Site Specific Time-series and Correlations of MODIS Aerosol Optical Depth and Surface PM2.5 Concentration

Sample site specific MODIS ta plot / In-situ PM2.5 time series and correlation plot

The site specific MODIS Aerosol Optical Depth (AOD)/ In-situ PM 2.5 mass concentration plot details the temporal behavior of the measurements made at a specific monitoring site location during the preceding 60 days. Correlations between the MODIS AOD observations and in-situ PM2.5 measurements are also reported. The site name, metropolitan statistical area (MSA), and site identifier are reported on the chart. The left vertical axis is mass concentration of PM 2.5 (scale 0 - 100) and the right vertical axis is MODIS aerosol optical depth (scale 0.0 - 1.6). In-situ PM2.5 concentrations from continuous monitors are shown at an hourly frequency for both 1-hour (solid line) and running 24-hour (dashed line) averages.

Correlations with MODIS AOD are reported for both 1-hour and 24-hour averaged surface PM2.5 concentration. The correlations are based on coincident MODIS AOD pixels acquired within one hour of the surface in-situ measurements.
Coincident values are represented by symbols:

	MODIS AOD
	1-hour average PM 2.5 mass concentration
	24-hour average PM 2.5 mass concentration

The number of possible MODIS observations during this 60-day period (i.e., the number of times the MODIS sensor views this location) is reported as N_poss. The number of actual AOD retrievals during this period is reported as N_MODIS. N_MODIS may be less than N_poss due to, for example, the presence of clouds which prevent an aerosol retrieval. The number of coincident MODIS pixels and in-situ PM 2.5 measurements used to determine the correlation are reported as N_corr.

The time-series/correlation plot provides site-specific information important to help interpret whether the MODIS AOD is indicative of PM concentrations at or near the surface as measured by the in-situ monitor. Higher correlations suggest the MODIS AOD pixel is reflective of in-situ surface PM 2.5 mass concentrations at the monitor location. The vertical distance between coincident points is an additional factor to be considered to help determine whether the aerosol viewed by MODIS is well mixed and at or near the surface, or whether the aerosol is aloft. If the distance is relatively small and the MODIS AOD falls on or near the hourly line plot, an inference can be made that the aerosols may be near the surface.