Information Visualization

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1 Tracking Air Quality Trends with SASIGRAPH David Mintz, U.S. EPA, Research Triangle Park, NC Terence Fitz-Simns, U.S. EPA, Research Triangle Park, NC Michelle Wayland, U.S. EPA, Research Triangle Park, NC Abstract Tracking and interpreting ambient air quality trends are the main functins f the Air Quality Trends Analysis Grup at EPA. SAs/GRAPH is instrumental in this prcess, allwing us t study trends using several graphing techniques. This paper discusses graphics we have generated using cmbinatins f the GMAP, GCHART, and GPLOT prcedures. We will shw hw these graphics help us t discver, and t sme extent explain, gegraphical, seasnal, and tempral patterns in the data. Intrductin As part f the US EPA air prgram, trends in the quality f the natins air are analyzed and published each year in the Natinal Air Quality and Emissins Trends Reprt. This reprt highlights trends fr six air pllutants fr which the EPA has set Natinal Ambient Air Quality Standards (NAAQS). Ten-year trends in carbn mnxide, lead, nitrgen dixide, (grundlevel) zne, particulate matter, and sulfur dixide are reprted with a natinal perspective. This paper discusses fur graphics we have fund t be very useful fr displaying these trends, thugh their use may be extended t ther types f data. Displaying Tempral Patterns Tile Maps The tile map is gd fr displaying data with a seasnal pattern. We call it a tile map because ne square, r tile, is pltted fr each day f the year. The clr f the tile shws the level f cncentratin fr that day, lighter shades indicating lwer cncentratins and darker shades indicating higher cncentratins. Due t the arrangement f all the days in a year, yu can quickly see any seasnal pattern that may exist. In additin, yu may lk at year-t-year trends by including multiple tile maps n the same page. Figure 1 shws zne trends in Ls Angeles frm 1982 t Yu can see that zne readings are much higher in the summer mnths. (This is due t the fact that zne is frmed in the presence f abundant sunlight and higher temperatures.) Als, yu can see that the number f high zne days has steadily decreased since This graphic helps us t shw that zne is a "summer pllutant". The tile map can als be used t display ther tempral patterns. Fr example, displaying diurnal patterns wuld wrk much the same way, except each tile wuld represent an hur f the day. Then day-ta-day trends, let's say fr ne week, culd be examined by including multiple tile maps n the same page. The tile map, if carefully cnstructed, can cnvey cmplicated tempral patterns in an easily understd manner. Bxplts The bxplt graphic is gd fr shwing trends in the distributins f data, as ppsed t just ne value. This graphic cnsists f several bxplts, ne fr each year, cnnected by a trend line fr a summary statistic, usually the mean. This allws yu nt nly t detect trends ver time, but als t tell if there are differences in the yearly distributins. Fr example, Figure 2 shws the natinal trend in carbn mnxide frm 1984 t Yu can see a dwnward trend in the mean ver the ten-year perid. Lking at the bxplts, yu can see that the dwnward trend is due t decreasing cncentratins at the high end f the distributins. The bxplt graphic gives yu the ability t view entire distributins, ften revealing t yu why a trend may exist. Displaying Gegraphical Patterns Ppulatjn-Cuntv Maps This graphic is a cmbinatin f tw plts - a map and a stacked bar chart. The map shws the levels f cncentratins acrss the cuntry. The adjacent stacked bar chart shws the ppulatins assciated with the ranges f cncentratins. Cmbining these tw graphs is effective because yu can see hw many peple live in areas at varius cncentratin levels and where thse areas are lcated acrss the U.S. In Figure 3, fr example, there were abut 6 t 7 millin peple living in cunties that exceeded the NAAQS fr PM-I 0 (particulate matter measuring s: 10 micrns in diameter) SESUO '95 Prceedings 344

2 Figure 1.IAN LOS ANGELES OZONE FEB MAR APFI MAY.IUN.lUL AUG SEP OCT NOV DEC _ , --"'DAY: PPM 1lllli PPM PPM PPM 345 SESUG '95 Prceedings

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4 PM-10 Air Quality Cncentratins, 1993 Highest Secnd Max 24-Hur Average i!! W.jl. ~ :i.5 c 90.g.!! 1 s 0 70! 60 f 1M 50 t/l 40 ~ C Cl ~..., " R n 0- :::J' e (J),. \,...,,,....c<# f."", \}~ f) Cncentratin < 55 55, >= :::J 0' 3 s 0' :::J < Vi' c: Q) t;i' -0' :::J

5 Figure 4 United states Envirnmental Prtectin Agency Air OffIce f Air Quality Planning and Standards Research Triangle Park Ne EPA 4541R Octber 1994 & EPA Natinal Air Quality and Emissins Trends Reprt, th Percenule = th Percentile = 38 ~-'-T-...J th Percentile = th Percentile = IS Particulate Matter Highest Cunty Annual Mean (micrgrams per cubic meter) SESUG '95 Prceedings 348

6 in This level is represented by the darkest shading. In additin, yu can see where thse cunties are lcated n the map. This graphic is very useful fr crss-referencing tw items f interest - ppulatin and gegraphic lcatin, based n a third variable (in this case, air quality cncentratin levels). Bxplt-Dt Maps This graphic als cmbines tw plts - a bxplt and a map. The bxplt separates the data int quartiles, each represented by a different clr. Arrws are drawn t shw the values at each quartile and at ther percentiles. Each dt n the adjacent map represents a mnitring site and is clr-cded t crrespnd t the quartile ranges f the bxplt. (In sme instances pltting t many pints n ne map may clutter the graphic, in which case splitting the map int fur separate maps may be desired - such as we have dne in Figure 4.) This display allws yu t see where a particular mnitr falls within the distributin. It als helps yu t detect any gegraphical patterns that may exist. Figure 4, fr example, shws that areas f highest PM-I 0 cncentratins tend t be lcated alng the west cast and the Ohi River Valley. Bxplt-dt maps can be very useful fr crss-referencing spatial and distributinal aspects f data. References Natinal Air Duality and Emissins Trends Re.prt.l221 EPA-45 O-R I, U.S. Envirnmental Prtectin Agency, Office f Air Quality Planning and Standards, Research Triangle Park, NC, Octber 1992 Natinal Air Ouality and Emissins Trends Re.prt..l.22l. EPA-4541R , U.S. Envirnmental Prtectin Agency, Office f Air Quality Planning and Standards, Research Triangle Park, NC, Octber 1994 Further Infrmatin If yu have questins r cmments, please cntact David Mintz at mintz.david@eparnail.epa.gv r the. fllwing mailing address: USEPAlOAQPSlEMAD/AQTAG Mail Drp 14 Research Triangle Park, NC Summary We develped all the graphics described abve with SAS/GRAPH sftware in the MVS envirnment In additin, mst f the graphics cde is in macr frm and is available upn request. AckDwledgments We wuld like t thank Nicle Garrisn fr her patient prfreading and thughtful cmments. SAS and SAs/GRAPH are registered trademarks r trademarks f SAS Institute, Inc. in the USA and ther cuntries. II) indicates USA registratin. 349 SESUG '95 Prceedings