1. INTRODUCTION

The Ozone Transport Assessment Group (OTAG) was initiated by the Environmental Council of the States (ECOS) in 1995 to develop a consensus on how to deal with the transport of ozone and precursors in the eastern United States. Among the issues confronting OTAG is the scope of regional transport of ozone and its precursors: how to accurately assess the influence of transport on ozone concentrations and to design emissions management strategies to account for it.

In the summer of 1995, a public-private consortium initiated the NARSTO-Northeast research program to enhance the understanding of factors governing ozone formation and transport in the Northeast United States. This study is part of the international public-private North American Research Strategy for Tropospheric Ozone (NARSTO). NARSTO-Northeast has provided an extensive database on the three-dimensional distribution and transport of ozone and precursors in the Northeast.

This draft report provides an overview of important transport and mixing phenomena identified during NARSTO-Northeast that were related to exceedances of the federal ozone standard during the 1995 measurement period. This report was prepared on a very short time schedule for submittal to the OTAG Air Quality Analysis Workgroup. The analyses described are based on three-dimensional air quality and meteorological data obtained during the summer of 1995. The report focuses on the ozone episodes of July 13-15 and July 31-August 1, 1995. It draws on work presented at the December 10-12, 1996 NARSTO-Northeast Data Analysis Symposium and on other analyses performed subsequently. The report represents an interim summary of findings from NARSTO-Northeast data. In many cases it uses data displays that were available rather than presentations that would better make the point but would have taken longer to prepare.

The measurements made for NARSTO-Northeast are described by Roberts et al. (1995), and the database and data access procedures are described by Korc et al. (1996). As part of NARSTO-Northeast in 1995, air quality and meteorological measurements were made at nine research stations in the northeast Ozone Transport Region (OTR). Observables (which varied by location) included: ozone, nitric oxide (NO), the sum of oxidized nitrogen compounds (NO_y), carbon monoxide (CO), sulfur dioxide (SO₂), hydrocarbon and carbonyl compounds, wind speed, wind direction, ambient temperature, relative humidity, precipitation, and UV radiation. In addition, routine surface air quality measurements were archived from about 200 existing monitoring stations in the region. During the same period, five radar wind profilers with radio acoustic sounding systems (RASS) were operated to provide hourly averaged vertical profiles of winds, virtual temperature, and related quantities such as the radar reflectivity structure parameter, which can be used to estimate mixing depth. Rawinsonde measurements were made at seven sites, and ozone tethersonde measurements were made at one site on the study days. Finally, three aircraft were instrumented to continuously measure observables (which varied by aircraft) including: ozone, NO, NO_y, SO₂, CO, light scattering, meteorological observables, position, and altitude. Two aircraft obtained integrated samples for hydrocarbons and carbonyls.

The locations of the surface air quality sites for the study are shown in Figure 1-1. The surface and upper-air meteorological sites are shown in Figure 1-2. The measurement sites and aircraft sampling locations discussed in the report are identified by name on one of these figures.

In preparing this assessment, we were asked to address the following questions.

• What are the major transport regimes during ozone episodes?
• How do transport speeds and directions differ between the surface and aloft?
• How far can ozone or precursors be transported aloft between episode days?
• How far are precursors that are emitted during the nighttime and morning transported in the near-surface layer during the day; at night?
• How do ozone layers aloft affect surface concentrations?
• What is the relative importance of long-range ozone and precursor transport for downwind ozone concentrations?

Section 2 below provides an overview of the major results and conclusions of this assessment and provides at least partial answers to the above questions. The subsequent sections provide supporting information.