The one-hour ozone National Ambient Air Quality Standard (NAAQS) is 0.120 parts per million (ppm), set by EPA to protect public health from high concentrations of ozone. A new, more stringent eight-hour standard of 0.080 ppm, which would be even more protective of those who spend many hours outdoors, was promulgated in 1997, but implementation of this new standard has been delayed by litigation. States are required to monitor their ozone levels, and if the allowable level (i.e., the NAAQS) is exceeded, they must develop/implement plans to reduce emissions and achieve the standards.
There are eighteen ozone monitors throughout the state of New Hampshire that continually monitor ozone levels. They operate either during the ozone season only (April through September) or year-round. DES also operates a monitor in Kittery, Maine. Although there are no major cities or significant industrial activity in the area, New Hampshire does have an ozone problem. By examining different meteorological factors, the state can begin to understand the conditions that lead to air pollution transport, and often result in high ozone concentrations in a region people tend to think of as clean.
Tables generated by DES and EPA provide the dates and monitors in New Hampshire that exceeded the eight-hour ozone standard during the years of 1995 to 2001. Exceedances of the one-hour and/or eight-hour ozone standard generally occur on the coast or in the southern portion of the state, and always between the months of May and September. Heat and solar radiation are required in order to drive the chemical reactions that form ozone.
Hourly ozone data was used to determine the hour of peak ozone concentration at each monitor on the days that exceeded the standard. Streamline plots and hourly meteorological data were then used to estimate the wind direction at the time of each exceedance. Wind direction is a good indicator of near-field sources of ozone and its precursors. A table was then created summarizing the date of the episode, a list of monitors that exceeded the standard on that day, the value of the peak eight-hour and one-hour ozone concentration, and the wind direction at the time of the exceedance. From this, the number of times the wind was blowing from each wind direction during an exceedance could be calculated for each monitor. Figure 1 gives a visual representation of the data summary for each one-hour exceedance within the state from 1995 to 1999 and Figure 2 summarizes the eight-hour ozone exceedances. Each circle represents a monitor in New Hampshire. The color of the circle indicates whether or not that monitor meets the ozone standard. The bars show how often the wind was blowing from a particular direction when that monitor exceeded the standard.
All of the ozone events in New Hampshire occurred when the wind came from somewhere between southeast and west-southwest. Throughout the majority of the state ozone, exceedances most often occurred under a south-southwest or southwest wind direction. On the coast however, a south-southeast wind was present during most episodes. This south-southeast wind brings emissions from Boston onto the New Hampshire coast by way of the seabreeze phenomenon. The seabreeze is a major factor leading to ozone exceedances within the state as 57 percent of all exceedances took place on the coast, 51 percent of those on a south-southeast wind.
Backward trajectory plots were used to get an idea of where the air mass that caused a monitor to exceed the standard had traveled prior to the time of arrival in New Hampshire. Trajectory plots give a longer-range estimation of the possible source regions of ozone and its precursors. These potential source regions were then added to the table mentioned above. The data from this study shows that the sea breeze often causes exceedances on the coast, whereas the Northeast corridor plays a large part in elevating ozone levels throughout the rest of the state. The Midwest and Canada also play a significant role in the transport of ozone and its precursors into the area.
It is also important to look at the amount of solar radiation reaching the earth on exceedance days, as sunlight is what drives the ozone formation chemistry. When solar radiation levels are low, local ozone formation is slow. Therefore a day with low local radiation levels and high ozone concentrations might be concluded as a day when ozone transport into the area was considerable. There were only a handful of days in which ozone exceedances occurred with low solar radiation (about 16 percent of all exceedances). This however, does not lessen the role of ozone transport into New Hampshire; it suggests that ozone continues to be formed over the state from the transport of ozone precursors.
The northern parts of the state rarely exceed the ozone standards, however the particular days that the north did see high ozone levels all share similar characteristics. The temperatures were around 87-90 degrees F, and there were also high levels of solar radiation reaching the surface of the earth at the time of the exceedances. The wind was blowing from a south-southwest direction and the trajectories showed that the air mass traveled at ground level up through the corridor before reaching New Hampshire. High concentrations of ozone in New Hampshire are caused by a combination of many factors that need to come together at the same time. The absence of any of these factors can significantly reduce ozone levels in New Hampshire. These factors include high temperature and solar radiation levels; air mass trajectories that travel over regions producing a large volume of emissions before reaching New Hampshire; and contributions of ozone and its precursors from nearby source areas (such as Boston). The formation of a seabreeze is also a major component contributing to the severity of ozone events along the seacoast. In conclusion, the ozone events in New Hampshire are due almost exclusively to the transport of ozone and its precursors from out of state.