This report presents a summary of air quality data collected throughout the state of Illinois during calendar year 1998. Data is presented for the six criteria pollutants (those for which air quality standards have been developedparticulate matter, ozone, sulfur dioxide, nitrogen dioxide, carbon monoxide, and lead) along with some heavy metals, nitrates, sulfates, volatile organic compounds and PM 2.5. Monitoring was conducted at more than 100 different site locations collecting data from more than 200 instruments. In terms of the Pollutant Standards Index, air quality during 1998 was either good or moderate more than 98 percent of the time throughout Illinois. There were four days statewide that exceeded an air quality standard for any pollutantone for particulate matter and three for ozone. These exceedances occurred in Cook, Jersey and St. Clair counties (ozone) and LaSalle County (particulate matter). Air quality trends for the criteria pollutants are continuing to show downward trends or stable trends well below the level of the standards. In 1998 monitoring was conducted at 10 locations in Cook and Madison counties for PM2.5 (fine particulate matter of size less than 2.5 microns). In July 1997 the U.S. EPA finalized new ambient air quality standards for particulate matter that included the fine particulates as measured by PM2.5. The monitoring network to fully assess these standards will be phased in during 1999 and 2000. Stationary point source emission data has again been included. The data in the report reflects information contained in the Emission Inventory System as of Dec. 31, 1998. Emission estimates are for the calendar year 1998 and are for the following pollutants: particulate matter, volatile organic material, sulfur dioxide, nitrogen oxides and carbon monoxide. Emission trends of these pollutants has been given for the years 1981 to the present. Emissions reported with the Annual Emissions Report have been provided starting with 1992. In general there has been a trend toward decreasing emissions over this time period.