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A large share of observations were from the United States, due to this country’s extensive network of air quality measurement stations. Other large contributor countries were China, Canada, and Japan. All in all, our analysis is based on over 2,600 observations from 293 observation stations in 109 cities around the world; these cities are located in 44 countries. Figure A.4 reveals the time period during which individual countries participated in the GEMS/AIR project. The countries are ranked by length of participation. Numerous countries provide more than fifteen years of observations, among them the United States, Canada, Germany, and Japan. In addition, table A.1 lists the cities in which the observation stations were located along with the number of stations in each city and the minimum and maximum concentrations measured at any of the stations in a given city read.

A.2 Data Sets

The data set was constructed from a variety of sources that are described in detail below and are summarized in the following diagram:
GEMS/AIR The primary source for our data is the AIRS Executive International database that contains information about ambient air pollution in nations that voluntarily provide data to the GEMS/AIR Programme sponsored by the United Nations World.

Health Organization. This package is provided by the U.S. Environmental Protection Agency (US-EPA) at http://www.epa.gov/airs/aexec.html. The US-EPA kindly provided a much more complete version of this dataset that included not only averages but also median and other percentiles of SO concentrations. We would like to express our gratitude to Jonathan Miller of the US-EPA for providing additional GEMS/Air data not contained in the public release of the database, and for patiently answering our numerous technical questions. We had problems with the identification of several observation stations. The longitude and latitude information provided in one of the ancillary files was in some cases incorrect and was corrected case-by-case based on the the description of the location.
PWT The Penn World Tables are described in Robert Summers and Alan Heston, “The
Penn World Table (Mark 5): An Expanded Set of International Comparisons, 1950-1988″QuarterlyJournalofEconomics, Vol. 100,May 1991,pp.322-336.Vari-ables obtained from this data set include GDP per capita, population, capital stock per worker, and trade intensity. Note that the PWT do not contain data for Cuba; thus, this country was dropped from our analysis. The PWT data are available in revision 5.6 from the NBER ftp site at ftp://ftp.nber.org/pwt56/.
CIESIN The Consortium for International Earth Science Information Network (CIESIN) Global Population Distribution Database contains the total population contained in each grid cell of 1° x 1° in the year 1990 for each country. This data set is only available for this single year. It can be obtained freely from the United Nations Environmental Programme server maintained by the U.S. Geological Survey.
The CIESIN data set was augmented by population counts of major urban agglomerations that is produced by the United Nations Population Division’s 1996 Global Population Estimates and Projections database on Urban Agglomerations 19502015.1 Additional data was obtained from the U.N. Demographic Yearbook (1994) and the Statistical Abstract of the United States (1994) to fill gaps in the data set.