International Corporation Air Sciences

DRAFT FINAL REPORT

DETERMINATION OF REPRESENTATIVENESS
OF 2002 OZONE SEASON FOR OZONE
TRANSPORT REGION SIP MODELING

Prepared for

Tom Frankiewicz Ozone Transport Commission 444 North Capitol St., NW, Suite 638 Washington, DC 20001

Prepared by

Till Stoeckenius
Sue Kemball-Cook ENVIRON International Corporation 101 Rowland Way, Suite 220 Novato, CA 94945

31 March 2005

101 Rowland Way, Suite 220, Novato, CA 94945 415.899.0700March 2005

i TABLE OF CONTENTS

Page

1. INTRODUCTION................................................................................................................ 1-1

2. DATA GATHERING AND IN ITIAL ANALYSIS........................................................... 2-1

Data ......................................................................................................................... ..... 2-1
Identification Of Ozone M onitoring Sub-Regions ................................................................ 2-1 Spatial Ozone Pattern Analysis.............................................................................................. 2-6 Meteorological Variab le Selection ........................................................................................ 2-7

3. EPISODE CLASSIFICATION ANALYSIS ..................................................................... 3-1

Clustering Analysis............................................................................................................ .... 3-1 Development Of An Objective Classification Procedure .................................................... 3-12 Classification Of 2002 Ozone Episodes .............................................................................. 3-15

4. CONCLUSIONS AND RE COMMENDATIONS............................................................. 4-1

5. REFERENCES.................................................................................................................. .. R-1

APPENDICES

Appendix A: Composites Representing Mean Meteorological Conditions During Each Ozone Episode Pattern Appendix B: Boxplots of Key Meteorological Variables Appendix C: Wind Direct ion Frequency Tables

TABLES

Table 3-1. Mean z-scores for the AvgEx00 ozone summary statistic within each monitoring s ub-region under four different
candidate sets of cluster designations. The episode pattern ID in the far right-hand column is keyed to the episode patterns
described in the text. ............................................................................................ 3-4 Table 3-2. Key characteristics of each OTR episode type. ................................................. 3-10 Table 3-3. Frequencies of occu rrence of OTR episode types. ............................................ 3-12March 2005

ii Table 3-4. Distribution of ep isode types during the 1997-2001/2003 historical period (as determin ed via the clustering analysis) for days in each terminal node of the classification tree shown in Figure 3-8. .......................................................................................... 3-14 Table 3-5. Number of days duri ng May-September with 8-hour daily maximum ozone greater than 0.08 ppm in each monitoring sub-region averaged over the 1997-2001/2003 historical period and in 2002. ............................................................................................ 3-18

FIGURES

Figure 1-1. Ozone monitoring site s in the Ozone Transport Region which is comprised of DC, CT, DE, ME, MD, MA, NH, NJ, NY, PA, RI, VT, and that portion of Virginia contained
within the DC Consolidated Metropolitan Statistical Area. ................................ 1-1 Figure 1-2. 8-Hour ozone nonattainm ent classifications in the OTR and adjacent areas. ............................................................................................... 1-2 Figure 2-1. Ozone monitoring station clus ter assignments for k = 5 clusters. ....................... 2-3 Figure 2-2. Ozone monitoring station clus ter assignments for k = 10 clusters. ..................... 2-4 Figure 2-3. Number of 8-hour oz one exceedance days at each monitoring site during the study period (1997 – 2001 and 2003). ...................... 2-5 Figure 2-4. Ozone monitoring sub-regions in the OTR.......................................................... 2-6 Figure 3-1. Dendrogram from applic ation of Ward’s hierarchical agglomerative clustering to the combined daily ozone and meteorological data. Each leaf at the bottom of the figure epresents one day; the vertical height at which pairs of leaves (or pairs of clusters of leaves) are joined represents a measure
of the distance between the l eaves (or cluster centroids) in the multivariate data space......................................................................................... 3-3 Figure 3-2. Composite 850 mb height and wind fields for each episode (pattern) type (pattern numbers refer to the episode types listed in Table 3-2)............................................................................................... 3-6 Figure 3-3. Composite 850 mb temperature fields for each episode (pattern) type (pattern numbers refer to the episode types listed in Table 3-2)............................................................................................... 3-7 Figure 3-4. Composite surface sea level pressure for each episode (pattern) type (pattern numbers refer to the episode types listed in Table 3-2)............................................................................................... 3-8 Figure 3-5. Composite surface temp erature and 10 m wind fields for each episode (pattern) type (pattern numbers refer to the episode types listed in Table 3-2). ....................................................................... 3-9 Figure 3-6. Deviance as a function of tree size (number of terminal nodes) for sequence of trees generated by the pruning algorithm. .............................. 3-13 Figure 3-7. Deviance from 10-fold cr oss-validation as a function of tree size (number of termin al nodes) for sequence of trees generated by pruning algorithm......................................................................... 3-14March 2005

iii Figure 3-8. Classification tree used to group days by episode type.
Variable names and values used to divide data at each splitting node are shown: days meeti ng the specified criterion are moved
down the left branch in each case. Terminal nodes are numbered 1 – 5 and are keyed to the summary in Table 3-4............................. 3-15 Figure 3-9. Percent of episode days by type in the 1997 – 2001/2003 historical period (as determ ined by the cluster analysis and by the classification tree) and in 2002 ............................................................... 3-17 Figure 3-10. Percent of episode days by type in the 1997 – 2001/2003 historical period (as determined by the cluster analysis and by the classification tree) and in 2002 with Type E events removed and frequencies re-normalized........................................................................... 3-17 Figure 3-11a. Boxplots of average da ily maximum ozone (ppb) in each monitoring sub-region du ring 2002 and during the 1997-2001 and 2003 “historical” period: T ype E (Pattern No. 1) events.. ......................... 3-19 Figure 3-11b. Boxplots of average da ily maximum ozone (ppb) in each monitoring sub-region du ring 2002 and during the 1997-2001 and 2003 “historical” period: T ype B (Pattern No. 2) events........................... 3-20 Figure 3-11c. Boxplots of average da ily maximum ozone (ppb) in each monitoring sub-region du ring 2002 and during the 1997-2001 and 2003 “historical” period: T ype A (Pattern No. 3) events........................... 3-21 Figure 3-11d Boxplots of average da ily maximum ozone (ppb) in each monitoring sub-region during 2002 and during the 1997-2001 and 2003 “historical” period: T ype D (Pattern No. 4) events........................... 3-22 Figure 3-11e. Boxplots of average da ily maximum ozone (ppb) in each monitoring sub-region du ring 2002 and during the 1997-2001 and 2003 “historical” period: T ype C (Pattern No. 5) events........................... 3-23 Figure 3-12. Key to boxplot symbols. .................................................................................... 3-24 Figure 3-13. Composite meteorological fi elds by episode type (pattern) in 2002: 850 mb heights and winds. .................................................................. 3-25 Figure 3-14. Composite meteorological fi elds by episode type (pattern) in 2002: 850 mb temperature. ............................................................................ 3-26 Figure 3-15. Composite meteorological fi elds by episode type (pattern) in 2002: sea level pressure. ................................................................................ 3-27 Figure 3-16. Composite meteorological fi elds by episode type (pattern) in 2002: surface temperature and 10 m winds. .................................................. 3-28March 2005

G:\OTC-EpisodeClassification\Report\ Draft_Final\Sec1(Intro).doc3/31/2005 1-1 1. INTRODUCTION

The Ozone Transport Commission is coordinati ng a photochemical modeling study of the Ozone Transport Region (OTR) in support of State Impl ementation Plan development for certain areas recently designated by the U.S. EPA as being in nonattainment of the 8-hour ozone National Ambient Air Quality Standard. The OTR is comp rised of 12 states (DC, CT, DE, ME, MD, MA, NH, NJ, NY, PA, RI, VT) and that portion of Virginia containe d within the Washington DC Consolidated Metropolitan Statistical Area (see Figure 1-1). Areas within the OTR designated nonattainment for 8-hour ozone are shown in Figure 1-2; detailed attainment demonstrations are required for the nonattainment areas within the OTR classified as “moderate”.

Figure 1-1. Ozone monitoring sites in the Ozone Transport Region which is comprised of DC, CT, DE, ME, MD, MA, NH, NJ, NY, PA, RI, VT, and that portion of Virginia contained within the DC Consolidated Metropolitan Statistical Area.March 2005

G:\OTC-EpisodeClassification\Report\ Draft_Final\Sec1(Intro).doc3/31/2005 1-2 Figure 1-2. 8-Hour ozone nonattainment classifications in the OTR and adjacent areas.

Development of effective 8-hour ozone attain ment strategies requires application of photochemical models to a set of episodes that ad equately represent the ra nge of meteorological
conditions associated with violations of the ambient standard. EPA’s 8-hour ozone modeling guidance (EPA, 1999) lists four criteria for episode selection:

1. Select episodes representing a variety of meteorological conditi ons, which frequently correspond to exceedances of the 8-hour standard. 2. Select episodes during which the daily maxi mum 8-hour averages are close to the 8-hour design value, i.e., the average annual 4 th highest daily maximum 8-hour average. 3. Select episodes for which extensive meteorologi cal and air quality data sets are available. 4. Select a sufficient number of episode days for modeling so that the modeled attainment test specified in EPA’s guida nce is based on several days.

In practice, it is difficult, if not impossible, to simultaneously meet all of these criteria. In general, it is important to include episodes that represent as completely as possible the full range of meteorological conditions asso ciated with exceedan ces of the ozone standard. Differences among episode types are important in so far as they influence the predicted effectiveness of alternative emission control strategies. CATEGORY/CLASSIFICATION

Severe 17 Los Angeles

Moderate

Marginal

Subpart 1 (Basic)

Subpart 1 EAC (Basic) Moderate EAC Greensboro, NC Serious - Riverside Co (Coachella Valley), San Joaquin, Sacramento, CA 

### 8-Hour Ozone Nonattainment

Classifications in the OTR

Source: http://www.epa.gov/oar/oaqps/greenbk/state_8hr_maps.zipMarch 2005

G:\OTC-EpisodeClassification\Report\ Draft_Final\Sec1(Intro).doc3/31/2005 1-3 The OTC recognizes that the OTR is a large regi on that experiences a wide variety of weather patterns associated with 8-hour exceedances and has decided to perform ozone SIP modeling of the full 2002 ozone season extending from May 15-September 15. This has the advantage of incorporating a fairly large number of episode days in different por tions of the OTR. Thus, there should be a good chance that all of the important episode types ar e covered within this period. However, the 2002 season is somewhat unique in that some of the most prolonged and severe ozone episodes in recent years occurred during this season. This raises the possibility that one or more episode types of interest are not adequately represented within the 2002 season. The goal of this study, therefore, is to assess the representativeness of conditions during the 2002 season with respect to exceedance events that have occu rred in other years and determine if there are any types of episodes which are not adequate ly represented within the 2002 season.

EPA’s 1999 draft guidance recommends joint use of subjective and sta tistical methods for characterizing and classifying 8-hr ozone episod es. Subjective methods include “typing” of episode meteorological conditions in which episodes are classified via inspection on the basis of similarities in meso- and synoptic-scale weather patterns. In contrast, statistical methods can produce objective classifications either by use of tree models 1 or and various forms of cluster analysis (often in conjunction with a principal compone nts analysis). A pr edictive classification procedure such as a classification tree model (whi ch can be viewed as a non-parametric form of least-squares regression) does not actually classify episodes, although it can be used to identify potential episodes with common mete orological features. This info rmation can then be used to inform the episode selection process. A cluste r analysis, on the other hand, is designed to identify natural groupings of conditions within th e set of candidate episodes. In either case, considerable expert judgment is required in variable selection, selection of different modeling methods, and interpretation of results so even the statistical methods are not wholly objective. Nevertheless, the these approaches are well su ited to the development of valid, defensible episode classification schemes that are sufficiently robust to explain the ma jor characteristics of ozone episode types.

In this study, we apply a combination of explorat ory statistical techniques , cluster analyses, and classification tree building algorith ms to ozone and meteorological data from the OTR to assess the representativeness of 8-hour ozone episodes occurring during the 2002 season. Data sources and preliminary analyses are described in Section 2. Procedures and results used to identify the major Northeastern U.S. ozone episode types an d their key characterist ics are presented in Section 3 along with a comparison of the frequenc y of occurrence and features of each episode type in 2002 versus those in other recent years. Our conclusions regarding the representativeness
of the 2002 season are detailed in Section 4.

1 A commonly used tree modeling approach is based on the CART methodology (Breiman et al., 1984).March 2005

G:\OTC-EpisodeClassification\Report\ Draft_Final\Sec2(Data).doc3/31/2005 2-1 2. DATA GATHERING AND INITIAL ANALYSIS

DATA

Daily ozone and meteorological da ta required for the episode repr esentativeness analysis were obtained from a variety of sources . To capture the full range of OTR episode characteristics and insure statistical significance, a seven year period (1997 – 2003) was chosen for analysis. Data prior to 1997 were not used to avoid any confoundi ng influences of long-term air quality trends. For purposes of this study, data from the warm season months (May – September) were used to capture most if not all high ozone events during the year.

Ozone and meteorological data were separated into two groups: data from 1997 – 2001 and 2003 were treated as the “historical” period and we re used to define the types of ozone episode conditions occurring in the OTR. Data from 2002 were treated as an independent data set with data in this year to be compar ed against the types of conditions found in the historical period.

Hourly ozone concentrations at monitoring sites throughout the OTR for the period 1997-2003 were provided by the New York State Department of Environmental Conservation. Stations missing more than one year of data were exclud ed from the study, leaving a total of 158 stations with nearly complete data. Daily maximum 8- hour averages were calculated from the hourly data using the data handling conventions specified in 40 CFR 50, Appendix I. Because the spatial pattern analysis procedure requires a comp lete data set, missing daily maxima were set to the station mean daily maximum (this confor ms to the procedure used by Cox, 1997).

Hourly surface meteorological data (winds, temper ature, etc.) from airpor ts and other locations
in the OTR were obtained from the National Ce nter for Atmospheric Research (NCAR) as dataset ds472. Upper air data were extracted from the ETA Data Assimilation System (EDAS)
files available from the National Climatic Data Center. EDAS contains 3-hourly objective analysis initialization and forecast fields from the National Center for Environmental Prediction's (NCEP) ETA model at 40 km resolution. By usi ng the EDAS data, we were able to obtain a consistent set of surface and upper air variables covering the entire eastern half of the U.S. at high temporal resolution.

IDENTIFICATION OF OZONE MONITORING SUB-REGIONS

Monitoring sub-regions were defined within the OTR to reduce the number of variables required to describe the spatial ozone distribution under different episode patter ns. Sub-regions were defined by combining results of a station cluste ring analysis with info rmation on typical ozone concentration patterns provided by air quality analysts from several OTR states. A variable clustering procedure (VARCLUS) ba sed on principal components analysis was used to group the OTR ozone monitoring sites into disjoint geographic clusters (Sarle, 1990, Harrell 1999). This procedure essentially divides the monitoring statio ns into groups of highly correlated sites.
Station clusters are selected so as to explain most of the day-to-d ay variation in ozone levels over the OTR using just a handful of station gr oups. VARCLUS works by performing a principal components analysis on the ozone values in each candidate cluster and se eks to find the set ofMarch 2005

G:\OTC-EpisodeClassification\Report\ Draft_Final\Sec2(Data).doc3/31/2005 2-2 clusters that maximize the total (across clusters) of the variance explained by the first principal
components.

The VARCLUS procedure requires one to specify as input the number of clusters to be formed. As with any clustering procedure, this introdu ces an element of subjectivity which can be minimized by repeating the analysis several times, each time varying the number of clusters to be formed and examining the robustness of the clus ter memberships as the number of requested clusters (k) changes.

Application of the clustering algor ithm for various values of k showed that, for a given value of k, the VARCLUS procedure produced several spatially coherent clusters as well as other clusters which were not spatially coherent. Clusters which were not spatially coherent were always made up of just 5 or fewer member stations. For ex ample, setting k=5 produced 2 coherent clusters (clusters 1 and 2) and 3 smalle r clusters (clusters 3-5) whos e members tended to be widely separated in space (see Figure 2-1). The version of VARCLUS used for our analysis assigns the lowest cluster identification numbers to the “tig htest” (i.e., most easily identifiable and robust) clusters. As the results in Figure 2-1 show, these lowest numbered clusters (in this case Clusters 1 and 2) turned out to also be the most spatia lly coherent (note that the clustering is based on ozone correlations only – the locations of each m onitoring site are not an input to the clustering algorithm). This is consistent with our expectati on that sites located clos e to one another will be highly correlated. Clusters 1 and 2 are similar to the two northeast clusters found by Cox (1997), who used a similar analysis technique applied over the entire eastern U.S.. Successive increases in k over the range 6-10 produced additional cohere nt clusters which subdivided the two large clusters seen in Figure 2-1. The smaller, non- contiguous clusters remained largely unchanged
for all values of k.March 2005

G:\OTC-EpisodeClassification\Report\ Draft_Final\Sec2(Data).doc3/31/2005 2-3 

### Ozone Spatial Clusters in the Ozone Transport Regio n

-84 -82 -80 -78 -76 -74 -72 -70 -68 36 38 40 42 44 46 

### k = 5

Cluster 1 Cluster 4

Cluster 2 Cluster 5

Cluster 3

Figure 2-1

. Ozone monitoring station cluster assignments for k = 5 clusters.

With k = 10, VARCLUS produced 6 spatially coherent clusters , and 4 smaller, non-coherent clusters (Figure 2-2). 1 As in the k=5 case, the spatiall y coherent clusters are the lowest numbered clusters, 1-5, and the non-contiguous clusters are 6-8, and 10. The k=10 case is unusual because cluster 9 (located on the Rhode Is land/Massachusetts coas t) turned out to be spatially coherent, even though the lower numbered clusters 6-8 were not. We investigated the possibility that cluster 9 should be treated as a separate sub-region. After examining the way exceedances in cluster 9 vary with those in surr ounding clusters, however, we concluded that this area could be adequately treated by including it in with cluster 4 (along the Washington – New York City corridor). In order to use only the cl usters which seemed robus t under variations in k, we therefore based the final ozone monitoring su b-regions largely on the first five clusters obtained under the k=9 scenario (which were sl ightly more coherent than those under the k=10

1 As k was increased beyond 10, the coherent clusters produced were judged to be too sm all in spatial dimension to be useful in classifying ozone exceedance regimes.March 2005

G:\OTC-EpisodeClassification\Report\ Draft_Final\Sec2(Data).doc3/31/2005 2-4 case). However, the RI – MA coastal sites were associated with the New York City metropolitan
area sites rather than th e other MA sites based both on the k=10 result described above and input from several state air quality analysts. In additi on, all stations on the ME coast were assigned to the southern New England group (Cluster 1) ba sed on input from state air quality analysts. Stations from the other higher numbered, non-con tiguous clusters were integrated into the surrounding clusters; there were no such stations for which the appropriate cluster assignment was ambiguous.

-84 -82 -80 -78 -76 -74 -72 -70 -68 36 38 40 42 44 46 

### Ozone Spatial Clusters in th e Ozone Transport Region

k = 10

Cluster 1 Cluster 6

Cluster 2 Cluster 7

Cluster 3 Cluster 8

Cluster 4 Cluster 9

Cluster 5 Cluster 10

Figure 2-2 . Ozone monitoring station cluster assignments for k = 10 clusters.

Another adjustment to the VARCLUS results was made along the Philadelphia – New York corridor. Figure 2-3 shows the number of 8-hour exceedances at each monitoring site during theMarch 2005

G:\OTC-EpisodeClassification\Report\ Draft_Final\Sec2(Data).doc3/31/2005 2-5 period analyzed. Exceedance events in the Wa shington – Philadelphia corridor are more frequent than within and downwind (northeast) of the New York City metropolitan area. Furthermore, based on our discussions with stat e ozone forecasters in the OTR, we expect transport of ozone and ozone pr ecursors along the I-95 co rridor to play an important role in exceedance events. This suggests that leaving th e entire Washington to New York City cluster intact might cause our final episode classificatio n scheme to overlook events in which transport northeast from Washington-Baltimor e-Philadelphia to New York is an important feature. We therefore decided to split this cluster into two parts as shown in the final ozone monitoring subregion assignments presented in Figure 2-4. Cl uster 5 extends from th e Washington area through
Trenton and a new cluster 6 c overs the New York City-Long Island-Southern Connecticut region.

-84 -82 -80 -78 -76 -74 -72 -70 -68 36 38 40 42 44 46

0 to 25

25 to 50

50 to 75

75 to 100

100 to 125 

### Number of 8-Hour Stati on Ozone Exceedances for 1997-2003 Excluding 2002

Figure 2-3 . Number of 8-hour ozone exceedance days at each monitoring site during the study period (1997 – 2001 and 2003).March 2005

G:\OTC-EpisodeClassification\Report\ Draft_Final\Sec2(Data).doc3/31/2005 2-6 -84 -82 -80 -78 -76 -74 -72 -70 -68 36 38 40 42 44 46 

### Ozone Spatial Clusters in th e Ozone Transport Region

Cluster 1 Cluster 4

Cluster 2 Cluster 5

Cluster 3 Cl uster 6

Figure 2-4. Ozone monitoring sub-regions in the OTR.

SPATIAL OZONE PATTERN ANALYSIS

An initial analysis of episode patterns was perf ormed based on just 8-hour ozone concentrations within the sub-regions (spatial clusters) descri bed above. For each day, a cluster was determined to be in exceedance if any one monitoring site in the cluster recorded an exceedance. We then counted the number of joint exceedance events between each pair of clusters and examined exceedance patterns across all six cl usters. Detailed results from this analysis were provided in a technical memorandum to the OTC (Stoeckenius and Kemball-Cook, 2004) but are not repeated here because this approach was ev entually discarded in favor of an integratedanalysis approach in which the daily ozone levels in each sub-regi on were combined with daily meteorological data to determine the key characteristics of the major types of ozone episodes occurring in the OTR.March 2005

G:\OTC-EpisodeClassification\Report\ Draft_Final\Sec2(Data).doc3/31/2005 2-7 METEOROLOGICAL VA RIABLE SELECTION

The extensive amount of meteorological data co llected for this study ha d to be reduced to a manageable size for to allow processing of days into groups with similar conditions as described
in Section 3. Selection of ke y meteorological variables that be st represent conditions across the OTR on exceedance days was based on a review of previous studies (D euel and Douglas, 1996; McHenry et al., 2004) and on disc ussions with state and local agency air quality personnel involved in ozone forecast programs within the OTR. Key variables focused on both surface conditions (maximum temperature, morning an d afternoon average wind direction and speed, pressure) and conditions aloft ( 500 and 850 mb heights, temperat ures, and winds). The final selected set of key daily mete orological parameters are:

Surface resultant wind speed and direction computed for both morning (05:00 – 10:00 EST) and afternoon (12:00 – 17:00 EST) hours at New York City (LaGuardia), Philadelphia, Boston, Buffalo, Albany, Washington DC, Portland ME, Atlantic City, Islip (Long Island), Hyannis (Cape Cod), Worcester MA, and Hartford CT. 2

Surface daily maximum temperatures at New York City NY (LaGuardia), Philadelphia PA, Boston MA, Buffalo NY, Albany NY, Wash ington DC, Portland ME, Atlantic City NJ, Islip NY (Long Island), Hyannis MA (Cap e Cod), Worcester MA, and Hartford CT. 2

Temperatures, heights, and winds at 850 mb pressure su rface at Washington DC, New York NY, Boston MA, Pittsburgh PA, Buffalo NY, and Portland ME.

Surface pressure gradients across the OTR computed as pressure differences between: Washington DC and New York City Washington DC and Boston Washington DC and Pittsburgh Pittsburgh and Buffalo
Buffalo and Boston

2 Surface wind and temperature data from Concord, NH an d New Haven, CT were also examined but these sites had a high frequency of missing data which prevented their use in this analysis.March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-1 3. EPISODE CLASSIFICATION ANALYSIS

In this section we describe a series of clustering and exploratory analyses performed on the ozone and meteorological data discussed in Sectio n 2. Clustering was performed with data from the historical (1997 – 2001 and 2003) period to iden tify the major types of ozone episodes in the OTR and their key characteristics. Once the key episode types were identified, we developed a decision rule for classifying any given day into one of the identified episode types based on ozone levels and meteorological conditions. This decision rule was then used to classify days during the 2002 ozone season by episode type. Th e resulting distribution of episode types and the ozone and meteorological condi tions occurring within each t ype in 2002 were subsequently compared with results from the historical peri od to determine the representativeness of the 2002 with respect to conditions during the historical period.

CLUSTERING ANALYSIS

Clustering was performed with data for the 329 days in the 1997-2001/20 03 historical period on
which an 8-hour ozone exceedance was recorded at one or more of the monitoring sites shown in Figure 2-4. As the clustering algorithms require nu merical variables, wi nd directions were decomposed into u (east-west) and v (north-s outh) components. Mete orological data were prepared for clustering by first filling in missing values with exceedance day means. This step was necessary as the clustering procedures cannot process any days that have missing values for
one or more variables. While the fraction of data that are missing for any individual variable is fairly small, roughly two-thirds of the 329 8-hour ozone exceedance days in our historical dataset had at least one missing value, so it was important to impute the missing values in some fashion even though the clustering results are not likely to be too sensitive to the exact method of imputation. All of the data were then standardized by computing z-scores (i.e., subtracting the mean and dividing by the standard deviation) prior to clustering so that va riables with different scales of measure are given equal weight.

Ozone data were also prepared at this time for use in the clustering anal ysis. Two daily ozone summary statistics, AvgEx08 and AvgEx00 were computed for each monitoring sub-region shown in Figure 2-4. AvgEx08 was defined as the average, over all sites in a given sub-region, of the amount by which the daily maximum 8-hour average exceeded 0.08 ppm (with values for sites below 0.08 ppm set equal to zero). A vgEx00 is identical to AvgEx08 but with the exceedance threshold set to 0 ppm. As with the meteorological data, z-scores were computed for the daily AvgEx08 in each sub-region for use in the clustering analysis. Preliminary clustering analyses were performed using the methods de scribed below with first the AvgEx08 measure and then the AvgEx00 measure. Cluster result s based on the AvgEx00 measure appeared to be more robust and physically meaningful then results based on the AvgEx08 measure so the AvgEx08 measure was dropped in favor of AvgEx00.

Initially, clustering was applied to the meteorol ogical variables only. Both agglomerative and divisive hierarchical clustering techniques were used. Classifications of days under the resulting meteorological clusters were compared with the classification of da ys by ozone exceedance pattern which had previously been reported (Stoeckenius and Kemball-Cook, 2004). These comparisons showed that, while some pairs of exceedance and meteorolog ical patterns showed aMarch 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-2 dominant one-to-one relationship, others did no t. In other words, some of the exceedance patterns were typically associated with more than one meteorological pattern and some meteorological patterns were typical ly associated with more than one exceedance pattern. This
result was found to be robust in the sense that it occurred under a variety of clustering approaches. We interpreted this to mean that at least some of the ozone exceedance patterns described by Stoeckenius and Kemball-Cook were not sufficiently unique from a meteorological perspective to serve as adequate archetypes of different types of ozone episodes. Given this result, we decided to examine clustering appro aches based on using bot h the meteorological and ozone (AvgEx00) data simultaneously.

Before proceeding further, we performed a principal components analysis (PCA) on the combined ozone and meteorological data set prio r to clustering to determine if it would be possible to reduce the number of variables require d for the analysis. Preliminary results showed, however, that the first four compon ents only explained 14% of the to tal variance. As a result, we did not pursue the PCA any further but simply reta ined all of the key vari ables in the clustering analysis.

Several different clustering pro cedures were applied to the da ta. Application of single and complete linkage hierarchical agglomerative clustering methods (Venables and Ripley, 1994) to the combined ozone and meteorological data re sulted in the formation of one large cluster containing most of the days in the dataset and a large number of additional clusters containing at most a few days each. Use of Ward's method (Ward, 1963) produced a more even distribution of cluster membership at each stage of the agglom eration but with fairly evenly spaced reductions in deviance (see resulting dendrogr am in Figure 3-1). In other words, these results did not provide much guidance as to what would constitu te a reasonable number of clusters to use in describing the data.March 2005

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Figure 3-1. Dendrogram from application of Ward’s hierarchical agglomerative clustering to the combined daily ozone and meteorological data. Each leaf at the bottom of the figure represents one day; the vertical height at which pairs of leaves (or pairs of clusters of leaves) are joined represents a measure of the distance between the leaves (or cluster centroids) in the multivariate data space.

Based on the agglomerative clustering results, we decided to apply Hartigan’s k-means clustering algorithm (Hartigan, 1979 ) several times, specifying a di fferent value for the number of clusters to form in each application. Under the k-means algorithm, data are arranged into a pre-specified number of clusters so as to minimize the total within-c luster sum of squares. Initial cluster centroids are determined via agglomerative hi erarchical clustering. After this initial step, each day is assigned to the nearest cluster centroid where “nearest” is in this case defined as the minimum least squares distance computed over all of the standardized variables. After this initial assignment phase, the algorithm iteratively reassigns days to different clusters until the sum of the within-cluster sums of squares is minimized. 1

Due to the large number of variables used in th e clustering procedure, it is difficult to obtain a complete picture of the meteorological and air qu ality conditions associated with days falling in each cluster, especially when looki ng at several alternat ive cluster configurations. As one of the most import features of each cl uster is the spatial ozone dist ribution, we tabulated the mean

1 As finding the global minimum of this objective functio n is not computationally feasible, Hartigan’s algorithm actually finds a local minimum such that switching any single observation from one cluster to another does not reduce the objective. As a result, the final cluster assignments may be sensitive to the selection of initial cluster
centroids.March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-4 values of the ozone measure described above (AvgExc00) for each sub-region within each cluster identified by the k-means algorithm when the data are divided into between 4 and 7 clusters (see Table 3-1). We also examined si milar sets of results for each key meteorological variable. Inspection of these resu lts revealed the presence of fi ve distinct sets of ozone and meteorological conditions that are robust in the se nse that they show up consistently whether the data are divided into 4, 5, 6 or 7 clusters.

Table 3-1 . Mean z-scores for the AvgEx00 ozone summary statistic within each monitoring sub-region under four different candidate sets of cluster designations. The episode pattern ID in the far right-hand column is keyed to the episode patterns described in the text. a) 4
clusters Episode
Pattern

Cluster # Sub-Region
1 Sub-Region
2 Sub-Region
3 Sub-Region
4 Sub-Region
5 Sub-Region
5a ID

1 0.51 0.08 0.40 -0.08 0.26 0.56 C

2 -0.86 0.49 -0.86 -0.40 -0.15 -0.37 B

3 0.27 0.05 0.22 0.30 0.30 0.20 A

4 -0.72 -0.57 -0.45 -0.31 -0.93 -0.96 E b) 5
clusters

Cluster # Sub-Region
1 Sub-Region
2 Sub-Region
3 Sub-Region
4 Sub-Region
5 Sub-Region
5a

1 -0.74 0.07 -0.40 -0.58 -0.34 -0.78 E

2 -0.70 0.45 -0.73 -0.37 -0.06 -0.17 B

3 0.45 0.16 0.32 0.35 0.45 0.46 A

4 -0.49 -0.97 -0.35 0.22 -1.07 -0.91 D

5 0.54 -0.04 0.47 -0.10 0.17 0.42 C c) 6
clusters

Cluster # Sub-Region
1 Sub-Region
2 Sub-Region
3 Sub-Region
4 Sub-Region
5 Sub-Region
5a

1 -0.83 0.47 -0.88 -0.47 -0.17 -0.27 B

2 0.38 0.21 0.23 0.44 0.49 0.46 A

3 0.05 -1.62 0.17 0.58 -1.32 -0.86 D

4 -1.13 -0.17 -0.91 -0.52 -0.89 -1.10 E1

5 0.49 0.03 0.43 -0.10 0.21 0.48 C

6 -0.19 -0.14 0.11 -0.39 -0.17 -0.48 E2 d) 7
clusters

Cluster # Sub-Region
1 Sub-Region
2 Sub-Region
3 Sub-Region
4 Sub-Region
5 Sub-Region
5a

1 0.59 -0.30 0.56 -0.39 -0.02 -0.01 C

2 0.08 0.02 0.02 0.32 0.32 0.10 D1

3 0.08 -1.77 0.21 0.65 -1.52 -1.00 D2

4 -1.17 -0.15 -0.97 -0.54 -0.88 -1.11 E1

5 0.60 0.44 0.39 0.45 0.60 1.03 A

6 -0.38 -0.12 -0.03 -0.45 -0.27 -0.58 E2

7 -0.89 0.47 -0.89 -0.47 -0.19 -0.39 BMarch 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-5 We prepared summaries of the meteorological char acteristics of each of th ese five episode types
as follows: 1) composite maps of surface and upper air ( 850 mb) meteorological variables for each cluster, 2) Side-by-side box plots compar ing the distributio ns of selected key meteorological variables within each cluster, and 3) Tables of morning and afternoon resultant wind direction frequencies within each cluster.
Full results of items 1 – 3 above are presented in Appendix A, B, and C, respectively. By way of example, we show the 850 mb height and wind fi elds, 850 mb temperature, surface pressure, and surface daily maximum temperatur e and 10 m wind fields composited for each episode type in Figures 3-2 to 3-5, respectively. Comparing these composite fields for different episode types reveals that each episode type is characterized by a distinct meteorol ogical pattern and these patterns are consistent with the oz one patterns noted in Table 3-1. Key characteristics of the five episode types are presented in Table 3-2. In the description of each episode type, “average” refers to averages over all OTR exceedan ce days used in the cluster analysis.March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-6 Figure 3-2 . Composite 850 mb height and wind fields for each episode (pattern) type. Pattern numbers refer to the episode types listed in Table 3-2: Pattern 1 = Episode Type E, 2 = B, 3 = A, 4 = D, 5 = C).March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-7 Figure 3-3 . Composite 850 mb temperature fields for each episode (pattern) type. Pattern numbers refer to the episode types listed in Table 3-2: Pattern 1 = Episode Type E, 2 = B, 3 = A, 4 = D, 5 = C).March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-8 Figure 3-4 . Composite surface sea level pressure for each episode (pattern) type. Pattern numbers refer to the episode types listed in Table 3-2: Pattern 1 = Episode Type E, 2 = B, 3 = A, 4 = D, 5 = C).March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-9 Figure 3-5 . Composite surface temperature and 10 m wind fields for each episode. Pattern numbers refer to the episode types listed in Table 3-2: Pattern 1 = Episode Type E, 2 = B, 3 = A, 4 = D, 5 = C).March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-10 Table 3-2 . Key characteristics of each OTR episode type. Episode

Type Pattern
No. Description

A 3 High ozone throughout the OTR. This pattern is characterized by strong high pressure over the southeastern st ates extending from the surface to 500 mb with high temperatures extending into New England and southwest surface winds throughout the OTR. 850 mb temperatures and heights, and surface temperatures are above average at all locations except Washington DC; winds are SW to W throughout the OTR except more variable at LaGuardia and magnitudes of resultant wind vectors are higher than average (indicative of a fairly steady, well defined flow regime), E-W surfacepressure gradients are near neutral but SW-NE gradients both along the I95 corridor and in the west (Pittsburgh to Buffalo) are positive which is
consistent with the SW flow. Ozone formation under these conditions is promoted throughout the OTR by the stable air mass and high temperatures.
B 2 High ozone confined to the extreme southeastern OTR. This pattern is characterized by an upper-level trough offshore of the OTR and a surface high centered over Kentucky. This results in cooler air advection over nearly all of the OTR with northwest flow aloft and a more westerly flow at the surface. 850 mb heights are lower than average (especially in New England) and surface winds are more frequently from NW along the I-95 corridor than under Type A. Temperatures at 850 mb along the I-95 corridor are only slightly cooler t han under Type A but inland temperatures, especially in the north, are much cooler (e.g., at Buffalo); similarly, surface temperatures along the I-95 corridor are about the same as under Type A but temperatures are cooler in Bu ffalo and Albany. Type B events have the strongest positive W – E surface pr essure gradients of any category, consistent with the NW winds but gradients from Washington to New York and Boston are positive. The cooler air over the western OTR and westerly to northwesterly flow result in the higher ozone levels being confined to just the extreme southern portion of the OTR under this pattern.
C 5 High ozone along I-95 corridor and northern New England. This pattern is characterized by an extension of t he semi-permanent Bermuda high into the southeastern U.S. and an area of high surface and 850 mb temperatures extending from Maryland to Maine; the 500 mb pattern is nearly zonal (east – west flow) while flow at the surface is generally from the SW. 850 mb heights intermediate between Type A and Type B but 850 mb temperatures are very high along the I-95 corridor and slightly cooler further inland. Winds are more consistently S - SW at all sites than under other episode types and almost no NW-N-NE winds are seen at LaGuardia in contrast to other types. Resultant wind vector magnitudes are much higher than average, consistent with the steady SW flow. SW – NE pressure gradients along I-95 corridor and from Pittsburgh to Buffalo are positive, consistent with the SW flow. Average E-W pressu re gradients are near zero. These conditions result in above average ozone levels all along the I-95 corridor with advection north into coastal and interior New England. Ozone levels are slightly below average in the extreme southeastern and western OTR (subregions 2 and 4).

D 4 High ozone in the western OTR . This pattern is characterized by an area of mean upper level divergence with associated cut-off low at 850 mb off the Outer Banks of North Carolina. A relatively vigorous mean low pressure
center can be seen at the surface. An east-west temperature gradient
across the OTR is evident at 850 mb. Surface temperatures along the I-95 corridor and in Albany are below aver age but surface temperature is above average at Buffalo. 850 mb heights ar e the highest of any episode type due
to a stron g rid g e over New En g land. Surface winds are mostl y E - NE alon gMarch 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-11 Episode

Type Pattern
No. Description

I-95 corridor from DC to NY but more variable further north. In contrast to episode types A, B, or C, SW – NE pressure gradients along the I-95 corridor are negative, consistent with the NE surface winds. W – E pressure gradients are flat. These conditions result in below average ozone in the eastern OTR (sub-regions 1, 2, 3, 5, and 6) due to the on-shore flow in the north and cyclonic conditions in the south but above average ozone levels in the western OTR (sub-region 4) due to stable, warm conditions with light winds.
E 1 Generally low ozone throughout OTR . This category includes days with moderately low to lowest average ozone readings of all OTR exceedance days included in the cluster analysis. The Bermuda high is shifted east relative to the other types and flow over the southeastern U.S. is only weakly anti-cyclonic with a nearly zonal flow pattern at the 850 and 500 mb levels over the OTR. Temperatures at the surface and aloft are the coolest of any episode type. While winds al oft are nearly westerly, surface winds are generally S – SE over most of t he OTR. SW – NE pressure gradients are negative along the I-95 corridor and E-W gradients are positive, consistent with the SE flow. These conditions result in below average ozone throughout the OTR due to the relatively low temperatures and southeasterly onshore flow at coastal locations.

The five episode types described in Table 3-2 exhibit characteris tics which are largely consistent
across the different cluster allocations noted in Tabl e 1 (4, 5, 6 or 7 clusters). When four clusters are specified, the Type D events are subsumed into the remaining four episode types. Finer division of days into six clusters results in a split of the Type E events into two groups (denoted as E1 and E2 in Table 1) with generally very similar meteorological cond itions but distinguished in part by E-W pressure gradient anomalies that are slightly greater under type E2. Further division into seven clusters appear s to preserve the Type A, B, and, to a lesser extent, Type C events along with the Type E1 and E2 events found in the seven cluster result while the Type D events are split into two new categories (denoted D1 and D2 in Table 1). Both D1 and D2 events are associated with high ozone in the west (sub -region 4) under S – SW flow as is typical of Type D but differ in the surface wind patter n, and hence ozone anomalies, along the I-95
corridor.

It is important to keep in mind that there is no a priori expectation that all ozone exceedance
events in the OTR fall into one of a finite num ber of distinct patterns: daily conditions differ from one another to varying degrees and some da ys will always have characteristics that cross over any predetermined classification boundaries. This means that an episode classification system will always have a certain degree of arbitrarin ess to it and division of days into bins will always result in some days that do not fit partic ularly well into any single bin. Nevertheless, for purposes of this study, we seek a reasonable clas sification system based on a handful of pattern types each of which is uniquely identifiable by a set of characteristics related to ozone formation
across the entire OTR. 2 Based on the clustering results de scribed above, it a ppears that the

2 It is worth reiterating here that we are seeking a general classification system applicable to the whole of the OTR. More precise classification systems could be developed fo r individual sub-regions within the OTR but the resulting two dimensional system (consisting of a unique set of episode types for each of several sub-regions) would not only be very time-consuming to develop but would lead to results from which it would most likely be very difficult to draw any conclusions regarding the representativeness of a single season with respect to conditions over the whole
of the OTR.March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-12 episode Types A – E meet these requirements re asonably well. Frequencies of occurrence for these five types are shown in Table 3-3.

Table 3-3 . Frequencies of occurrence of OTR episode types. Type A Type B Type C Type D Type E No. Days 46 50 123 44 66
Pct. 14% 15% 37% 13% 20%

DEVELOPMENT OF AN OBJECTIVE CLASSIFICATION PROCEDURE

In order to complete our analysis, we needed to develop a final episode classification rule based on results of the above analysis of the 1997 – 2001 and 2003 data which can then be applied to
the 2002 data to determine the classification of episodes in 2002 to the five ozone event types
described above. A classificati on tree model was created for th is purpose using the ozone and meteorological data from 1997-2001 and 2003 as predic tors and the episode pattern type as the response variable. In the classification tree mode l, data from all exceedance days start out together in the root node of th e tree and are then split into two daughter nodes based on the value of one of the predictor variables. For example, a split might consist of sending all days with resultant afternoon wind speed at Hartford, CT less than 4.8 m/s to one node and all remaining days to the other. The variable and value of that variable used to perform a split is determined by examining all possible splits and finding the one which results in the greatest reduction in deviance in the response variable (deviance is a measure of the degree of heterogeneity of the response variable in a node). Th e splitting process is then rep eated for each resulting daughter node and so on until a stopping criteri on is reached. The daughter nodes resulting from the last split along each branch of th e tree are referred to as terminal nodes. The resulting classification tree, grown using the 1997-2001/2003 data as the learning dataset, can then be applied to the 2002 data for which the episode classificat ions are unknown by running the 2002 daily data down the tree, separating days at each node accord ing to the previously determined splitting criteria. Each day from the 2002 data will fall into one of the terminal nodes of the tree, and the probability of that day belonging to the ith episod e type is estimated from the fraction of days from the learning dataset in the terminal node belonging to the ith episode type. The predicted episode type for days in 2002 falling in the terminal node is taken to be the episode type with the highest probability of occurrence.

Initially, the classification tree was grown by making successive sp lits until only a small number of days (in this case five) ends up in each termin al node. This results in a relatively large tree with many terminal nodes, each of which will typically be very homogeneous: most of the days in any one terminal node will belong to the same ep isode type. This large tree represents an over fit to the data in the learning dataset. In other wo rds, if the tree were to be validated against an independent set of days for which the episode ty pes are known (i.e., a test dataset) the frequency of misclassification will generally be higher than the low miscla ssification frequency determined by applying the full tree against the learning dataset. Thus, a smalle r tree (one with fewer splits and therefore fewer terminal nodes), is likely to perform at least as well against a test dataset as the initial, large tree. We therefore applied a recu rsive tree-pruning algorithm known as costcomplexity pruning to the large tree (Venables and Ripley, 1994). This results in a sequence of trees, each of which can be characterized by the number of terminal nodes and the costcomplexity parameter, which is a measure of the trade off between growth in tree size and reduction in deviance. The resulting tree sequence is shown in Figure 3-6. As this figure shows,March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-13 there is a diminishing return in deviance reductio n as the size of the tr ee increases beyond about
5 terminal nodes.

To further evaluate the relative value of different size trees, we performed a ten-fold crossvalidation using the learning datase t. The ten-fold cross-validation consists of setting aside 1/10th of the days in the learning sample as a test sample, building a tree using the remaining 90% of days, and evaluating the deviance reducti on using the reserved days. This process is repeated 10 times with a different set days se t aside in each case. Results from the crossvalidation (Figure 3-7) suggests th at the residual devi ance is minimized at a tree size of about five or six terminal nodes. These results, togeth er with an examination of the misclassification rates from the learning dataset for the pruned tree sequence shows that the 6 terminal node tree is about the optimal size.

sizedeviance200 400 600 800 1000

5 10 15 20 190.0 65.0 32.0 26.0 19.0 14.0 13.0 8.9

Figure 3-6 . Deviance as a function of tree size (number of terminal nodes) for sequence of trees generated by the pruning algorithm.March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-14 sizedeviance800 900 1000 1100 1200

5 10 15 20 190.0 65.0 32.0 26.0 19.0 14.0 13.0 8.9

Figure 3-7 . Deviance from 10-fold cross-validation as a function of tree size (number of terminal nodes) for sequence of trees generated by pruning algorithm.

The selected classification tree is shown in Figur e 3-8; Table 3-4 summari zes the distribution of days by episode type in each terminal node. Two nodes are made up of predominantly Type E days, each of the rest are most representative of one of the four other episode types. Each terminal node has a dominant episode type accoun ting for between 64 and 81% of days assigned
to the node. To use the classification tree fo r assigning an episode t ype to a previously unclassified day, we define the predicted episode type for all days reaching a given terminal node as the dominant episode type for the node as shown by the shaded boxes in Table 3-4. When this rule is applied to the 329 episode da ys during the historical period, a comparison of the predicted episode types with the episode t ypes assigned by the cluster analysis shows an
overall misclassification rate of 23%.

Table 3-4 . Distribution of episode types during the 1997-2001/2003 historical period (as determined via the clustering analysis) for days in each terminal node of the classification tree shown in Figure 3-8. Episode Type

Node No. 1 2 3 4 5 Total

4 16 3 6 0 0 25

5 0 16 100 1 9 126

7 5 3 5 0 53 66

8 2 25 1 0 3 31

10 19 0 5 4 0 28

11 4 3 6 39 1 53

Total 46 50 123 44 66 329March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-15 | resPMwd.KPHL.u<0.156411

resPMws.KBOS<4.84

sfcTmax.KHRT<300.5 resPMwd.KISP.v<0.0958421 H850.BUFFALO<1520.75

1 3 5 2 1 4 Classification Tree

Figure 3-8 . Classification tree used to group days by episode type. Variable names and values used to divide data at each splitting node are shown: days meeting the specified criterion are moved down the left branch in each case. Terminal nodes are numbered 1 – 5 and are keyed to the summary in Table 3-4.

CLASSIFICATION OF 2002 OZONE EPISODES

Data from the 2002 ozone season were analyzed us ing the classification tr ee described above to yield a division of the ozone exceedance days in to the five episode types. The resulting frequency distribution of episode types in 2002 wa s then compared with the historical episode type frequency distribution shown in Table 3-4, thereby providing an indication of the degree to which conditions during 2002 are representative of conditions observed in other years. We also compared ozone concentration dist ributions and composite meteorol ogical fields by episode type in 2002 with those during the historical period as a way of further evaluating the representativeness of conditions du ring the 2002 ozone season.

Episode Type Classification

We applied the 6-node tree shown in Figure 3-8 to all 8-hour ozone exceedance days in 2002. Of the 71 exceedance days, 69 could be assigned to terminal nodes on the tree; missing data prevented classification of two of the days. Examination of the cl assification results showed thatMarch 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-16 if surrogate splits 3 were used to assign these two days to one of the terminal nodes, the number of days falling into the node w ould change by no more than 3 per centage points, so the two days with missing data were simply ignored. The pr edicted episode type for each exceedance day in 2002 was taken to be the predominan t episode type in the terminal node to which it was assigned (as indicated by the shaded boxes in Table 3-4. The resulting distributions of days by episode type for the 2002 season and the 1997-2001/2003 historical pe riod are shown in Figure 3-9. 4 For the historical days, both the episode type assignm ents based on the classification tree and the episode types as originally assi gned in the clustering analysis ar e shown. The overall pattern of episode type occurrence frequencie s for the historical period is si milar between the classification tree and the clustering analysis, as we would expect. Frequencies of occurrence of the episode types are within two percentage points of each other except for Type D events (slightly more Type D days assigned by the classification tree) and Type B events (about a third fewer Type B days determined by the cl assification tree).

Comparison of the occurrence frequencies over th e historical period with the 2002 data also suggest a generally similar pattern of episode types. Note that th e error bars in Figure 3-9 show
the 10 th and 90 th percentile range in the frequencies of occurrence of each episode type observed within individual years during the historical period: an individual year would be expected to fall within this range with 80% probability. The 200 2 type frequencies generally fall within these error bars except for a somewhat higher frequenc y of Type C events and a lower frequency of Type E events. As Type E events are characterized by below average ozone (relative to all exceedance days) throughout all but the southernmo st OTR, this difference reflects the higher frequency of exceedance days in 2002 relative to the historical period as noted above. If we ignore the Type E events and renormalize (see Figure 3-10), the occurrence frequencies in 2002 of the remaining episode types are found to be sim ilar to those in the historical period and fall
within the 10 th to 90 th percentile range in each case. Thus , each of the event types A – D appear to be well represented within the 2002 season.

3 Surrogate splitting uses the best alternative splits (based on the non-missing variable that produces nearly the same split as the primary splitting variable).

4 The bars in this figure are scaled to the fraction of OTR exceedance days assigned to each episode type. Thus, these comparisons are not effected by the above average number of exceedance days in 2002 noted earlier.March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-17 Episode Type Frequencies

37%

15% 20%

13% 14% 38%

9% 20%

16% 16% 33%

12% 29%

17%

9%

0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50%

Type A Type B Type C Type D Type E Clust ering (Hist orical Dat a)

Classif icat ion Tree (Hist orical Dat a)

Classif icat ion Tree (2002 Dat a)

Figure 3-9 . Percent of episode days by type in the 1997 – 2001/2003 historical period (as determined by the cluster analysis and by the classification tree) and in 2002; error bars show
10 th and 90 th percentiles of annual frequencies of occurrence during 1997-2001&2003.

Episode Type Frequencies

( Re nor ma li z e d wi t h Type E Ev ent s Remov ed)

43%

18% 23%

16% 46%

11% 24%

19% 37%

13% 32%

19%

0% 10% 20% 30% 40% 50% 60%

Type A Type B Type C Type D Clust ering (Hist orical Dat a)

Classif icat ion Tree (Hist orical Dat a)

Classif icat ion Tree (2002 Dat a)

Figure 3-10 . Percent of episode days by type in the 1997 – 2001/2003 historical period (as determined by the cluster analysis and by the classification tree) and in 2002 with Type E events removed and frequencies re-normalized; error bars show 10 th and 90 th percentiles of annual frequencies of occurrence during 1997-2001&2003.March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-18 Ozone Concentration levels

An exceedance of the 8-hour ozone standard occurre d at one or more sites in the study region on 71 days during 2002, representing 46% of the 153 days during the May – September season analyzed in this study. For the 1997 – 2001/2003 hi storical period, the co rresponding percentage was 36% so exceedances were more frequent during 2002. The greater frequency of ozone exceedance events was distributed throughout the OTR as shown by the comparison by monitoring sub-region in Table 3-5. Exceedances occurred with 20 – 50% greater frequency in 2002 in all sub-regions (100% grea ter in sub-region 3). This di fference in the frequency of exceedances in 2002 as compared to the historic al period does not necessarily mean, however,
that the exceedance events themselves have char acteristics that signifi cantly differ from those seen during the historical period.

Table 3-5 . Number of days during May-September with 8-hour daily maximum ozone greater than 0.08 ppm in each monitoring sub-region averaged over the 1997-2001/2003 historical period and in 2002. Sub-Region

1 2 3 4 5 6 >0.08 ppm

1997-2001/2003 22.3 31.0 8.5 27.3 42.0 30.8

2002 34 38 17 39 58 44

Pct. Difference 52% 23% 100% 43% 38% 43%

Distributions of daily maximum 8- hour average ozone concentrati ons averaged over monitors in each sub-region (the AvgExc00 stat istic) in 2002 and the historical period are compared for each event type in Figures 3-11(a-e). A key to th e boxplot symbols used to summarize the ozone distribution is shown in Figure 3- 12. Overall, the range of oz one under each event type in 2002
is similar to that under the corr esponding event type in the histor ical period. The most notable exceptions are higher ozone levels during Type D events in 2002 along the Washington – New York City corridor (sub-regions 2, 5, and 6). This is consistent with a less pronounced low pressure center off the NC coast in the 2002 Type D events as compared to the historical period (see further discussion below). Aside from this difference, the overall ozone levels during the 2002 exceedance events were generally very consiste nt with those observed during the historical period, not withstanding the f act that exceedance days were more frequent during 2002.March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-190.03 0.04 0.05 0.06

2002 Historicalppb Sub-Region 10.04 0.06 0.08

2002 Historicalppb Sub-Region 20.02 0.04 0.06

2002 Historicalppb Sub-Region 30.03 0.05 0.07

2002 Historicalppb Sub-Region 40.04 0.06 0.08

2002 Historicalppb Sub-Region 50.02 0.04 0.06

2002 Historicalppb Sub-Region 6

Figure 3-11a . Boxplots of average daily maximum ozone (ppb) in each monitoring sub-region during 2002 and during the 1997-2001 and 2003 “historical” period: Type E (Pattern No. 1)
events.March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-200.03 0.04 0.05 0.06 0.07

2002 Historicalppb Sub-Region 10.05 0.06 0.07 0.08 0.09

2002 Historicalppb Sub-Region 20.03 0.04 0.05 0.06

2002 Historicalppb Sub-Region 30.04 0.06 0.08

2002 Historicalppb Sub-Region 40.05 0.07 0.09

2002 Historicalppb Sub-Region 50.02 0.04 0.06 0.08 0.10

2002 Historicalppb Sub-Region 6

Figure 3-11b . Boxplots of average daily maximum ozone (ppb) in each monitoring sub-region during 2002 and during the 1997-2001 and 2003 “historical” period: Type B (Pattern No. 2)
events.March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-210.04 0.06 0.08 0.10

2002 Historicalppb Sub-Region 10.04 0.06 0.08 0.10

2002 Historicalppb Sub-Region 20.03 0.05 0.07 0.09

2002 Historicalppb Sub-Region 30.04 0.06 0.08 0.10

2002 Historicalppb Sub-Region 40.04 0.06 0.08 0.10

2002 Historicalppb Sub-Region 50.04 0.06 0.08 0.10

2002 Historicalppb Sub-Region 6

Figure 3-11c . Boxplots of average daily maximum ozone (ppb) in each monitoring sub-region during 2002 and during the 1997-2001 and 2003 “historical” period: Type A (Pattern No. 3)
events.March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-220.03 0.05 0.07

2002 Historicalppb Sub-Region 10.03 0.05 0.07 0.09

2002 Historicalppb Sub-Region 20.03 0.05 0.07

2002 Historicalppb Sub-Region 30.04 0.06 0.08

2002 Historicalppb Sub-Region 40.02 0.04 0.06 0.08

2002 Historicalppb Sub-Region 50.03 0.05 0.07

2002 Historicalppb Sub-Region 6

Figure 3-11d . Boxplots of average daily maximum ozone (ppb) in each monitoring sub-region during 2002 and during the 1997-2001 and 2003 “historical” period: Type D (Pattern No. 4)
events.March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-230.03 0.05 0.07 0.09

2002 Historicalppb Sub-Region 10.03 0.05 0.07 0.09

2002 Historicalppb Sub-Region 20.03 0.05 0.07

2002 Historicalppb Sub-Region 30.03 0.05 0.07 0.09

2002 Historicalppb Sub-Region 40.05 0.07 0.09

2002 Historicalppb Sub-Region 50.04 0.08

2002 Historicalppb Sub-Region 6

Figure 3-11e . Boxplots of average daily maximum ozone (ppb) in each monitoring sub-region during 2002 and during the 1997-2001 and 2003 “historical” period: Type C (Pattern No. 5)
events.March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-24 Median 75th
Percentile

25th

Percentile IInter-Quartile
Distance
(IQD) Largest value < 75%'tile + 1.5 * IQD

Smallest value > 25%'tile - 1.5 * IQD Extreme Value

Figure 3-12 . Key to boxplot symbols.

Meteorological Conditions

Selected composite meteorological fields fo r each episode type in 2002 as predicted by application of the classification tree were co mputed and displayed for comparison with the historical period composite fields. Results are shown in Figure 3-13 through 3-16. Comparing
of these results with those for th e historical period (Figures 3- 2 to 3-5), we see a remarkable degree of similarity: 5 the surface and upper air meteorological patterns for a given episode type in 2002 are very similar to those for the same epis ode type observed in the historical period. In other words, the key characteristics of each type observed in the historical dataset are reproduced within the 2002 data. Perhaps th e most significant difference is the less pronounced low pressure center off the NC coast under Type D events in 2002 which allowed for the formation of higher ozone concentrations along the Washington – New York City corri dor for these event types in 2002 as compared to the historical period. Overall, however, the close match in weather patterns associated with each event type in 2002 a nd the historical peri od strongly supports the conclusion that the 2002 ozone episodes, although more numerous than in other years, are of substantially similar character.

5 In making these comparisons, note that different color and wind vector scales had to be used in some plots of the
2002 data.March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-25 Figure 3-13 . Composite meteorological fields by episode type (pattern) in 2002: 850 mb heights and winds. Pattern numbers refer to the episode types listed in Table 3-2: Pattern 1 = Episode Type E, 2 = B, 3 = A, 4 = D, 5 = C).March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-26 Figure 3-14 . Composite meteorological fields by episode type (pattern) in 2002: 850 mb temperature. Pattern numbers refer to the episode types listed in Table 3-2: Pattern 1 = Episode Type E, 2 = B, 3 = A, 4 = D, 5 = C).March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-27 Figure 3-15 . Composite meteorological fields by episode type (pattern) in 2002: sea level pressure. Pattern numbers refer to the episode types listed in Table 3-2: Pattern 1 = Episode Type E, 2 = B, 3 = A, 4 = D, 5 = C).March 2005

G:\OTC-EpisodeClassifica tion\Report\Draft_Final\Sec3 (Analysis).doc3/31/2005 3-28 Figure 16 . Composite meteorological fields by episode type (pattern) in 2002: surface temperature and 10 m winds. Pattern numbers refer to the episode types listed in Table 3-2: Pattern 1 = Episode Type E, 2 = B, 3 = A, 4 = D, 5 = C).March 2005

G:\OTC-EpisodeClassification\Report\Dr aft_Final\Sec4(Conclusions).doc3/31/2005 4-1 4. CONCLUSIONS AND RECOMMENDATIONS

Results from the application of statistical cluste ring analyses presented in Section 3 show that regional ozone episode conditions over the OTR can be reasonably well described by a set of five different episode types. Our examinati on of mean ozone and me teorological conditions shows that each of these episode types is associated with a unique set of distinguishing characteristics. While we would not expect ev ery exceedance day to fu lly exhibit all of the characteristics of one type or another, our resu lts provide no clear evidence for the existence of any other additional sufficiently unique type s which occurred frequently enough to be distinguishable within the six year historical period analyzed.

Data from the 2002 ozone season were analyzed within the framework of the five identified episode types with respect to: a) frequencies of occurrence of each type and b) characteristics of the ozone and meteorological conditions within each type in 2002 as compared to the 1997 – 2001/2003 historical period.

A key feature of the 2002 season is that ozone ep isodes (defined as an exceedance of the 8-hour ozone standard at one or more monitoring sites w ithin the OTR) occurred more frequently than during the historical period (71 exceedance days during the May – Septem ber season in 2002 as compared to an average of 55 days per season during the historical period). Taken by itself, however, this difference does not necessarily me an that region-wide meteorological and ozone concentration patterns during exceedance days were significantly different in 2002 as compared to other years: the greater number of exceedance days in 2002 may just reflect a lower than average frequency of days with meteorological conditions not conducive to ozone formation in 2002. As these days are of little in terest to us in terms of modeling in support of ozone SIP development, however, the higher than average exceedance rate in 2002 is by itself not an indication of any lack of representativeness of the 2002 exceedance events.

Our examination of conditions during exceedance days in 2002 showed that:

• Except for the Type E events during which oz one exceedances are typically confined to the extreme southeastern corner of the OTR, each of the five episode types identified in the historical period was found to occur on about as many days in 2002 as one would expect based on their rate of occurrence during the hist orical record. Thus the meteorological conditions on episode days in 2002 exhibit a normal range of variation and each of the five types of episodes are well represented.

• Type E events are under represented in the 2002 season. This is consistent with the higher than average frequency of exceedance days in 2002. The relative lack of Type E events in
2002 should not be of concern from a SIP m odeling standpoint, however, as these events are characterized by relatively low ozone leve ls throughout nearly all of the OTR (except the Washington and Virginia area).

• The distribution of da ily maximum 8-hour average ozone le vels during each event type in 2002 is generally very similar to that within the same event type during the historical period. The only significant exception is hi gher ozone along the Washington – New York City corridor under Type D events in 2002 as compared to the historical average.March 2005

G:\OTC-EpisodeClassification\Report\Dr aft_Final\Sec4(Conclusions).doc3/31/2005 4-2 • Regional-scale meteorological conditions during each event t ype in 2002 exhibit the same key characteristics as observed for the even t types during the historical period. A less pronounced low pressure center off of the NC coast under the 2002 Type D events appears to be responsible for the higher Washington – New York City ozone levels under this event type noted in the previous bullet.

In summary, while ozone exceedances were mo re frequent during 2002, conditions during the 2002 exceedance events were for the most part ve ry similar to those found to occur in other years. This leads us to concl ude that the 2002 season can be consid ered to be representative for purposes of photochemical modeling in support of SIP development.March 2005

G:\OTC-EpisodeClassification\Re port\Draft_Final\Sec5(refs).doc R-1 REFERENCES

Breiman, L., J.H. Friedman, R.A. Olshen and C.J. Stone, 1984. Classification and Regression Trees. Wadsworth, Belmont, CA

Cox, W.M., 1997. Spatial Clustering of Ambi ent Ozone in Eastern U.S., Draft, U.S. Environmental Protection Agen cy, January (available at http://www.epa.gov/ttn/naaqs/oz one/areas/misc/ozclus2a.htm )

Deuel, H.P. and S.G. Douglas, 1996. “ Regional Ozone Patterns in the Eastern U.S. 1985-1995: Statistical Pattern Analysis and Classificati on and Regression Tree (CART) Analysis in Support of the Ozone Transport and Asse ssment Group (OTAG) Modeling Effort.” Systems Applications Inc. San Rafael, CA October.

EPA, 1999. “Draft Guidance on the Use of M odels and Other Analyses in Attainment Demonstrations for the 8-Hour Ozone NAAQ S.” EPA-54/R-99-004, U.S. Environmental Protection Agency, May.

Harrell, F.E., 1999. Hmisc S-PLUS function libra ry. Programs available from lib.stat.cmu.edu.

Hartigan, J. A. and M.A. Wong, 197 9. A k-means clustering algorithm. Applied Statistics 28

, 100-108.

McHenry, J.N., W.F. Ryan, N.L. Seaman, C.J. Coat s, J. Pudykiewicz, S. Arunachalam and J.M. Vukovich, 2004. A real-time Eulerian photoche mical model forecast system: Overview and initial ozone forecast performance in the northeastern U.S. corridor. Bulletin of the American Meteorological Society , 85(4), April.

Sarle, WS: The VARCLUS Procedure. SAS/STAT User's Guide, 4th Edition, 1990. Cary NC: SAS Institute, Inc.

Stoeckenius, T.E. and S. Kemball-Cook, 2004. Ozone episode classifi cation project: spatial ozone pattern analysis (Task 2a). Memo to T. Frankiewicz, Ozone Transport Comission, 21 September.

Venables, W.N. and B.D. Riple y, 1994. “Modern Applied Statisti cs with S-Plus.” SpringerVerlag, New York.

Ward, J.H., 1963. Hierarchical groupings to optimize an objective function. J. Amer. Statistical

Assoc. , 58:236-244.Appendix A

Composites Representing Mean Meteorological Conditions During Each Ozone Episode PatternComposites Representing Mean Meteorological Conditions During Each Ozone Episode Pattern

Mean meteorological fields were computed over days falling into each of the five ozone episode patterns in the Ozone Transport Regi on defined in the text. The five episode patterns and their composite pattern identifiers are:

Composite Pattern Episode Type

3 Type A: High ozone throughout the OTR

2 Type B: High ozone confined
to extreme southeastern OTR

5 Type C: High ozone along I95 corridor and northern New England

4 Type D: High ozone in the
western OTR

1 Type E: Generally low ozone throughout the OTR

Mean fields were computed for the following parameters extracted from the EDAS data:

Parameter ID Description H850 850 mb height

850 mb Wind Resultant wind vector at 850 mb T(850 mb) 850 mb temperature (deg K) MSLP Mean sea level pressure (mb) TSFC Surface temperature (deg K)

10m Wind Resultant wind v ector at 10 m height w_500 w (vertical) component of 500 mb wind

vector H500 500 mb heightsAppendix B

Boxplots of Key Mete orological VariablesBoxplots Of Key Meteorological Variables

Boxplots in this Appendix summarize distri butions of the sub-re gional ozone summary statistic, AvgEx00, described in the text al ong with selected key daily meteorological variables by episode pattern membership for th e five cluster case. Pattern membership identifiers (“Met Cluster”) used in these pl ots correspond to the ep isode types described
in the text as follows:

Met Cluster Episode Type 1 Type E 2 Type B 3 Type A 4 Type D 5 Type C

Ozone and meteorological variables are:

Variable Description clnx AvgEx00 ozone summary statistic for ozone monitoring cluster x (x = 1,2...6; see Figure 1 in text) DelPsfc.edas.DCtoNYC Surface pressure gradient: Washington DC – New York City DelPsfc.edas.DCtoBOSTON Surface pressu re gradient: Washington DC to

Boston DelPsfc.edas.DCtoPITTSBURGH Surface pres sure gradient: Washington DC to Pittsburgh DelPsfc.edas.BUFFALOtoBOSTON Surface pr essure gradient: Buffalo to Boston

DelPsfc.edas.PITTSBURGHtoBU FFALO Surface pressure gradient: Pittsburgh to Buffalo H850.DC 850 mb height: Washington DC H850.BOSTON 850 mb height: Boston H850.PITTSBURGH 850 mb height: Pittsburgh H850.BUFFALO 850 mb height: Buffalo H850.PORTLAND 850 mb height: Portland, ME H850.NYC 850 mb height: New York City T850.DC 850 mb temperature: Washington DC T850.BOSTON 850 mb temperature: Boston

T850.PITTSBURGH 850 mb temperature: Pittsburgh T850.BUFFALO 850 mb temperature: Buffalo T850.PORTLAND 850 mb temperature: Portland, ME T850.NYC 850 mb temperature: New York City sfcTmax.KLGA Daily max surface temperature: La Guardia sfcTmax.KPHL Daily max surf ace temperature: Philadelphia sfcTmax.KBOS Daily max surface temperature: Boston sfcTmax.KBUF Daily max surface temperature: Buffalo sfcTmax.KALB Daily max surface temperature: Albany sfcTmax.KDCA Daily max surface temperature: Washington DC0.04 0.06 0.08

12345

Met Clustercln10.04 0.06 0.08 0.10

12345

Met Clustercln20.02 0.04 0.06 0.08

12345

Met Clustercln30.04 0.06 0.08 0.10

12345

Met Clustercln40.02 0.04 0.06 0.08 0.10

12345

Met Clustercln50.02 0.06 0.10

12345

Met Clustercln6-6 -4 -2 0 2 4

12345

Met ClusterdelPsfc.edas.DCtoNYC-5 0 5

12345

Met ClusterdelPsfc.edas.DCtoBOSTON-6 -4 -2 0 2 4 6

12345

Met ClusterdelPsfc.edas.DCtoPITTSBURGH-15 -10 -5 0 5 10

12345

Met ClusterdelPsfc.edas.BUFFALOtoBOSTON-6 -4 -2 0 2 4 6 8

12345

Met ClusterdelPsfc.edas.PITTSBURGHtoBUFFALO1450 1500 1550 1600 1650

12345

Met ClusterH850.DC1400 1450 1500 1550 1600

12345

Met ClusterH850.BOSTON1400 1500 1600

12345

Met ClusterH850.PITTSBURGH1350 1450 1550 1650

12345

Met ClusterH850.BUFFALO1400 1450 1500 1550 1600

12345

Met ClusterH850.PORTLAND1450 1500 1550 1600

12345

Met ClusterH850.NYC282 286 290 294

12345

Met ClusterT850.DC275 280 285 290 295

12345

Met ClusterT850.BOSTON280 285 290 295

12345

Met ClusterT850.PITTSBURGH275 280 285 290 295

12345

Met ClusterT850.BUFFALO270 275 280 285 290

12345

Met ClusterT850.PORTLAND275 280 285 290 295

12345

Met ClusterT850.NYC290 295 300 305 310

12345

Met ClustersfcTmax.KLGA290 295 300 305 310

12345

Met ClustersfcTmax.KPHL285 290 295 300 305 310

12345

Met ClustersfcTmax.KBOS290 295 300 305

12345

Met ClustersfcTmax.KBUF290 295 300 305

12345

Met ClustersfcTmax.KALB295 300 305 310

12345

Met ClustersfcTmax.KDCA290 295 300 305

12345

Met ClustersfcTmax.KPWM290 295 300 305 310

12345

Met ClustersfcTmax.KHVN290 295 300 305 310

12345

Met ClustersfcTmax.KACY290 295 300 305 310

12345

Met ClustersfcTmax.KISPAppendix C

Wind Direction Frequency TablesWind Direction Frequency Tables

Contingency tables showing resultant surface wind direction frequencies were prepared for the five cluster membership cases. These results show relative frequency of days with the indicated wind direction in each cluster, i. e., the values for each cluster (column) sum
to 100%. Tabulations are shown for both mo rning (AM) and afte rnoon (PM) resultant
wind direction. Site location codes refere nced in these tables are shown below.

Site Code Location KLGA LaGuardia airport, New York, NY KPHL Philadelphia, PA KBOS Boston, MA KBUF Buffalo, NY KALB Albany, NY KDCA Washington, DC KPWM Portland, ME KHVN New Haven, CT KACY Atlantic City, NJ KISP Islip, Long Island, NY KHYA Hayannis, Cape Cod, MA KWOR Worcester, MA (KORH) KHRT Hartford, CT (KHFD)Table B-1 . Morning and afternoon daily resultan t wind direction frequencies (%) by cluster membership for the five cluster cas e (columns sum to 100%). Header row for
each table indicates AM or PM and four letter site ID as described in text (e.g., KLGA = LaGuardia, NY). Cluster identifier (A, B, C, D, E) is shown in first row of each table.

a) Morning wind directions

$resAMwd.KLGA:

E B A D C

E 2 2 5 47 3

N 4 4 1 0 0

NE 27 4 7 35 0

NW 7 42 10 0 0

S 33 6 12 5 6

SE 11 0 7 7 2

SW 16 10 34 2 76

W 0 32 23 5 14 $resAMwd.KPHL:

E B A D C

E 9 0 2 32 2

N 4 4 1 2 0

NE 7 0 0 34 0

NW 4 28 4 5 0

S 24 4 17 9 29

SE 20 0 4 7 2

SW 26 34 55 9 64

W 7 30 17 2 5 $resAMwd.KBOS:

E B A D C

E 2 0 4 9 2

N 2 0 1 0 0

NE 9 4 1 14 0

NW 9 22 13 20 0

S 17 8 11 16 11

SE 15 2 2 7 3

SW 22 26 43 20 77

W 24 38 25 14 8
$resAMwd.KBUF:

E B A D C

E 16 2 3 16 0

N 0 0 1 0 0

NE 13 0 0 5 3

NW 0 10 0 0 5

S 31 29 44 16 17

SE 24 0 24 57 6

SW 16 40 25 7 59

W 0 19 2 0 11 $resAMwd.KALB:

E B A D C

E 2 0 8 6 0

N 10 7 5 20 2

NE 2 0 1 3 0

NW 10 20 4 3 0

S 34 20 55 49 87

SE 15 0 9 6 3

SW 20 26 12 9 8

W 7 28 5 6 0 $resAMwd.KDCA:

E B A D C

E 4 0 0 26 0

N 2 14 2 2 0

NE 7 0 1 28 0

NW 9 24 5 5 0

S 46 24 38 16 68

SE 4 0 5 16 2

SW 22 28 45 2 29

W 7 10 5 5 2
$resAMwd.KPWM:

E B A D C

E 10 8 3 2 3

N 2 0 1 7 0

NE 7 2 0 0 2

NW 12 17 14 17 2

S 12 8 11 5 27

SE 2 4 7 2 5

SW 15 8 16 29 50

W 39 52 49 38 12 $resAMwd.KACY:

E B A D C

E 7 0 1 26 0

N 2 2 2 10 0

NE 5 0 1 23 0

NW 5 22 7 10 0

S 29 0 17 10 41

SE 12 2 6 8 3

SW 22 29 45 8 52

W 17 45 22 5 5 $resAMwd.KISP:

E B A D C

E 9 0 5 14 0

N 9 9 6 19 0

NE 9 0 7 36 0

NW 0 34 10 5 0

S 32 2 8 2 11

SE 14 0 3 14 2

SW 11 21 44 7 80

W 16 34 18 2 8
$resAMwd.KHYA:

E B A D C

E 6 0 0 8 0

N 3 13 3 14 0

NE 11 2 1 43 0

NW 6 20 6 0 0

S 14 7 6 11 3

SE 8 4 6 3 0

SW 31 22 41 19 60

W 22 33 38 3 37 $resAMwd.KWOR:

E B A D C

E 5 2 0 7 0

N 0 4 3 10 0

NE 10 0 1 15 2

NW 5 23 10 17 0

S 5 2 0 5 2

SE 5 0 2 7 0

SW 33 4 19 12 62

W 38 65 65 27 35 $resAMwd.KHRT:

E B A D C

E 5 5 5 19 0

N 5 2 2 6 0

NE 8 2 2 9 0

NW 3 7 0 0 0

S 51 36 71 41 94

SE 5 12 13 16 3

SW 19 19 2 6 3

W 3 17 5 3 0Table B-1 (concl). b) Afternoon wind directions

$resPMwd.KLGA:

E B A D C

E 9 0 7 20 0

N 0 6 1 0 0

NE 30 0 15 68 0

NW 4 68 26 0 8

S 30 6 12 2 8

SE 13 2 8 7 0

SW 11 6 17 0 48

W 2 12 14 2 36 $resPMwd.KPHL:

E B A D C

E 13 2 4 57 0

N 0 8 2 0 0

NE 4 0 0 32 0

NW 2 40 5 0 2

S 22 0 20 2 8

SE 26 4 3 9 2

SW 30 22 50 0 73

W 2 24 16 0 17 $resPMwd.KBOS:

E B A D C

E 20 2 13 34 5

NE 9 2 2 9 0

NW 4 30 2 0 0

S 17 4 23 18 8

SE 15 2 20 25 0

SW 26 10 24 7 65

W 9 50 16 7 23

$resPMwd.KBUF:

E B A D C

E 7 2 3 19 0

N 0 2 0 0 0

NE 11 4 1 5 5

NW 2 6 6 0 8

S 28 12 20 33 3

SE 11 6 2 16 0

SW 37 51 61 28 68

W 4 16 7 0 17 $resPMwd.KALB:

E B A D C

E 2 0 3 8 0

N 2 2 2 11 0

NE 4 2 2 3 0

NW 7 41 6 0 2

S 59 2 45 50 58

SE 15 2 5 18 2

SW 7 20 17 8 24

W 4 31 20 3 15 $resPMwd.KDCA:

E B A D C

E 7 4 5 23 0

N 0 4 2 2 0

NE 9 6 2 48 0

NW 2 34 4 0 5

S 57 14 53 9 59

SE 11 6 9 11 3

SW 11 6 17 2 21

W 4 26 8 5 12
$resPMwd.KPWM:

E B A D C

E 11 8 17 20 5

N 0 4 2 0 0

NE 13 2 2 7 2

NW 11 42 11 0 2

S 20 6 24 27 29

SE 20 4 16 25 6

SW 16 14 15 14 33

W 9 20 13 7 24 $resPMwd.KACY:

E B A D C

E 15 2 3 42 0

N 0 6 1 2 0

NE 7 4 1 47 0

NW 2 48 11 0 0

S 33 4 21 0 23

SE 24 0 10 9 0

SW 13 8 26 0 55

W 7 28 27 0 22 $resPMwd.KISP:

E B A D C

E 11 0 6 36 0

N 2 10 5 2 0

NE 11 0 3 36 0

NW 2 52 11 2 3

S 38 0 17 2 14

SE 20 0 8 18 0

SW 11 8 39 2 68

W 4 29 13 0 15
$resPMwd.KHYA:

E B A D C

E 16 11 7 9 0

N 0 9 2 7 0

NE 9 4 5 27 0

NW 0 11 6 0 0

S 13 9 12 20 5

SE 18 6 8 20 0

SW 38 21 48 16 75

W 7 30 11 0 20 $resPMwd.KWOR:

E B A D C

E 14 4 3 17 2

N 5 4 1 2 0

NE 14 4 4 22 0

NW 9 35 9 2 0

S 9 0 7 22 5

SE 5 0 3 10 0

SW 37 4 28 7 54

W 7 49 44 17 40 $resPMwd.KHRT:

E B A D C

E 11 2 16 16 0

N 4 9 3 3 0

NE 0 4 7 22 0

NW 2 22 1 0 0

S 56 13 43 22 52

SE 13 11 18 24 2

SW 11 11 9 11 35

W 2 28 4 3 11