February 2010 3-29 DRAFT - Do Not Quote or Cite 

1 3.3.5 Exclusion of Hours with Relative Humidity Greater than 90 Percent from PM 

2 Light Extinction NAAQS Scenarios and Most Results 

3 

4 As advised by CASAC as part of its comments on the first public review draft of this 

5 assessment, EPA staff considered whether to structure the PM light extinction NAAQS scenarios 

6 so that ambient data obtained during daylight hours in which relative humidity was greater than 

7 90 percent would play no role in the indicator/form of the NAAQS, i.e., so that those data would 

8 not enter into the calculation of the design value. EPA staff obtained hourly meteorological 

9 parameters from National Weather Service monitoring sites near the 15 study sites (usually a 

10 major airport), for 2005 through 2007, for all days in this period including days for which PM 

11 observations to support estimates of PM light extinction are not available. Using these data, 

12 EPA staff compared the occurrence of liquid precipitation, hail, other frozen precipitation, fog, 

13 mist, and smoke/haze during daylight hours with humidity greater than 90 percent and during all 

other daylight hours. 36 14 The first five of these conditions are generally considered natural causes 

of reduced visibility. Table 3-6 presents this comparison.37 15 The percentages of hours with each 

16 of these five conditions individually are shown for the two sets of daylight hours. NWS 

17 observations of these conditions are instantaneous, and are generally made about 50 minutes 

18 after the hour. The relative humidity observations are made at the same time. It should be noted 

19 that this analysis of the co-occurrence of high relative humidity and these five conditions uses 

20 data from NWS sites other than the AQS sites that provided the data used to estimate PM light 

21 extinction. AQS sites could not be used for this analysis because they generally do not report 

22 similar weather condition data. 

23 The comparison for the 15 sites shows that in the set of hours with relative humidity 

24 above 90 percent, the frequencies of liquid precipitation (rain), other frozen precipitation (snow 

25 and sleet), or fog ranged as high as 68 percent, and were considerably higher for the same 

26 condition than in the set of hours with lower relative humidity. The frequencies of hail and mist 

27 were all less than 0.5 percent and thus too low for meaningful comparisons. Moreover, except in 

 36 The “smoke/haze” category is not an original 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 distinct and clear meanings that are uniformly applied across 

observation sites, so they have been combined in this presentation. 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 

1 Tacoma, the frequency of rain or fog at the observation moments during the hours with relative 

2 humidity less than or equal to 90 percent was less than 8 percent. Also, a separate analysis (not 

3 shown) indicated that rainy hours with lower relative humidity experience considerably less 

4 accumulation than rainy hours with higher relative humidity. Based on this assessment, the 90% 

5 relative humidity cutoff criteria is effective in that on average less than 6 percent of the daylight 

6 hours are removed from consideration, yet those hours have on average about 10 times the 

7 likelihood of rain, 6 times the likelihood of snow/sleet, and 34 times the likelihood of fog 

8 compared to hours with 90% or less relative humidity. 

9 Rain, snow/sleet, and fog cause a natural reduction in visibility, independent of PM 

10 concentrations. To reduce the likelihood that a design value for a secondary PM NAAQS could 

11 be affected by measurements made under natural weather conditions that reduce visibility, for 

12 this assessment EPA staff eliminated from the design value definition any contribution from PM 

13 light extinction values that come from any daylight hours with relative humidity above 90 

percent.38 14 Also, because PM light extinction during such hours is not as likely to be the primary 

15 cause of adverse effects on the public, all figures and tables in the body of this document and in 

16 Appendices that present PM light extinction values or statistics exclude values for such hours 

17 (unless explicitly stated to include them), so that the patterns of PM light extinction during the 

18 remaining daylight hours can be seen clearly. Figures and tables that present PM component 

19 concentrations and relative humidity values are based on all daylight hours, however. 

20 More information on this topic can be found in Appendix G, which reports by study area 

21 the percentages of daylight hours that were excluded from design values, the distribution of the 

22 excluded hours by time of day, and the percentage of days that had one or more daylight hours 

23 eliminated. Appendix G also contains box plots which contrast the distributions of daylight 1-

24 hour PM light extinction values (and maximum daily daylight 1-hour PM light extinction, see 

25 section 3.3.6) before and after this elimination step. The tile plots in Figure 3-12 also present 

26 additional detailed information on the specific hours that had relative humidity values above 90 

27 percent, and on the PM light extinction values during those and other daylight hours. 

 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 Cite 

1 Table 3-6 Comparison of Meteorological Parameters for Daylight Hours with Relative Humidity G2 Percent and Other Daylight Hours, During 2005 -2007 3 

Daylight Hours with Relative Humidity <= 90% Daylight Hours with Relative H

Percentage of Hours with Weather or Other 

Condition Percentage of Hours with We

Study Area Number of 

Hours Liquid 

Precip. 

Hail 

Other 

Frozen Precip. 

Fog Mist

Smoke/

Haze Any 

Number 

of 

Hours Liquid 

Precip. 

Hail

Other 

Frozen Precip. 

Fog 

Tacoma 10,326 12% 0% 0% 0% 0% 4% 14% 1,756 36% 0% 1% 10% Fresno 11,758 3% 0% 0% 1% 0% 15% 17% 342 25% 0% 1% 60% Los Angeles 11,419 2% 0% 0% 0% 0% 8% 9% 713 25% 0% 0% 12% Phoenix 12,123 1% 0% 0% 0% 0% 0% 1% 43 67% 0% 0% 30% Salt Lake City 11,810 4% 0% 2% 1% 0% 4% 8% 304 28% 0% 40% 42% Dallas 11,827 4% 0% 0% 1% 0% 5% 8% 223 68% 0% 2% 20% Houston 11,525 6% 0% 0% 1% 0% 6% 9% 645 42% 0% 0% 25% St. Louis 11,590 5% 0% 1% 1% 0% 10% 14% 583 56% 0% 8% 48% Birmingham 11,590 5% 0% 0% 1% 0% 9% 11% 486 56% 0% 0% 41% Atlanta 11,337 5% 0% 0% 1% 0% 10% 13% 867 50% 0% 0% 45% Detroit 11,484 5% 0% 3% 1% 0% 9% 14% 676 51% 0% 16% 39% Pittsburgh 10,603 5% 0% 3% 1% 0% 9% 14% 1,261 46% 0% 9% 12% Baltimore 11,321 4% 0% 1% 2% 0% 12% 14% 858 53% 0% 5% 38% Philadelphia 11,125 4% 0% 1% 1% 0% 8% 11% 878 47% 0% 3% 33% New York 11,799 7% 0% 1% 1% 0% 10% 14% 397 66% 0% 8% 48% Average 11,442 5% 0% 1% 1% 0% 8% 11% 669 48% 0% 6% 34%