November 28, 2007
Special Feature: Multiple monitors needed to describe air quality in Cape Town
As our special coverage of South Africa continues during the GEO Summit, we consider that one of the biggest challenges in assessing atmospheric issues using Earth observation data is timely data access. There are a dozen (likely more) satellite sensors that provide global or hemispheric data relevant to air quality and other atmospheric conditions (not even counting weather satellites). There are tens (maybe even hundreds) of ground-based networks around the world consisting of thousands of ground-based monitors for various air pollutant concentrations, visibility, and aerosol optical depth. Yet, much of the data is not available when trying to assess conditions on a near real-time basis for specific countries, such as we do here at the Smog Blog. And much of it is not available at all.
So, as we consider the air quality in South Africa for Wednesday, I'll mention a few potential data sources that were not available to help us today. But I'll also include some new data from ground monitors in Cape Town, that shows that making friends in the global air quality community can give us new data, information, and insights.
Wednesday was stunningly clear over Cape Town and western South Africa. Thanks to the NASA RapidFire site, we have MODIS images over much of the world by early afternoon, including CapeTown. The UV index was 11, so the sunshine was really coming through.
Satellite sensors are great for telling us about regional scale pollution and wildfires. Air quality overall in western South Africa looked pretty good on Wednesday. Yet, conditions on the ground can vary significantly minute by minute, hour by hour, especially in urban areas. The City of Cape Town was generous enough to share their PM10 monitoring data with us for Wednesday. PM10 is a measure of particles in the air that are less than 10 microns in diameter--fine enough to cause health problems when we breath them in. The 24-hour average health standard for PM10 in Cape Town is 50 micrograms per cubic meter (ug/m3) concentration. Below is a time series graph of 1 minute averages from two monitors in Cape Town: one near the Waterfront and one near Khayelitsha, a busy urban area. (Please note that these data are preliminary "live data" that have not been validated and that the standard is an average of these points, not any single value).
[NOTE (29 Nov 07): Figure revised from original post to include 24 hours of data and correct graphing error.]
Clearly, on the ground conditions are more dynamic than we can see from orbit with the satellite sensor! However, by having both the regional view from the satellite and the concentrations right on the ground, we now know that the air pollution is very local and much higher during the times when people are commuting to work--thus it is likely from urban sources, such as cars, road dust, and industry, instead of large wildfires or dust storms.
Although the true color image is available from MODIS, we do not have any near real-time aerosol optical depth data (a unitless measure related to particle air pollution) for this region from MODIS, POLDER, MISR, OMI, CALIPSO, AERONET, or other aerosol measuring instruments. To the credit of ESA/KNMI's Tropospheric Emission Monitoring Internet Service (TEMIS), we do have OMI instrument NO2 concentrations (averaged over the bottom layer of the atmosphere) and SO2 concentrations (over the total atmosphere), as well as global UV index from the SCIAMACHY instrument. Of those, only the UV index values are of significant concern.
So, what is the conclusion for Cape Town and South Africa on Wednesday? Sunny with high UV, good air quality regionally, but higher levels of pollutants during the day in the busy city areas. And that there is tremendous value in using multiple sensors to learn about our air every day.
Posted by Jill Engel-Cox at November 28, 2007 8:30 PM | TrackBack