## Hourly Variation of PM Component Fractions at the JST and BHM SEARCH Sites during 2005

- 07

Ivar Tombach 9 November 2010



### Introduct ion

In the c omments I provided to the EPA on 30 August 2010, concerning the second draft Policy Assessment Document (PAD) for the PM 2.5 NAAQS, 1 Figure 3 was intended to display an example of the hourly variation of the fractions of PM 2.5 mass concentration attributable to ammonium sulfate and organic carbonaceous particulate matter at the Jefferson Street (JST) SEARCH site in Atlanta. However, due to errors in calculations, this figure and its textual description do not correctly represent these fractions an d so they should be ignored.
This memo provides several figures that correctly portray the hourly variation of the actual mass fractions for representative months in 2005

- 07 at both the JST and Birmingham (BHM) SEARCH sites. The mass fraction for any com ponent of PM 2.5 is just the mass concentration of that component divided by the mass concentration of PM 2.5 . The component (or species) mass concentrations of sulfate, nitrate, organic carbon and elemental carbon were determined hourly from continuous meas urements at the two sites. Hourly measurements of fine soil constituents were not available, so the average mass concentration determined by chemical analysis of a 24

- hr filter sample was assumed to apply for every hour of the day. Since fine soil is gener ally a relatively minor component of PM 2.5 at these sites, 2 the error due to not using an hourly concentration should be small except possibly during isolated dust episodes.

1 Comments Concerning the Visibility

- Related Portion of the Second External Review Draft of the Policy Assessment for the Review of the PM NAAQS , by Ivar Tombach, 30 August 2010.
2 See Table 1 in my 30 August report for average fractions at JST . For average fractions at BHM,

see Figure 1 in my supplementary comments, Testing of the Simplified Light Extinction Estimates at the SEARCH Site in Birmingham Alabama , submitted on 2 November 2010.2 For calculating the fraction s , the denominator was just the PM 2.5 mass concentra tion, taken to be the sum of the concentrations of ammonium sulfate and ammonium nitrate (as defined in the original IMPROVE algorithm), organic carbonaceous matter (OMC, determined by multiplying the measured organic carbon concentration by 1.7), elementa l carbon, and fine soil. The se calculated fractions are therefore the hourly analogs of the corresponding fractions derived from 24

- hr CSN filter samples in Step v of the Speciated PM 2.5 Mass

- calculated Light Extinction (SPMLE) method described in the seco nd draft PAD. Calculations were carried out for all daylight hours with RH < 90%.

The results of these calculations for ammonium sulfate, ammonium nitrate, and OMC ( labeled as OMC1.7 to indicate the non

- standard 1.7 multiplier) are shown in Figures 1 and 2 for three example months during the 20 0 5

- 07 period at JST and BHM, respectively. The three selected months

-- February, June, and October

-- are evenly spaced over the year to give an indication of seasonal behavior, and the completeness of data was a factor in the choice of year for each monthly example. Specifically, since JST data were relatively complete in all three years, the three months displayed in Figure 1 (JST) were selected from measurements during each of 2005 through 2007. For Figure 2 (BHM), the three months were selected from 2006 and 2007 samples because of gaps in the data for 2005. In any given month, the species fractions were similar in all three years. 3 In Figures 1 and 2, t hree

- year average fractions of ammonium sulfate, ammo nium nitrate, and OMC1.7 for each of the three months are indicated with dashed l i n e s

. 4
Figure 1 show s a much more chaotic situation at JST than the original, flawed Figure 3 in the 30 August comments suggested, especially for the winter month of February . In February, the ammonium sulfate fraction (blue dots) tends to increase , sometimes by a considerable amount (sometimes nearly a factor of 2) during many days , although it decreases or shows little change during other days. T he ammonium nitrate fraction (red dots) varies widely from day to day and often has behavior opposite to that of ammonium sulfate. The OMC1.7 fraction (orange dots) mostly tends to show an increase during the day in winter.
In June, the behavior at JST is more organized and systemati c. In June , which has more hours of daylight than in winter, the sulfate fraction is greater than in February and grows substantially during the course of the day . The relatively small ammonium nitrate fracti on in this warmer month decreases c onsistently d uring almost every day. The OMC1.7 fraction tends to grow on most days.
October at JST has many of the same characteristics as June, except that the ammonium nitrate fraction is of ten greater than it was in June, because cooler weather increases the fract ion of nitrate that is in particle form.

3 Fraction values and plots for all three years are provided in the Excel workbook that accompanies this memo.
4 Values of the monthly average fractions for JST were presented in Table 1 of the 30 August report. Those for BHM were presented in Table 1 of the 2 November supplementary comments.3 Figure 1. Daylight hour, RH < 90%, species fractions at JST during 3 months in 2005

- 07

. Dashed lines indicate three

- year average fractions for the month. JST February 2005 Species Fractions

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

2/1/05 2/6/05 2/11/05 2/16/05 2/21/05 2/26/05 AmSulf Fraction AmNitr Fraction OMC1.7 Fraction

JST June 2006 Species Fractions

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

6/1/06 6/6/06 6/11/06 6/16/06 6/21/06 6/26/06 AmSulf Fraction AmNitr Fraction OMC1.7 Fraction

JST October 2007 Species Fractions

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

10/1/07 10/6/07 10/11/07 10/16/07 10/21/07 10/26/07 10/31/07 AmSulf Fraction AmNitr Fraction OMC1.7 Fraction4 Figure 2. Daylight hour, RH < 90%, species fractions at BHM during 3 months in 2005

- 06

. Dashed lines indicate three

- year average fractions for the month. BHM February 2007 Species Fractions

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

2/1/07 2/6/07 2/11/07 2/16/07 2/21/07 2/26/07 AmSulf Fraction AmNitr Fraction OMC1.7 Fraction

BHM June 2006 Species Fractions

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

6/1/06 6/6/06 6/11/06 6/16/06 6/21/06 6/26/06 AmSulf Fraction AmNitr Fraction OMC1.7 Fraction

BHM October 2007 Species Fractions

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

10/1/07 10/6/07 10/11/07 10/16/07 10/21/07 10/26/07 10/31/07 AmSulf Fraction AmNitr Fraction OMC1.7 Fraction5 The JST data displayed In Figure 1 often show great variability from day to day, and sometimes there is substan tial variability during the course of a single day. In all periods, hourly fractions often differ greatly from their long

- term av erages. At BHM, in Figure 2 the hourly patterns during the day are similar to those at JST i n all three example months, except that the BHM patterns show more variability from day to day than those of JST.
These examples show that the hourly fractions of ammonium sulfate, ammonium nitrate, and OMC1.7 deviate considerably

– often by more than ±50% of the fraction value

– from a c onstant average during a month. Therefore, a monthly average fraction is likely to be a poor representation of hourly fractions for these species.
A Microsoft Excel workbook accompanying this report contains species fraction plots for full calendar years 2005, 2006, and 2007. It also contains the species fraction data use d to construct these plots, which can be used to prepare similar plots for any period of time.