# Data Assimilation of AOD and Estimation of Surface Particulate Matters over the Arctic
## 3 Results and Discussions
### Estimations of Surface PMs from Assimilated AODs over the Arctic
With low biases ranged from \(-1.52\) to \(-0.92\) ug m\({}^{-3}\) except for Vindeln, these estimated values were close to the observations. The temporal variations of \(PM_{S,\text{CMAQ(w/OI)}}\) were also calculated at several sites of Thule, Hyytiala, Horsund, Kuopio, Andenes, and the North Pole, as shown in Figure 6. The estimated surface concentrations of \(PM_{10,\text{CMAQ(w/OI)}}\) (\(PM_{2,5,\text{CMAQ(w/OI)}}\)) averaged for April-September were 5.88, 3.78, 6.37, 2.19, 7.99, 0.61 ug m\({}^{-3}\) (2.21, 3.09, 2.54, 1.79, 2.25, 0.41 ug m\({}^{-3}\)), respectively. As shown in Figure 6, the estimated values increased by approximate 170-670% for all regions except the North Pole, compared to those from the CMAQ model simulations. The temporal variations of model-calculated PM\({}_{10}\) and PM\({}_{2.5}\) (colored as bright and dark green lines, respectively) remained flat. However, the estimated _PM\({}_{10,\text{CMAQ(w/OI)}}\)_ and _PM\({}_{2.5,\text{CMAQ(w/OI)}}\)_ (colored as red and purple lines, respectively) fluctuated considerably. In February, March, October, and November, the similar concentrations between the model-calculated PM\({}_{\text{S}}\) and the inferred _PM\({}_{5,\text{CMAQ(w/OI)}}\)_ over Thule and Hornsund indicate that the effect of data assimilation is insignificant due to the absence of MODIS observed data originated from high surface reflectivity. As shown in Figure 6, there were two separate groups divided in terms of the ratio of PM\({}_{2.5}\) to PM\({}_{10}\). The first group, having relatively low ratios, included Thule, Hornsund, and Andenes. Their ratios were 0.35, 0.35, and 0.30, respectively. These areas located near the coastal line have geographical conditions favorable for generating the coarse particle of sea salts. On the other hand, the second group, showing high ratios of PM\({}_{2.5}\) to PM\({}_{10}\) concentrations, were Hyytiala and Kuopio. The ratios were 0.80 and 0.77, respectively. These areas located in inland Finland are easily affected by anthropogenic sources. To confirm the origin of the air mass, we conducted the 5-day backward trajectory analysis for April-September using the HYSPLIT v5.0 [92]. From the trajectory analysis, we found that air masses at Hyytiala and Kuopio were mainly transported from Europe (39-47%) and the Arctic regions (38-46%) while those arriving at Thule, Hornsund, and Andenes were originated dominantly from the Arctic regions (81-92%). Figure 5: Spatial maps of monthly average PMs calculated from CMAQ simulation and estimated from the linear relationship between assimilated AODs and PMs for May. (**a**) PM\({}_{10}\) and (**c**) PM\({}_{2.5}\) calculated from the CMAQ simulations. (**b**) PM\({}_{10}\) and (**d**) PM\({}_{2.5}\) estimated from the Equations (11) and (12). Daily mean variations of observed (black closed squares), modeled (green lines), and estimated (red lines) PM\({}_{10}\) at (**e**) Hyytiala and (**g**) Vindeln stations and PM\({}_{2.5}\) at (**f**) Hyytiala and (**h**) Virolahi stations. Mean values (M, \(\mu\)g m\({}^{-3}\)), mean bias (MB, \(\mu\)g m\({}^{-3}\)), Index of Agreement (IOA), and slope (S) from the observation (M\({}_{\text{in-situ}}\)), CMAQ simulation (M\({}_{\text{CMAQ}}\)), and linear estimation (M\({}_{\text{Assim}}\)) for April-September. The closed triangles in Figure (a) and (c) indicate the locations of the monitoring stations.