Effects of the NO(x) SIP Call program on ozone levels in New York.

This study looks at the effects of the major U.S. Environmental Protection Agency (EPA)-mandated ozone (O3) control program implemented by New York State Department of Environmental Conservation since 2003. The study is based on ozone concentrations from eight sites representative of a range of geographic and land use conditions in the period 1995-2012. All sites show lower sample maximums of daily maximum 8-hr ozone concentrations. For example, in the period 2003-2012 compared to the period 1995-2002, the New York Botanical Gardens site experienced an 81%, 56%, and 25% drop in the number of days with daily maximum 8-hr ozone exceeding 85 ppb, 75 ppb, and 65 ppb respectively. For the same site, the frequency of hot days (with temperatures above 32 degrees C) was about the same in both periods. However, a hot day from the period 1995-2002 was 2.1 times more likely to have daily maximum 8-hr ozone exceeding 75 ppb than a hot day in the period 2003-2012. Other sites showed similar results. A comparison of the underlying distributions of ozone and temperature indicates a broad-based reduction of expected ozone values and variability, confirmed as significant by bootstrap tests. Most of the sites exhibit significant albeit small (30%) increases in expected values of maximum afternoon temperatures, favoring increased ozone production. The contrary actually happened. Consequently, trends in observed concentrations are caused by reduced ozone production rather than by favorable meteorological conditions. The study findings are consistent with previous studies that relied on different data sets and analysis methods. Taken together they demonstrate the effectiveness of the NO(x) emission reduction programs in the New York state.