Spaceborne observations of low surface aerosol concentrations in the Stockholm region

Abstract

A B S T R A C TThis article investigates the feasibility of using spaceborne observations of aerosol optical thickness (AOT) derived with the Moderate Resolution Imaging Spectroradiometer (MODIS) for monitoring of fine particulate matter (PM2.5) in an environment of low aerosol loading. Previous studies of the AOT-to-PM2.5 relationship benefit from the large range of observed values. The Stockholm region features a comprehensive network of ground-based monitoring stations that generally show PM2.5 values B20 mg m (3 . MODIS AOT at 555 nm is usually B0.20 and in good agreement with ground-based sun photometer observations in this region. We use MODIS Collection 5 AOT data with a horizontal resolution of 10 km )10 km and ground-based in-situ PM2.5 observations to derive an AOT-to-PM2.5 relationship that can be used to estimate fields of PM2.5. This has been carried out with respect to the months from April to September of the period 2000Á2013. Relative average absolute deviations of 33Á55 % (mean of 45 %) are obtained between MODIS-retrieved and ground-based PM2.5. The root mean square error is 0.2159 mg m (3 between retrieved and measured PM2.5. From spaceborne lidar observations, it is found that elevated aerosol layers are generally sparse in the Stockholm region. This favours remote sensing of PM2.5 from space. The deviations found between measured and retrieved PM2.5 are mainly attributed to infrequent situations of inhomogeneous aerosol layering for which column-integrated observations cannot be connected to surface conditions. Using MODIS Collection 6 data with a resolution of 3 km)3 km in a case study actually gives far fewer results than the coarser Collection 5 product. This is explained by the complex geography of the Stockholm region with a coastline and an abundance of lakes, which seems to induce biases in the retrieval of AOT at higher spatial resolution.

Effect of Wind Speed on Moderate Resolution Imaging Spectroradiometer (MODIS) Aerosol Optical Depth over the North Pacific

“…Zhang et al [62] analyzed cloud artifacts over remote oceans and found that cloud contamination explains 60-90% of the correlation between MODIS AOD and cloud fraction. Similar deviation as the one found here in MODIS AOD derived from observations over the current coastal sites has also been reported in previous studies [43,[63][64][65][66]. Mélin et al [64] suggest that further improvements of atmospheric correction in coastal waters may require an additional level of detail to describe the complex aerosol mixtures more accurately, and specific water bio-optical models for an appropriate representation of the boundary condition.…”

Trends in MODIS and AERONET derived aerosol optical thickness over Northern Europe

“…Values of R 2 ¼ 0.86 and RMSD rel ¼ 31% were found for the MODIS c051 and ground-based collocated cases. The present results are in agreement with Glantz and Tesche (2012) who included 20 AERONET stations in Europe in the validation of MODIS c051 and Tesche et al (2016) who focussed on long-term MODIS observations over the Stockholm region. In a global validation of the MODIS c051 darktarget aerosol products against AERONET, Levy et al (2010) conclude that MODIS compares best (negligible bias and good correlation), for light aerosol loading (AOT <0.15), over sites that are both moderately 'dark' and moderately 'green'.…”

“…The satellite derived time series of AOT have been compared to trends in AOT derived from AERONET measurements in the same region. Tesche et al (2016) show that cloudfree conditions, required for passive remote sensing, only make up 20-40% of the daytime observations for the months from April to September in eastern middle Sweden. Therefore, we have investigated if the AOT availability is higher over the Baltic Sea compared to the surrounded land areas and what this means for the representativity of the data.…”

“…Since the majority of the aerosols that contribute to AOT in the investigation area are probably accumulation-mode particles (Levy et al, 2010), relatively small spatial variations are expected for Northern Europe (e.g. Tesche et al, 2016). This, together with the finding that Baltic Sea provides better statistical representation of AOT than the surrounding land, support a focus on specific regions over the Baltic Sea in order to estimate trends in AOT (Section 3.5) as well as to investigate transport patterns of clean and polluted air masses (Section 3.6).…”

“…In addition, it was found here that collocated matches over land between MODIS and AERONET are substantially higher for c051 than c061. Using c061 and MODIS 3-km in the study by Tesche et al (2016) also reduces the available AOTs over Sweden compared to MODIS c051. The MODIS c061 retrieval uses more aggressive tests for spatial variability of reflectance and normalized difference vegetation index (NDVI) to mask clouds and inland water bodies (Levy et al, 2013).…”