OTAG Air Quality Analysis Workgroup Volume I: EXECUTIVE SUMMARY Dave Guinnup and Bob Collom, Workgroup co-chair Telling the OTAG Ozone Story with Data Draft, June 2, 1997

Workgroup Objective The Workgroup is to provide assessments of air quality and meteorological data relevant to the mission of OTAG. OTAG mission: To understand the role of transported ozone and precursors in the current ozone nonattainment problem

Description of the Air Quality Analysis WG AQA WG members were affiliated with EPA, state agencies, industry (power,transportation), consultants, academia Members were analysts or research managers generally representing their organizations Interaction occurred through meetings, conference calls (monthly), and Sharing of reports, data and comments was conducted through the AQA-WG interactive web site. (

Types of Analyses Spatial pattern percentile analyses Trajectory residence time analyses Spatial, temporal correlation analyses Statistical cluster analyses Model/data comparisons Tracer analyses Temporal pattern and trends analyses Meta analysis: analysis of analysis Results integration

Problem Statement Some nonattainment areas (e.g. NE corridor, Lake Michigan) experience considerable influx of ozone across their boundaries They cannot demonstrate nonattainment by local measures only Significant ozone reductions at their boundaries will also be necessary From the OTAG Background Document:

Counties not meeting the 80 ppb standard are more numerous than 120 ppb nonattainments. Transport impacts at 80 ppb are more likely.

Area source NO x emissions are highest near cities. Point sources dominate the center of OTAG. Area Source Density of NO x Point Source Density of NO x

The OTAG domain corners are at tropospheric O 3 levels. The highest avg. O 3 is over the megalopolis and Ohio Valley. There is an increasing trend from west to east.

Highest (90 %-ile) O 3 occurs near urban areas. Lowest (10 %-ile) O 3 is high in the center of the domain. 10th percentile of daily max. O 3 90th percentile of daily max. O 3

Northeast O 3 exceedances have been declining. OTAG domain exceedances show less decline. Ten year station-day exceedances for the Northeast. Ten year station-day exceedances for the OTAG domain.

At slow wind speeds, O 3 accumulates near source areas. At high wind speeds, O 3 is dispersed from sources. The dispersion leads to long range transport and regional O 3. Average ozone during high (>6 m/s) wind speeds. Average ozone during low ( < 3 m/s) wind speeds.

In the Northeast, regional O 3 is transported mainly through synoptic and channeled flows while local O 3 is moved by near surface flows.

On high O 3 days, the transport winds are slow with clockwise circulation around the south-center of the domain. On low O 3 days, the swift transport winds are from outside the domain. Transport winds during high (90%-ile) local ozone days. Transport winds during low (10%-ile) local ozone days.

During regional episodes, air masses meander over the high emission regions and accumulate O 3. The ‘88,‘91,‘95 modeling episodes lasted 6-9 days. Ozone pattern and air mass histories during the 1995 episode. Daily maximum ozone averaged over all monitors in the domain.

OTAG-wide episodes tend to be associated with stagnation followed by transport.

The 4 episode avg. model concentration shows high O 3 over the central section of the domain. The measured O 3 pattern roughly corresponds to the model. Model-average daily maximum O 3 during the four episodes. Measured average daily maximum O 3 during the four episodes.

The model underpredicts O 3 in the North and overpredicts in the South by ppb. The modeling periods over-represent O 3 in the North and under-represent O 3 in the South. Difference between UAM-V model prediction and measured O 3. Difference between the OTAG domain episodes and the 90 th percentile O 3.

Transport winds during the ‘91,‘93,‘95 episodes are representative of regional episodes. OTAG episode transport winds differ from winds at high local O 3 levels. Comparison of transport winds during the ‘91, ‘93, ‘95 episodes with winds during regional episodes in general. Comparison of transport winds during the ‘91, ‘93, ‘95 episodes with winds during locally high O 3.

OTAG is a well defined control region. Low O 3 air comes from outside, high O 3 air from inside OTAG. Back trajectory frequencies for low ozone days. Back trajectory frequencies for high ozone days.

The transport winds on high O 3 days are slow in the center of the domain. At many sites, the avg. O 3 is higher when the wind blows from the center of the domain. Ozone roses for selected 100 mile size sub-regions. Superposition of O 3 contours and transport winds during high (90 th percentile) O 3 conditions.

Emission changes do change O 3 levels. 120 ppb exceedances are 3 times higher on Fridays than on Sundays. Map of exceedances on Fridays.Map of exceedances on Sundays.

Conclusions O 3 transport is real, characterized by time and space scales of miles Transport from central portion of OTAG domain more closely associated with high ozone levels downwind Model may understate transport impacts -- interpret results accordingly