There are three types of data here:
city-specific data, city characteristics data, and
metadata. The city-specific data consists of daily
time-series of pollution and meteorological data. Each
city's data is stored in a separate compressed comma
delimited (UNIX) text file. You can see descriptive
statistics for a small number of variables available
for each city here and can download
the data below.
Along with the city-specific data you
can also download city characteristics data. This is a
collection of tables giving information about each
city, such as, their latitude and longitude, census
demographics, how cities are defined in terms of
couties etc. Such data becomes increasingly important
as you move from analyzing one city to analyzing the
whole collection of cities.
Finally you can download the
metadata. This includes "data about the data", such as
a list of all the tables included in NMMAPS, a list of
all the variables, labels for variables, and labels for
The documentation page gives a better
idea of what the data look like.