
\[z_{\mathrm{a}}=\frac{\sum_{i=1}^{n}\beta_{ext,i}z_{i}}{\sum_{i=1}^{n}\beta_{ ext,i}} \tag{1}\]  
Here, \(\beta_{ext,i}\) represents the aerosol extinction of a bin, \(z_{i}\) the lidar bin altitude, and \(n\) the number of vertical bins within the profile. To filter outliers, bin extinction coefficients less than 0 or greater than 5 km\({}^{-1}\) were omitted from the calculations. The extinction-weighted altitude provides an indication of where the peak aerosol loading is located in the profile  
Figure 6: Map of the domains featured including the 15\({}^{\circ}\)\(\times\) 15\({}^{\circ}\) Southeast Asian CATS domain. The MPLNET site at Kuching, Malaysia, is denoted by a point. and provides a useful metric to compare aerosol altitudes between various instruments and models [62; 63; 64]. Processes that influence the vertical distribution of aerosols will in turn impact the evolution of the extinction-weighted altitude, such as the diurnal evolution of the planetary boundary layer (PBL), and interact with convective processes. It is worth noting that this calculation yields a single extinction-weighted altitude, which can introduce bias for complex, multi-layer aerosol scenes that are not contiguous in altitude. This metric is meant to provide a general characterization of the diurnal cycle of aerosol vertical profiles.