February 2010 3-29 DRAFT - Do Not Quote or Cite 3.3.5 Exclusion of Hours with Relative Humi dity Greater than 90 Percent from PM 1 Light Extinction NAAQS Scenarios and Most Results 2

3 As advised by CASAC as part of its comments on the first pu blic review draft of this 4 assessment, EPA staff considered whether to st ructure the PM light ex tinction NAAQS scenarios 5

so that ambient data obtained dur ing daylight hours in which rela tive humidity was greater than 6 90 percent would play no role in the indicator/form of the NAAQS, i.e., so that those data would 7

not enter into the calculation of the design value. EPA staff obtained hourly meteorological 8 parameters from National Weather Service mon itoring sites near the 15 study sites (usually a 9 major airport), for 2005 through 2007, for all days in this period including days for which PM 10 observations to support estimates of PM light extinction are not available. Using these data, 11 EPA staff compared the occurren ce of liquid precipitation, hail, other frozen precipitation, fog, 12 mist, and smoke/haze during daylight hours with humi dity greater than 90 percent and during all 13 other daylight hours. 36 The first five of these conditions ar e generally considered natural causes 14 of reduced visibility. Table 3-6 presents this comparison. 37 The percentages of hours with each 15 of these five conditions individually are show n for the two sets of daylight hours. NWS 16

observations of these conditions are instantane ous, and are generally made about 50 minutes 17 after the hour. The relative humidity observations ar e made at the same time. It should be noted 18 that this analysis of the co-occurrence of hi gh relative humidity and these five conditions uses 19 data from NWS sites other than the AQS sites that provided the data used to estimate PM light 20 extinction. AQS sites could not be used for this analysis because they generally do not report 21

similar weather condition data. 22 The comparison for the 15 sites shows that in the set of hours with relative humidity 23 above 90 percent, the frequencies of liquid preci pitation (rain), other fr ozen precip itation (snow 24 and sleet), or fog ranged as high as 68 percen t, and were considerably higher for the same 25

condition than in the set of hours with lower relative humidity. The frequencies of hail and mist 26 were all less than 0.5 percent and thus too low fo r meaningful comparisons. Moreover, except in 27

36 The “smoke/haze” category is not an orig inal NWS reporting category. It is a combination of two original NWS weather categories: smoke and haze. The explanation of these categories in the NWS documentation does not allow EPA staff to be confident that these terms have distin ct and clear meanings that are uniformly applied across observation sites, so they have been combined in this pres entation. As best EPA staff can determine, the combined category reflects some mix of smoke from burning biomass, smoke from industrial processes, dust from wind storms, volcanic ash, and general urban haze. Also, the reported conditions may be at some distance from the observation site.

37 Compared to the version of this table in the second external review draft, this version correctly omits non-daylight hours which were inadvertently included in the earlier version, reports results for four study areas for which results were missing in the earlier version, and separates “mist” from “smoke/haze” to reflect that “mist” is a natural condition while “smoke/haze” is not always a natural condition.February 2010 3-30 DRAFT - Do Not Quote or Cite Tacoma, the frequency of rain or fog at the observation moments duri ng the hours w ith relative 1 humidity less than or equal to 90 percent was less than 8 percent. Also, a separate analysis (not 2 shown) indicated that rainy hours with lower relative humidity experience considerably less 3 accumulation than rainy hours with higher relative humidity. Based on this assessment, the 90% 4 relative humidity cutoff criteria is effective in th at on average less than 6 percent of the daylight 5

hours are removed from consid eration, yet those hours have on average about 10 times the 6 likelihood of rain, 6 times the likelihood of snow/sleet, and 34 times the likelihood of fog 7 compared to hours with 90% or less relative humidity. 8 Rain, snow/sleet, and fog cause a natural reduction in visibility, independent of PM 9

concentrations. To reduce th e likelihood that a design value for a secondary PM NAAQS could 10 be affected by measurements made under natural weather conditions that reduce visibility, for 11

this assessment EPA staff eliminated from the design value definition any contribution from PM 12 light extinction values that co me from any daylight hours with relative humidity above 90 13

percent. 38 Also, because PM light extinction during su ch hours is not as likely to be the primary 14 cause of adverse effects on the public, all figures and tables in the body of this document and in 15 Appendices that present PM light extinction values or statistic s exclude values for such hours 16 (unless explicitly stated to include them), so th at the patterns of PM light extinction during the 17 remaining daylight hours can be seen clearly. Figures and tables that present PM component 18 concentrations and relative humidity values are based on all daylight hours, however. 19 More information on this topic can be found in Appendix G, which reports by study area 20 the percentages of daylight hours that were excluded from design values, the distribution of the 21 excluded hours by time of day, and the percentage of days that had one or more daylight hours 22 eliminated. Appendix G also cont ains box plots which contrast th e distributions of daylight 1- 23 hour PM light extinction values (and maximum daily daylight 1-hour PM light extinction, see 24 section 3.3.6) before and af ter this elimination step. The til e plots in Figure 3-12 also present 25 additional detailed information on the specific ho urs that had relative humidity values above 90 26 percent, and on the PM light extinction valu es during those and ot her daylight hours. 27

38 Another consideration is that instruments used to measure light extinction could be adversely affected if allowed to operate without heating or other protective method (such as diffusion drying of incoming air) when relative humidity is very high. If protected, however, the measured light scattering would not reflect actual ambient conditions.February 2010 3-31 DRAFT - Do Not Quote or CiteTable 3-6Comparison of Meteorological Parameters for DaylightHours with Relative Humidity Greater than 901Percent and Other Daylight Hours, During 2005 -200723Daylight Hours with Relative Humidity <= 90%Daylight Hours with Relative Humidity > 90%Percentage of Hours with Weather or OtherConditionPercentage of Hours with Weather or Other ConditionStudy AreaNumberofHoursLiquidPrecip.HailOtherFrozenPrecip.FogMistSmoke/HazeAnyNumberofHoursLiquidPrecip.HailOtherFrozenPrecip.FogMistSmoke/HazeAnyTacoma10,326 12% 0% 0% 0% 0% 4% 14% 1,756 36% 0% 1% 10% 0% 43% 63%Fresno11,758 3% 0% 0% 1% 0% 15% 17% 342 25% 0% 1% 60% 0% 65% 93%LosAngeles11,419 2% 0% 0% 0% 0% 8% 9%713 25% 0% 0% 12% 0% 52% 73%Phoenix12,123 1% 0% 0% 0% 0% 0% 1%43 67% 0% 0% 30% 0% 40% 74%Salt LakeCity11,810 4% 0% 2% 1% 0% 4% 8% 304 28% 0% 40% 42% 0% 69% 85%Dallas11,827 4% 0% 0% 1% 0% 5% 8%223 68% 0% 2% 20% 0% 82% 91%Houston11,525 6% 0% 0% 1% 0% 6% 9%645 42% 0% 0% 25% 0% 64% 75%St. Louis11,590 5% 0% 1% 1% 0% 10% 14% 583 56% 0% 8% 48% 0% 82% 91%Birmingham11,590 5% 0% 0% 1% 0% 9% 11% 486 56% 0% 0% 41% 0% 79% 86%Atlanta11,337 5% 0% 0% 1% 0% 10% 13% 867 50% 0% 0% 45% 0% 81% 88%Detroit11,484 5% 0% 3% 1% 0% 9% 14% 676 51% 0% 16% 39% 0% 76% 92%Pittsburgh10,603 5% 0% 3% 1% 0% 9% 14% 1,261 46% 0% 9% 12% 0% 72% 85%Baltimore11,321 4% 0% 1% 2% 0% 12% 14% 858 53% 0% 5% 38% 0% 80% 90%Philadelphia11,125 4% 0% 1% 1% 0% 8% 11% 878 47% 0% 3% 33% 0% 64% 84%New York11,799 7% 0% 1% 1% 0% 10% 14% 397 66% 0% 8% 48% 0% 86% 96%Average11,442 5% 0% 1% 1% 0% 8% 11% 669 48% 0% 6% 34% 0%69% 84%