# Data Assimilation of AOD and Estimation of Surface Particulate Matters over the Arctic
## 2 Experiments
### Description of Remote-Sensed Observations  
The mission of the MODIS instrument equipped on the NASA Earth Observation System (EOS) platforms of Terra and Aqua satellites is to capture the vertical columnar aerosol loading and aerosol properties such as AOD, Angstrom exponent, and single scattering albedo over the cloud- and snow-free land and ocean. The MODIS/Terra observes the atmosphere with a descending orbit, passing over the equator at about 10:30 local sun time while the MODIS/Aqua flies northward across the equator at about 13:30 local sun time. In particular, the vertically-integrated aerosol extinction (i.e., AOD, \(\tau_{MODIS}\)) observed by the MODIS sensor has been used to evaluate the performance of 3D-CTM and conduct the data assimilation of the model-calculated AODs [69; 70; 71]. For such purpose of study, we obtained the daily level 2 AOD of Terra satellite (MOD04_L2) from the NASA LAADS ([[https://ladsweb.modaps.eosdis.nasa.gov/search/](https://ladsweb.modaps.eosdis.nasa.gov/search/)]([https://ladsweb.modaps.eosdis.nasa.gov/search/](https://ladsweb.modaps.eosdis.nasa.gov/search/))). The AOD products were retrieved at a wavelength of 550 nm through the NASA Collection 6 algorithm (C6). Although, latest product via the C6.1 algorithm were released, there is no change in the combined dataset between the C6 and C6.1 collections [72]. Figure 1: Bottom–up emissions of (**a**) SO\({}_{2}\), (**b**) NO\({}_{x}\), (**c**) EC, (**d**) OC, (**e**) PM\({}_{10}\), and (**f**) PM\({}_{2.5}\) for the CMAQ simulations over the Arctic. The red triangles (**a**) in the panel of (**a**) represent the locations of five AERONET stations of Thule, Hornsund, Andenes, Hyytiala, and Kuopio. The C6 includes three retrieval algorithms, which are (i) Dark Target (DT) over vegetated/dark-soiled land retrieved in the visible wavelength [73; 74], (ii) Deep Blue (DB) over the brighter surface in the visible wavelength (e.g., deserts) [75], and (iii) DT over water surface in the visible and longer wavelength [74; 76]. The expected uncertainty of AOD was reported to be \(\pm\) (0.05 \(\pm\) 15%) over land and (0.04 \(\pm\) 10%, \(-\)0.02 \(\pm\) 10%) over the ocean [74]. [PERSON] et al. also reported that in comparison with AERONET-retrieved AODs (\(\tau_{AERONET}\)), 69.4% of \(\tau_{MODIS}\) fall within expected error [74]. In this study, to use more gap-filled data set, we used a merged AOD product combining the DT with DB over ocean and land, which is \"AOD_550_Dark_Target_Deep_Blue_Combined\" data with a spatial resolution of 10 km \(\times\) 10 km at the nadir (and 48 km \(\times\) 20 km near the swath edge) and the temporal resolution of 5 min [74; 77]. In the analysis, only pixels recommended quality assurance, good and very good flag (QA = 2 and 3) were used [77]. The MODIS-retrieved AOD (\(\tau_{MODIS}\)) were re-gridded to a model grid of 18 km \(\times\) 18 km on a daily basis. Figure S1 shows a spatiotemporal map of the monthly availability of data for the entire domain, using the merged AOD product. As expected, there were large numbers of missing pixels over the Arctic Ocean and Greenland because of high surface albedo due to sea ice and snow cover. The low data availability over the Arctic Ocean region greatly varies from season to season. On the other hand, higher than 30% of data availability was found frequently over some parts of the Greenland Sea, Barents Sea, and the Norwegian Sea. The results indicate that data integration (or assimilation) between model-calculated and satellite-observed AODs is needed for the regions showing high surface albedo such as the Arctic to address the defects of each model estimation and satellite observation and to maximize their advantages. The AOD product retrieved from the C6 algorithm has been validated with the AERONET-observed AOD (\(\tau_{AERONET}\)), which is a global network of ground-based sun/sky-photometers [78]. The sun-photometers measures the sun radiances at the eight typical channels of 340, 380, 440, 500, 675, 870, 1020, and 1640 nm.