Hourly Variation of PM Component Fractions at the 

JST and BHM SEARCH Sites during 2005-07

Ivar Tombach

9 November 2010

Introduction

In the comments I provided to the EPA on 30 August 2010, concerning the second draft 

Policy Assessment Document (PAD) for the PM2.5 NAAQS,1 Figure 3 was intended to 

display an example of the hourly variation of the fractions of PM2.5 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 and 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 component of PM2.5 is 

just the mass concentration of that component divided by the mass concentration of 

PM2.5. The component (or species) mass concentrations of sulfate, nitrate, organic carbon 

and elemental carbon were determined hourly from continuous measurements 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 generally a relatively minor 

component of PM2.5 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 fractions, the denominator was just the PM2.5 mass concentration, 

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), elemental 

carbon, and fine soil. These calculated fractions are therefore the hourly analogs of the 

corresponding fractions derived from 24-hr CSN filter samples in Step v of the Speciated 

PM2.5 Mass-calculated Light Extinction (SPMLE) method described in the second 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 2005-07 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, three-year average fractions of ammonium sulfate, ammonium nitrate, and 

OMC1.7 for each of the three months are indicated with dashed lines.

4

Figure 1 shows 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. The 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 systematic. 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 fraction 

in this warmer month decreases consistently during 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 often greater than it was in June, because cooler weather increases the 

fraction 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 Fraction
4

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 Fraction
5

The JST data displayed In Figure 1 often show great variability from day to day, and 

sometimes there is substantial variability during the course of a single day. In all periods, 

hourly fractions often differ greatly from their long-term averages.

At BHM, in Figure 2 the hourly patterns during the day are similar to those at JST in 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 constant 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 used to 

construct these plots, which can be used to prepare similar plots for any period of time.