# A Geostationary Instrument Simulator for Aerosol Observing System Simulation Experiments
## 2 Methodology and Data
### GEOS-5 Nature Run Aerosol Simulation
#### 2.2.2 Evaluation of G5 NR Aerosols  
Aerosol concentrations in the G5 NR simulation were evaluated with the following datasets [13]: (1) global concentrations from the NASA/GMAO MERRA Aerosol Reanalysis (MERRAero) [31], (2) statistics from the AeroCom Phase I models [32], and (3) CALIOP aerosol attenuated backscatter vertical profiles [33]. The evaluation was meant to assess the overall realism of the simulated aerosols,and mainly focused on broad performance metrics of the simulation such as aerosol residence times, and the spatial and temporal distributions of aerosol composition and AOD. As the G5 NR is a free running simulation, differences are to be expected between the distributions of observed and simulated atmospheric composition due to meteorological variability. G5 NR aerosol residence times, and wet and dry removal rates are listed in Table 2. The residence time is defined as the global aerosol total column mass divided by the removal rate (See Section 6 in [32]). Also included in the table are the AeroCom multi-model mean parameters and their standard deviations reported in [PERSON] et al. [32]. The residence times of sea salt, dust, and black carbon agree well with the AeroCom multi-model means. The residence time of organic carbon in G5 NR is roughly one day less than the AeroCom mean, but lies well within one standard deviation of the mean. However, the residence time of sulfate aerosol is significantly (36%) lower than what is reported by the AeroCom models. The shorter sulfate residence time in G5 NR is due to the relatively high wet removal rate of the simulation of 0.34 days\({}^{-1}\). The 2006 seasonal mean 550 nm AOD (AOD\({}_{550}\)) is shown in Figure 3 from the MERRAero Reanalysis and G5 NR. The MERRAero aerosol reanalysis includes assimilation of AOD observations from the MODIS sensor on both Terra and Aqua, and provides a climatology of the aerosol spatio-temporal distribution. Overall, the seasonal and spatial patterns in G5 NR are very similar to MERRAero. Patterns of aerosol enhancements agree well with the major sources and transport routes of aerosols. However, over Africa and along the westward transport route of Saharan dust, G5 NR AOD\({}_{550}\) is systematically higher than MERRAero. Likewise, G5 NR AOD\({}_{550}\) is higher than MERRAero in the regions affected by dust that originate from sources in the Middle East. Figure 4 shows the monthly time series of global mean AOD\({}_{550}\) for MERRAero and G5 NR. The G5 NR temporal variability and the magnitude of the global mean AOD\({}_{550}\) are similar to the aerosol climatology, which is expected given the similarities in the primary emissions. However, there are significant differences between G5 NR and MERRAero in speciated global mean AOD\({}_{550}\). Underestimated carbon and sulfate were compensated by overestimated dust. This indicates that the wind driven dust emissions and resolution dependent biomass burning emissions should be re-tuned for the mesoscale resolution of G5 NR. \begin{table}
\begin{tabular}{l c c c} \hline \hline  & **Residence Time** & **Dry Removal Rate** & **Wet Removal Rate** \\  & **[days]** & **[day\({}^{-1}\)]** & **[day\({}^{-1}\)]** \\ \hline Organic Carbon & 5.33 (6.54 \(\pm\) 27\%) & 3.55 \(\times\) 10\({}^{-2}\) & 1.52 \(\times\) 10\({}^{-1}\) \\ Black Carbon & 7.31 (7.12 \(\pm\) 33\%) & 3.08 \(\times\) 10\({}^{-2}\) & 1.06 \(\times\) 10\({}^{-1}\) \\ Sulfate & 2.67 (4.12 \(\pm\) 18\%) & 3.18 \(\times\) 10\({}^{-2}\) & 3.43 \(\times\) 10\({}^{-1}\) \\ Sea Salt & 0.42 (0.48 \(\pm\) 58\%) & 1.13 & 1.24 \\ Dust & 4.22 (4.14 \(\pm\) 43\%) & 1.55 \(\times\) 10\({}^{-1}\) & 8.16 \(\times\) 10\({}^{-2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Residence and removal rates of aerosols in G5 NR. The residence times in parenthesis are multi-model means and standard deviations from the AeroCom project reported in [32]. Figure 3: 2006 seasonal mean 550 nm AOD from the MERRAero aerosol reanalysis (left column) and the G5 NR (right column).