To assess surface level PM2.5, hourly PM2.5 mass concentrations from the ground stations, Terra-MODIS AOT at 0.55?m and meteorological fields from hourly Rapid Update Cycle (RUC) reanalysis over AirNow stations in the United States are used.
Surface PM2.5 Mass Concentration
The United Stated Environmental Protection Agency (EPA) and its state partners maintain several air quality monitoring networks in the United States. These networks monitor the mass concentration of particulate air pollutants at the ground. PM2.5 data from these networks include 24-H average (daily) and hourly PM2.5 mass concentrations. Both hourly and daily mean PM2.5 mass concentration data sets from more than 400 ground stations over 8-10 year time period have analyzed to develop statistical models. PM2.5 mass concentration over these stations is measured using a Tapered-Element Oscillating Microbalance (TEOM) instrument with an accuracy of ?1.5 ?gm-3 for hourly averages although it may underestimate PM2.5 mass concentration due to volatilization of ammonium nitrate and organic carbon. Hourly average PM2.5 mass concentration values are used to derive air quality categories whereas daily mean values are used to monitor and assess the air quality. Air quality categories represent ranges of air quality index that are used by USEPA to establish a relationship between mass concentration and human health.
Other Particulate Matter Data Sets for Research
- IMPROVE -Interagency Monitoring of Protected Visual Environments
- SEARCH –South Eastern Aerosol Research and Characterization Study experiment.
Satellite Aerosol Data
The MODIS instruments onboard NASA’s Terra (equatorial crossing time of 10:30 a.m.) and Aqua (equatorial crossing time of 1:30 PM) satellites provide routine retrievals of cloud and aerosol properties over land. MODIS provides spectral information of aerosol optical properties in seven different wavelengths over ocean and in three wavelengths over land. The AOT, representing the columnar loading of aerosols in the atmosphere, is an important aerosol parameter retrieved from satellite observations. Recently, a new version of the MODIS algorithm (collection 5, V5.2) has replaced an older version (collection 4, V5.1). The new algorithm uses new aerosol models and improved estimation of surface reflectance. This algorithm applies stringent criteria to select appropriate pixels in the retrieval process thereby reducing the total number of data points. Several validation studies over AERONET locations conducted over global land reveal that 57% of MODIS AOT retrievals (Collection 4) are within expected uncertainty levels of ?0.05?0.15*AOT. Preliminary results from a validation exercise of MODIS collection 5 shows that 72% of the retrievals fall within that expected uncertainty over land, which is an improvement over the previous Collection 4 data sets. MODIS aerosol product also provides the fractional cloud cover for each pixel and it is obtained during cloud masking process in the aerosol retrieval algorithm. More details on the algorithm can be found in ATBD at MODIS Atmosphere
Aerosol information is also available from many more polar and geostationary satellites some of them of listed here
- MISR – Multi-angle Imaging Spectro Radiometer provides aerosols optical depth in four spectral bands with 17.6x17.6 km2 spatial resolution. MISR also provides AOD values on bright surface but its global coverage is limited due to narrow swath width (~360 km). For more details on PM2.5 research based MISR data please refer to Liu et al., 2007
- CALIPSO - Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation is two channel space born LIDAR, which provides vertical profiles of aerosol information in the atmosphere. Aerosol data sets from CALIPSO are available since April 2006. This is a part of A-Train constellation of satellites.
- OMI – Ozone Monitoring Instrument is onboard on another A-Train satellite AURA, which is an EOS chemistry mission. OMI uses UV part of solar spectrum to measure aerosols optical properties. It provides aerosol index, total AOD and AOD from absorbing aerosols. Spatial resolution is 13x24 km2 at nadir.
- POLDER - Polarization and Directionality of the Earth’s Reflectances is a wide-field imaging radiometer/polarimeter, which retrieve aerosol and cloud information by measuring directionality and polarization of light reflected by the Earth-atmosphere system.
- GOES – Geostationary Orbiting Environmental Satellite provides GOES Aerosol and Smoke Product (GASP) at a 30-minute interval and 4 km x 4 km spatial resolution during the sunlit portion of the day over only United States. GASP is very useful product if you required studying diurnal cycles of aerosols and pollutions.
Meteorology from RUC Analysis
The Rapid Update Cycle (RUC or RUC20 with 20km grid resolution) is an operational atmospheric prediction and assimilation system comprised of a numerical forecast model and an analysis system to initialize that model. The RUC has been developed by the Earth System Research Laboratory at NOAA to serve users needing short-range weather forecasts. RUC runs operationally at the National Centers for Environmental Prediction (NCEP). The reanalysis version of RUC data has a horizontal resolution (20km), 50 vertical computational levels typically from surface to 45-60 hPa) and state of the art analysis and model physical parameterizations. Hourly analysis data of air temperature at 2 meter height (TMP), surface relative humidity (RH), wind speed at 10 meter (WS), and height of planetary boundary layer (HPBL) at a 20x20 km2 spatial resolution are used in this study. Inter-comparison studies of RUC analysis with METAR provides a RMS difference of 1.5 ms-1 and >1.5K in wind speed and temperature respectively and varies as a function of the season. HPBL is one of the important parameters in evaluating air quality from satellite data. It is a diagnostic variable in the RUC reanalysis and it is calculated using vertical profiles of virtual potential temperature. To our knowledge validation of RUC HPBL is not available. However, we use this as a surrogate for aerosol height since routine measurements are not available over the entire area of study.