The ambient air quality data is used to verify compliance with state and national ambient air quality standards, to support development of regulations designed to reduce ambient air pollution, to assess the effectiveness of existing air pollution control strategies, and to provide aerometric data for special research.

The federal Clean Air Act requires that states which are non-attainment develop and implement strategies for attaining the standard. The State Implementation Plan (SIP) is the mechanism for documenting this process, and all revisions to the SIP must be approved by the U.S. EPA. Measures contained within the existing Massachusetts SIP or awaiting U.S. EPA approval include:

• Cutting back on volatile organic compound (VOC) emissions from industrial sources and from the transfer of gasoline;
• Introducing reformulated gasoline which is a cleaner fuel with a lower vapor pressure (evaporation rate);
• Requiring biannual inspection and maintenance (I/M) of light duty cars and trucks;
• Reducing vehicle miles traveled by encouraging use and investment in mass transit systems.

Massachusetts has made significant progress in attainment of the CO standard through the implementation of air pollution control programs. The last violation of the CO NAAQS occurred in Boston in 1986. In December, 1994, after a year-long study, Massachusetts requested that U.S. EPA designate the Boston metropolitan area as being in attainment for CO. Effective April, 1996, U.S. EPA formally redesignated the Boston area as in attainment of the CO federal air quality standard. Springfield, Worcester, Lowell, and Waltham will remain non-attainment for CO, despite significant improvement, until such time as appropriate studies and analyses are completed which support a request for redesignation.

The ozone one hour standard of 0.12 ppm was exceeded at two sites, in Truro and Fairhaven, out of the seventeen sites at which ozone was monitored during 1996. There were two exceedance days (days ozone exceedances occurred) during the year. Figure 2 shows the ten year trends for number of exceedance days and total ozone exceedances.

Primary standards protect against adverse health effects. Secondary standards protect against welfare effects such as damage to crops, vegetation, and buildings.

Standards other than those based upon the annual arithmetic mean must not be exceeded more than once a year.

* The O3 standard is based upon an estimated exceedance calculation described in Appendix H of 40 CFR Part 50. Estimated exceedances (number of days with maximum hourly O3 concentration above 0.12 ppm) must not exceed 1 per year.

** The PM-10 standard is based upon an estimated exceedance calculation described in Appendix K of 40 CFR Part 50. The annual standard is attained if the estimated annual arithmetic mean concentration does not exceed 50 mg/m?. The 24-hour standard is attained if the estimated number of days per calendar year above 150 mg/m? does not exceed 1 per year.

mg/m? = micrograms per cubic meter; ppm = parts per million; mg/m? = milligrams per cubic meter

(1) PM-10 encompasses particulate matter with an aerodynamic size less than or equal to 10 microns. It replaced TSP as the ambient particulate standard effective July 31, 1987.

Air Quality Trends: How is the Air Quality?
Trend data provides a means to address the question "How is the air quality?" As the figures below display, ten-year trends indicate that the air quality is improving - and very substantially for some pollutants. When interpreting trends it must be recognized that air quality is influenced by many factors. For instance, the state of the economy, as reflected by industrial and commercial activity - and the resultant levels of pollutant emissions, as well as meteorological conditions should be considered when evaluating ozone trends. In recent years, while the Massachusetts economy has been strengthening, the meteorological conditions have been favorable for lower O3 levels. With meteorological conditions more conducive to O3 formation (such as higher temperatures) the levels would have been higher.

While current data trends are downward for many pollutants, MADEP believes that in order to maintain these levels, and reduce them further (to attain the ozone NAAQS for example), it is necessary to maintain and improve existing emission control programs. The challenge is to effectively balance the goals of continued emission reductions and promoting conditions beneficial to economic growth.

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The O3 data indicates the trend has been relatively stable except for 1988 when meteorological conditions were conducive to O3 formation. The CO data shows the trend fluctuates but is clearly in a downward direction. CO as indicated by the 8-hour 2nd maximum concentration has decreased 42% over the ten-year period.

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The NO2 trend is downward - the annual mean concentration has decreased 21% over the ten-year period. The SO2 data also indicates a downward trend with the annual mean concentration decreasing 55% over the period.

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PM-10 shows a downward trend as concentrations have decreased 19% over the period. Lead monitoring in Massachusetts was ended in 1995. As figure 8 indicates, the concentration of lead (Pb) in the air decreased dramatically over the ten-year period. This success is primarily the result of the increased usage of unleaded gasoline in cars.

The PAMS monitoring for VOC has been conducted for only three years. Preliminary analysis of the ambient concentration levels indicate a decline of certain toxic VOC. The decrease is likely the result of the use reformulated gas beginning in January, 1995, which lessened the emissions of toxic pollutants from gasoline.