Abstract
This study examined the role of meteorological processes in two of the highest ozone (O3) episodes within the last decade at monitoring sites in southern New Hampshire (NH), USA. The highest O3 levels occurred on 14 August 2002 at Thompson Farm (TF) and 22 July 2004 at Castle Springs (CS). Ozone mixing ratios in the 2002 episode showed continual high values (>100 ppbv) at the beginning of the episode, and reached 151 ppbv on 14 August. The 2004 episode consisted of one day of high O3 (>100 ppbv) on 22 July at CS with the peak level of 111 ppbv. Our analysis suggested that the August 2002 high O3 event at TF occurred under stagnant synoptic high-pressure conditions that prevailed over the entire eastern USA for an unusually extended time period. The clear skies and stable meteorological conditions resulted in accumulation of pollutants in the boundary layer. At the same time, the mesoscale low-level-jet (LLJ) played an important role in transporting air masses from the polluted Mid-Atlantic areas to the Northeast. Local land-sea-breeze circulations also added to the impact of this event. Our examination showed that the unprecedented high levels of O3 on 22 July 2004 at CS was driven by two mechanisms, stratospheric intrusion and the Appalachian lee trough (APLT), which was not found during other O3 episodes at the site in the decade long data record. This study demonstrated that unusually high O3 levels at New England rural sites were driven by multi-scale processes, and the regional/local scale processes controlled the magnitude and timing of the local pollution episodes.
Highlights
Ground-level ozone (O3) formation is dependent on a series of complex chemical reactions with nitrogen oxides (NOX) and volatile organic compounds (VOCs), and is sensitive to meteorological conditions, such as solar radiation, temperature, and wind speed/direction
We focus on the meteorological conditions and the air mass transport mechanisms which shaped this extremely high O3 episode at Thompson Farm (TF)
This paper presents an analysis of meteorological processes on the synoptic and mesoscale for
Summary
Ground-level ozone (O3) formation is dependent on a series of complex chemical reactions with nitrogen oxides (NOX) and volatile organic compounds (VOCs), and is sensitive to meteorological conditions, such as solar radiation, temperature, and wind speed/direction. Mao and Talbot [15] speculated that the LLJ transported O3 and O3 precursor-rich air masses overnight from the northeast urban corridor to New England in the residual layer, which were brought down to the surface during the day Another well-known regional scale transport mechanism is the Appalachian lee trough (APLT). 1995 tropospheric O3 pollution event in southern New England and New York State Another high O3 case associated with the sea breeze was identified on 12–16 July 1995 in the northeastern United. In 2004, Castle Springs (CS) had one day of high O3 (>100 ppbv) on 22 July These events resulted in the highest O3 mixing ratios at these sites during the last decade, the dominant meteorological conditions were quite different. We investigated associated meteorology and chemistry using a synthesis of measurement data from AIRMAP and other sources in addition to meteorological reanalysis data
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