Atmos. Chem. Phys., 8, 2729–2739, 2008

www.atmos-chem-phys.net/8/2729/2008/

© Author(s) 2008. This work is distributed under

the Creative Commons Attribution 3.0 License.

Atmospheric

Chemistry

and Physics

Weekly patterns of aerosol in the United States

D. M. Murphy1, S. L. Capps1,2,*, J. S. Daniel1, G. J. Frost1,3, and W. H. White4

1Earth System Research Laboratory, Chemical Sciences Division, National Oceanic and Atmospheric Administration

Boulder, CO 80305, USA

2Vanderbilt University, Nashville, TN 37240, USA

3Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, CO 80309, USA

4Crocker Nuclear Laboratory, University of California, Davis Davis, CA 95616, USA

*now at: School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA

Received: 4 December 2007 – Published in Atmos. Chem. Phys. Discuss.: 10 January 2008

Revised: 13 March 2008 – Accepted: 3 April 2008 – Published: 22 May 2008

Abstract. Data from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network of aerosol

samplers and NOAA monitoring sites are examined for

weekly cycles. At remote and rural sites, fine particle elemental carbon, crustal elements, and coarse particle mass

had pronounced (up to 20%) weekly cycles with minima on

Sunday or Monday. Fine particle organic carbon and mass

had smaller amplitude cycles, also with Sunday or Monday

minima. There was no statistically significant weekly cycle in fine particle sulfate despite a 5 to 15% weekly cycle in power plant SO2 emissions. Although results for nitrate may be more susceptible to sampling artifacts, nitrate

also showed a pronounced weekly cycle with an amplitude

similar to elemental carbon. The only species found with a

weekend maximum was Pb, probably from general aviation

on weekends. Aerosol optical properties at NOAA monitoring sites were consistent with the IMPROVE chemical data,

with significant weekly cycles in aerosol light absorption but

not light scattering. These results support a large role of

diesel emissions in elemental carbon aerosol over the entire

United States and suggest that a large fraction of the airborne

soil dust is anthropogenic. They also suggest that studies of

weekly cycles in temperature, cloudiness, precipitation, or

other meteorological variables should look for causes more

in light-absorbing particles and possible ice nucleation by

dust rather than sulfate or total aerosol. There are also implications for personal exposure and epidemiological studies

of aerosol health effects.

Correspondence to: D. Murphy

(daniel.m.murphy@noaa.gov)

1 Introduction

Observations of the weekly cycle are a powerful tool for distinguishing anthropogenic from natural causes. Only anthropogenic activities are likely to influence concentrations, temperatures, or other atmospheric variables on a seven-day cycle. Weekly trends have been found for diurnal temperature

range over the United States (Forster and Solomon, 2003)

and China (Gong et al., 2006) as well as cloudiness and precipitation in Germany (Baumer and Vogel, 2007). White  ̈

et al. (1990, 1991) studied transport of urban pollutants using weekly cycles. Blanchard and Tanenbaum (2003) found

significantly less NO2 on weekends in Southern California.

Beirle et al. (2003) found weekly cycles in column NO2 measured from satellite, with different amplitudes and phases on

various continents that could be related to various societies.

A number of studies have examined weekly cycles of ozone

concentrations and photochemistry (e.g. Elkus and Wilson,

1977; Marr and Harley, 2002; Murphy et al., 2007).

Several studies have specifically studied weekly cycles of

aerosol. Almeida et al. (2006) found that chemical elements

such as Ca, Si, and Fe associated with soil were significantly

higher on weekdays at a site in a suburb of Lisbon. Motallebi

et al. (2003) found weekly cycles at selected urban sites in

California. Lough et al. (2006) and Harley et al. (2005) found

significant weekly trends in aerosol components in the Los

Angeles area. In particular, both found much lower ratios

of elemental carbon to organic carbon on weekends. Jin et

al. (2005) used satellite data to discern a midweek maximum

in aerosol optical thickness near New York City. Baumer et  ̈

al. (2008) examined aerosol optical thickness over Europe.

Delene and Ogren (2002) found slightly higher single scattering albedos on Sundays and Mondays at Bondville, Illinois

Published by Copernicus Publications on behalf of the European Geosciences Union.
2730 D. M. Murphy et al.: Weekly patterns of aerosol





	


	







	 







	



	











	






	



	


	













	









Fig. 1. Clusters generated from the IMPROVE site locations. Each

cluster is shown by a circle with lines to the positions of the individual sites in that cluster. Next to each cluster is the number of elemental carbon samples for the analysis in this paper; other species

have similar numbers of samples. Also shown as squares are the

Bondville and Southern Great Plains NOAA monitoring sites discussed in this paper.

and Southern Great Plains, Oklahoma than on other days of

the week.

With the exception of Delene and Ogren (2002), studies on

weekly aerosol properties have concentrated on urban sites.

In this paper we use data from the IMPROVE network and

NOAA aerosol monitoring sites to examine weekly cycles in

aerosol across the United States. An important feature of the

IMPROVE aerosol data set is the chemical analysis, allowing much better weekly analysis than a simple mass measurement.

2 Data

2.1 IMPROVE network

IMPROVE data in this paper are from 1 September 2000

through 31 August 2006. This is an integral number of weeks

and years, which helps keep annual cycles from being aliased

into weekly cycles. We use data after summer 2000 because

the network shifted then from a Wednesday–Saturday sampling schedule to a once every third day schedule that rotates

through the days of the week, an important feature for this

work. There were also few sites in the eastern United States

before 2000. Except as noted for coarse mass, all data in the

paper are for fine mode aerosol smaller than 2.5μm.

The IMPROVE monitoring program is described by Malm

et al. (2004). Routine monitoring began in 1988 with the

objectives of establishing current background haze levels,

identifying the chemical composition and emissions sources

of regional aerosols, and documenting long-term trends in

aerosol concentrations. Most of the samplers are in locations

representative of the 156 National Parks and Wilderness Areas where the Clean Air Act provides special protections for

visibility. The network now includes about 170 rural or remote sites in the United States.

Data, standard operating procedures and site descriptions

are all available in detail at http://vista.cira.colostate.edu/

improve/. Every third day (all sites sample on the same days),

a set of three PM2.5 samples and one PM10 sample is collected by four independent filter sampling trains. The PM10

filter is analyzed only for mass. The PM2.5 modules sample

behind AIHL cyclones (John and Reischl, 1980) at nominal

flow rates of 22.8 l min−1. One module collects PM2.5 on a

37 mm Nylon filter behind an annular denuder coated with

Na2CO3 and glycerol. This filter is analyzed by ion chromatography for the anions chloride, nitrate, and sulfate. A

second module collects undenuded PM2.5 on a 25 mm Teflon

filter for weighing and energy-dispersive x-ray fluorescence

analysis (XRF). The third module collects undenuded PM2.5

on a 25 mm quartz-fiber filter for thermal fractionation of organic and elemental carbon with an optical correction for pyrolysis.

The methods used for elemental and carbon analysis have

undergone some minor changes since August 2000. Before

December 2001, the elements lighter than iron were determined by proton-induced x-ray emission, and since January

2005 the XRF analysis for elements lighter than nickel has

been done under vacuum. Carbon analyses since January

2005 have been conducted with new instrumentation according to a more precise protocol (Chow et al., 2007). These

one-time method transitions should have no effect on multiyear weekly cycles.

There is a weekly pattern of sampler operations: the local operator checks the system’s operation each Tuesday,

and replaces the cartridge of exposed filter cassettes in each

module with one of unexposed filter cassettes. This schedule imposes extra handling for Tuesday samples, which are

briefly interrupted while the operator transfers the active filter cassette from the old cartridge to the new one. It also imposes sample-day-dependent delays between exposure and

unloading, when losses might potentially occur within the

sampler. (Any potential for contamination would presumably exist both before and after exposure, and so be independent of sample day.) Such hypothetical effects have not

been detected in previous testing. With the exception of winter nitrate, they would not explain the calendar-week cycles

presented here, whose extremes usually occur away from the

Tuesday sampler maintenance.

The sites in the IMPROVE database were combined into

25 groups using hierarchical cluster analysis (Fig. 1). Fifteen urban sites were eliminated (ATLA1, BALT1, CHIC1,

DETR1, FRES1, HOUS1, NEYO1, OLTO1, OMAH1,

PHOE1, PITT1, PUSO1, RUBI1, SAGU1, and WASH1), as

were some sites that were closed before 2000. Then great

circle distances were calculated between all site pairs and

the closest pair of sites or clusters combined with each other.

The process was repeated until the closest clusters were more

Atmos. Chem. Phys., 8, 2729–2739, 2008 www.atmos-chem-phys.net/8/2729/2008/
D. M. Murphy et al.: Weekly patterns of aerosol 2731

than 390 km apart. This threshold was chosen to keep separate clusters in California. Single sites that had not ended up

in a cluster were paired together (in Louisiana and the Carolinas) or combined with the nearest cluster (Everglades and

Big Bend). Of sites outside of the continental United States,

only Hawaii was kept because there were fewer samples in

Alaska or the Virgin Islands.

Use of clusters is essential to this work. Weekly cycles at

single sites are often not statistically significant when samples are taken every third day. Several sampling sites must be

averaged to distinguish weekly cycles from random variability. Yet one cannot average the entire United States without

first looking at regional patterns. It does not make sense to

average together mineral dust concentrations from Arizona

and Maine or sulfate concentrations in California and Ohio.

The exact technique used to generate the clusters is less important than having some sort of regional averaging.

To start with a consistent set of data, all samples were required to have valid values for mass, elemental carbon, and

sulfate. About 0.1% of the data were eliminated as high

outliers. These are probably valid data influenced by local

events such as fires or nearby dust generation. For example, fine mode mass was required to be less than 50μg m3

.

The results are not sensitive to these cutoffs but they do help

keep a single sample from skewing one day of the week for

an entire cluster. This left 92744 samples in the IMPROVE

data during the time period after eliminating the urban sites.

The actual number of samples for a given species may be

slightly smaller, since not all analyses were performed on

every sample. Data below detection limit were retained as

zeros but species with many values below detection limit are

not discussed in the paper. Notably, our mineral dust analysis does not consider aluminum because of a poor detection

limit compared to silicon and iron.

Most of the subsequent results are presented in terms of the

average weekly cycle in a cluster. For each day of the week,

this is calculated as the average of all observations on that

day of the week during the September 2000 to August 2006

period, at all sites in the cluster. For visual presentation, and

for network averages, each cluster’s cycle is normalized to

that cluster’s overall September 2000 to August 2006 mean.

There is an option whether or not to normalize data from

individual sites before calculating the weekly cycles for each

cluster. Normalizing prevents a few sites with high absolute

concentrations from dominating the averages, but has the disadvantage of amplifying noise from sites with low concentrations that may be near detection limits. Normalizing can

also overemphasize sites with incomplete records. We have

chosen to directly average all data within a cluster. We have

done the calculations both ways and the calculated weekly

cycles are not sensitive to whether or not the sites are normalized before averaging. In part, this is because the exclusion

of urban sites leaves sites within each individual cluster that

usually have similar absolute concentrations. For a particular species, the standard deviations of the concentrations of

the sites in individual clusters are usually about half as large

as the standard deviation of the clusters from the continental

average.

Judging the statistical significance of the weekly cycles is

difficult. The most important problem is the number of independent measurements. The standard errors in subsequent

figures are calculated as usual by dividing the standard deviation of individual measurements by the square root of the

number of measurements. The samples are not completely

independent of each other, however, as this calculation implicitly assumes. Concentration variations are driven largely

by meteorological variables, which are correlated in both

time and space. The IMPROVE protocol of samples every

third day should reduce but not completely eliminate the autocorrelation between successive samples (Anderson et al.,

2003). Similarly, multiple sites within a cluster will often lie

in the same synoptic airmass.

Dilution can cause statistical significance to be underestimated. Consider the simple case of two sites downwind of

a consistent weekly source with rapid transport. Both would

show the same weekly cycles, but the site further downwind

would have lower absolute values. If the sites with different

absolute concentrations are averaged together, the standard

deviations for each day would be large even though the sites

were sampling the same source.

To assess the statistical significance, we compared the amplitude of the observed weekly cycle with the amplitude of

the weekly cycle for synthetic time series generated from

randomized versions of the same data. To preserve shortterm correlations, data within each month were kept as a

block and each month was shifted by a random number of

days of the week. The resulting statistics for 500 randomized samples of the data are shown in Table 1. The weekly

cycle inferred from random data varies by species because

of different variance and skew in the concentrations of each

species.

Since we are considering the absolute value of the weekly

cycle in the randomized data, the distributions are not centered on zero. This is why the 90th and 99th percentiles

may appear closer than expected. Also, these percentiles

of the absolute values of weekly cycles represent the amplitude required for confidence that a cycle of any phase is real.

They are not the same as the 90 percent confidence limits on

the amplitude of the derived cycles (these are not computed

here). The reason the two quantities are different is that random noise will always generate a positive weekly cycle of

some phase whereas it can generate either a positive or negative error in a cycle whose phase is already determined.

We also checked for cycles of 5, 8, and 10 days using

the same software as the 7-day cycle (6 and 9 day cycles do

not work well with samples taken every third day). For the

species with cycles that are statistically significant compared

to the randomized data, the 7-day cycle was much larger than

the other periods. For example, by one measure (maximum

two consecutive days minus minimum two consecutive days)

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2732 D. M. Murphy et al.: Weekly patterns of aerosol

Table 1. Statistics of weekly cycles for selected species averaged over the continental United States.

Species Overall continental Weekly range Weekly two-day Randomized data weekly two-day range

US average (%) range∗(%) (%)

(μg m−3) 90th percentile 99th percentile

Silicon 0.149 22.4 15.3 9.1 11.4

Iron 0.040 26.5 18.1 10.2 14.2

Calcium 0.046 24.3 17.4 9.6 12.8

Coarse mass 4.91 23.1 18.4 9.8 12.8

Elemental carbon 0.230 14.2 11.4 6.3 8.1

Organic carbon 1.159 7.9 5.6 4.9 6.2

Fine mass – EC 5.90 7.9 6.2 5.1 6.4

Fine mass – (EC+soil) 5.17 5.5 4.8 5.4 7.2

Sulfate 1.835 4.5 2.3 5.4 7.2

Nitrate 0.592 13.4 11.0 8.0 9.7

Pb 0.0012 9.1 6.7 5.0 6.6

Pb/(Pb + Zn) 0.26 (ratio) 8.4 7.0 3.6 5.2

Zn 0.0041 8.3 6.6 4.3 5.3

V 0.0008 4.5 3.2 6.6 8.0

Se 0.0005 3.3 1.8 4.4 5.9

∗ maximum two consecutive days – minimum two consecutive days.

Bold: exceeds 99th percentile of randomized data; italics: less than 90th percentile.

the weekly cycle for silicon was 15.3%. The 5, 8, and 10 day

cycles were 2.6%, 2.7%, and 5.6% respectively. For sulfate,

which was not statistically significant, the weekly cycle was

2.3% and the 8-day cycle was 2.5%. Finally, all of the weekly

patterns shown here are robust with respect to changing the

time period to various subsets of 2000 to 2006.

2.2 NOAA aerosol optical properties

The National Oceanic and Atmospheric Administration

(NOAA) continuously measures aerosol optical properties at

a number of baseline and regional monitoring stations (Delene and Ogren, 2002). Bondville, Illinois, and Southern

Great Plains, Oklahoma, are relevant to this work. Their locations are shown on Fig. 1. Daily averaged data were used

to construct the weekly cycles.

Aerosol light absorption was measured at 565 nm using

Particle Soot/Absorption Photometers (PSAP, Radiance Research). The data were corrected for scattering using Bond

et al. (1999). Aerosol light scattering was measured using TSI 3563 nephelometers. Only the 550 nm data are

shown here. The sample stream was heated, if necessary,

to maintain the relative humidity at about 50%. Further

descriptions are available at http://www.esrl.noaa.gov/gmd/

aero/instrumentation/instrum.html.

The PSAP filters were changed less frequently on weekends at the Southern Great Plains sit and therefore had lower

average total filter transmission on Sundays and Mondays (J.

Ogren, personal communication, 2007). The Bondville site

was less affected. More recent work on PSAP corrections

by Virkkula et al. (2005) suggests that the Bond et al. corrections have an accurate dependence on filter transmission

except for highly absorbing aerosols (albedo less than about

0.7) that are not likely to be important for these sites.

2.3 Emissions inventories

Weekly cycles in SO2 and NOx emissions from power plants

were computed for comparison with observed weekly cycles

in aerosols. US power plant emission data were obtained

from the Environmental Protection Agency’s Clean Air Markets Division, http://camddataandmaps.epa.gov/gdm/index.

cfm?fuseaction=emissions.wizard. 1999 and 2005 hourly

emissions recorded by continuous emission monitors at each

power plant were aggregated over large geographic regions

containing 50 to 300 individual plants. The hourly emissions

for each region were averaged for each day of week over the

entire year, and the day-of-week averages were normalized

using the average emissions in each region. Weekly cycles in

1999 were very similar to the 2005 data shown in Fig. 6.

3 Results

3.1 Crustal elements

The largest percentage weekly cycles in aerosol concentrations were observed for the crustal elements Si, Ca, Ti, and

Fe (Fig. 2). All had weekly cycles with 20% or greater range.

Minima were on Sunday for most locations and maxima were

usually on Thursday. Aluminum also had similar patterns

but more values below the detection limit. The weekly cyAtmos. Chem. Phys., 8, 2729–2739, 2008 www.atmos-chem-phys.net/8/2729/2008/
D. M. Murphy et al.: Weekly patterns of aerosol 2733



  	







  











	

 


      









  	 









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Fig. 2. Weekly cycles of crustal elements in the fine mode (below

2.5μm). The top panel shows weekly cycles for silicon at each cluster of IMPROVE sites. For each cluster of sites, Saturday through

Friday averages are shown in red or orange as departures from the

weekly mean, with standard errors. Red curves indicate statistical

significance is more likely than for orange. Weekly cycles in medians are shown in blue. Rectangles are proportional in size to the

annual mean for each cluster. The lower left panel shows the normalized weekly cycles for silicon for each cluster in the continental

United States. The heavy line is the average over all clusters. The

lower right panel shows the average weekly cycle for the continental

United States for various crustal elements and coarse mass, defined

as the difference between fine and total (PM10) mass.

cles were consistent throughout the country except for a few

locations where the absolute values were small. One of the

largest weekly cycles was observed for the cluster in Arizona

that had the largest annual averages for these elements of any

cluster in the continental United States. That cluster had low

values on both Saturday and Sunday, perhaps because the

soil particles were generated locally so the atmospheric concentrations could respond more quickly to low emissions on

the weekend. A curious feature for many locations is that

the weekly maximum was on Thursday rather than Friday,

as might be expected if weekday emissions built up in the

atmosphere throughout the week. It is not clear to what extent this is a statistical fluctuation or a consequence of lower

emissions on Friday caused by four-day work weeks.

On Fig. 2 and subsequent figures, the average for the continental United States was generated from the normalized averages of the clusters. This means that individual sites in areas

with a dense network of sites contribute less to the average

than sites where the network is sparse. Averaging the normalized weekly cycles means that regions with low absolute



  	






  	 



  



  	








 


  

Fig. 3. Weekly cycles for fine mode elemental carbon and the balance of fine mass. As in Fig. 2, red lines are average percentage

daily departures from the mean for that cluster and blue lines are

departures of the medians. Rectangles are proportional to the annual mean for each cluster of sites.

concentrations contribute as much to the continental average

as regions with high concentrations. As a guide to the eye,

normalized weekly cycles for individual clusters are plotted

in orange or red if the maximum of two consecutive days is

less or more than four standard errors from the minimum of

two consecutive days.

3.2 Elemental carbon, organic carbon, and aerosol mass

Weekly cycles for elemental carbon, organic carbon, and

several measures of aerosol mass are shown in Figs. 3 and

4. Elemental carbon is subtracted from fine mode mass because for climate forcing calculations it is sensible to separate absorbing and non-absorbing aerosols. This also facilitates comparison with the optical measurements discussed

later. The weekly cycles for fine mass are not statistically

significant for individual clusters. Although Fig. 3 shows one

highly significant cluster, testing with the randomized series

shows that by chance one out of 25 clusters can exceed the

four standard error threshold for the color code on Fig. 3.

However, the continental US average weekly cycle for fine

mass is statistically significant (Table 1) at about the 95%

level.

www.atmos-chem-phys.net/8/2729/2008/ Atmos. Chem. Phys., 8, 2729–2739, 2008
2734 D. M. Murphy et al.: Weekly patterns of aerosol











	
 
	



	 
 
 
  
 


	



 





 



 







		
 












	
 
	



	 
 
 
  
 


	



 







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Fig. 4. A comparison of the weekly cycles of elemental and organic carbon as well as various measures of the fine and coarse mode mass.

Each line is the weekly cycle for one cluster of sites, colored by the annual mean. The heavy black lines are the averages for all clusters in

the continental United States.

Elemental carbon has a much larger weekly cycle than organic carbon or the fine mode mass. Figure 4d compares

some of the weekly cycles. In view of the large weekly cycle for crustal elements, a curve is shown for fine mass after removing the contribution from soil calculated using an

equation modified slightly from Malm and Hand (2007):

Soil=(2.42+1.5)·Fe+1.63·Ca+2.2·Al+2.49·Si+1.94·Ti (1)

The factors account for oxides, and in the case of Fe,

crustal elements such as K that cannot easily be separated

from other sources. The additional factor of 1.5∗Fe is a very

rough estimate of a contribution from total carbon in the soil

estimated from examining some strong dust events. We infer

from this curve that the weekly cycle for fine mass is only

partially due to the strong cycle in soil dust. It is also due to

organic carbon and nitrate.

Figure 5 shows histograms of elemental carbon concentrations for one cluster. The lower averages on Sunday and

Monday were caused by a shift of the entire distribution

rather than changes in a few extreme values. This is the case

for other clusters as well. The histograms also show the form

of the distributions: on a log scale such as Fig. 5 the distributions are skewed left. On a linear scale they are skewed

to the right, so the distributions are between log-normal and

normal.

The IMPROVE network uses a thermo-optical method

to separate organic and elemental carbon. For all thermooptical methods, there are questions about the absolute accuracy of how organic carbon that chars during heating is separated from elemental carbon (Novakov et al., 2005; Chow

et al., 2007). Despite these questions, weekly cycles depend

more on consistency of the carbon analysis than the absolute

cut point between elemental and organic carbon. In addition, the different weekly cycles for elemental and organic

carbon show that the operational definition of elemental and

organic carbon in the IMPROVE protocol does represent a

useful separation for species with differing sources and behavior in the atmosphere.

Atmos. Chem. Phys., 8, 2729–2739, 2008 www.atmos-chem-phys.net/8/2729/2008/
D. M. Murphy et al.: Weekly patterns of aerosol 2735















 	 






  



  







   



	


	 
 




Fig. 5. Histograms of elemental carbon concentrations for one cluster of IMPROVE sites centered around Pennsylvania and West Virginia. The histograms compare Sunday and Monday with Wednesday and Thursday.

3.3 Sulfate and nitrate

Sulfate has a very different pattern than elemental carbon.

There is no statistically significant weekly cycle in sulfate,

despite a weekly cycle in SO2 emissions (Fig. 6). This

is probably caused both by the emissions and the chemistry of SO2. Emissions of SO2 from power plants decline by 5 to 15% on weekends compared to weekdays, a

much smaller change than the approximately 75% reduction

in diesel emissions on weekends in California (Chinkin et

al., 2003; Motallebi et al., 2003). Electric power generation accounted for about 70% of US SO2 emissions during

the period 2000–2006 (National Emissions Inventory (NEI)

Air Pollutant Emissions Trends Data, http://www.epa.gov/

ttn/chief/trends/index.html). Unlike crustal material or elemental carbon, the SO2 emissions must be chemically transformed to produce sulfate. This will delay and spread out

any weekly pattern. Also, oxidation of SO2 to sulfate takes

place by both gas phase reactions and liquid phase chemistry

in clouds. These pathways proceed at different rates, further

spreading out any weekly pattern. The variable presence of

clouds may add to the daily scatter in sulfate and make cycles

harder to discern.

We compared the weekly cycles for winter and summer

sulfate concentrations. Sulfate concentrations are generally

higher in summer than in winter, when photochemistry is

slower and liquid phase clouds are less common in the northern United States. Distinguishing seasons did not reveal any

statistically significant cycles.











	
 


 	

	 
 
 
  
 

	






		  


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Fig. 6. Weekly cycles of sulfate in the IMPROVE network and

emissions of SO2 from power plants. Symbols and lines are the

same as Fig. 2.

There are two main types of aerosol nitrate: ammonium

nitrate and nitrate chemically bound to crustal cations such

as calcium (Murphy et al., 2006). With few exceptions, ammonium nitrate is only stable when the sulfate has been completely neutralized. Otherwise it dissociates to gas phase ammonia and nitric acid (Bassett and Seinfeld, 1984). Because

of the widespread oxidation of SO2 to sulfuric acid, ammonium nitrate is more often limited by ammonium than the

availability of gas phase nitric acid. The absolute amounts

of nitrate vary greatly, with much larger concentrations in

southern California and the Midwest than the mountain west.

California and the Midwest are locations with substantial

sources of ammonia and, especially for California, small

emissions of SO2. The reaction of nitric acid with calcium in

mineral dust is much less sensitive to sulfate.

www.atmos-chem-phys.net/8/2729/2008/ Atmos. Chem. Phys., 8, 2729–2739, 2008
2736 D. M. Murphy et al.: Weekly patterns of aerosol



 	
 






  











	 



      

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Fig. 7. Weekly cycles of nitrate in the IMPROVE network. The

lower right panel shows the average for the continental US separated

by season. Symbols and lines are the same as Fig. 2.

In contrast to sulfate, nitrate shows a weekly cycle (Fig. 7).

Some of the cycle in nitrate is driven by the strong weekly

cycle in crustal elements, but examination of the absolute

values shows that calcium nitrate and similar species cannot account for all of the observed cycle in nitrate. The

weekly nitrate minimum is Sunday through Tuesday, later

than other species. The presence of a weekly cycle in nitrate

is a little surprising because of the possible limitations by ammonia. For example, emissions of ammonia from farm animals should be fairly constant. NOx emissions from power

plants have a similar cycle to that shown for sulfate in Fig. 6.

NOx emissions from vehicles have a significant component

from heavy vehicles, so vehicular emissions will have a much

stronger weekly cycle than NOx from power plants (Chinkin

et al., 2003). The fraction of US NOx emitted by electric power generation decreased from 24% in 2000 to 20%

in 2006. During the same period, on-road mobile sources

produced about 37% while the contribution of off-highway

vehicles has increased from 18% to 22% (National Emissions Inventory (NEI) Air Pollutant Emissions Trends Data,

http://www.epa.gov/ttn/chief/trends/index.html). A weekly

cycle for nitrate with a Sunday–Monday minimum has also

been found in some urban areas (Millstein et al., 2007).

Although the data are noisy when separated by season,

there does appear to be a shift in the timing from a Monday minimum in summer to a Tuesday minimum in winter

(Fig. 7). There are two possible explanations for this shift.

First, conversion of NOx to HNO3, followed by rapid equilibration with aerosol nitrate, will be much slower in winter



  	







  











	
 




      

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Fig. 8. Weekly cycles for Pb and the ratio of Pb to Pb+Zn. As for

other figures, the red lines are the departures of the daily averages

from each cluster mean, blue departures for medians, and the boxes

are proportional to the annual mean for each cluster.

than in summer. That could shift the phase of the weekly cycle. Second, there could be an unrecognized artifact associated with the extra handling Tuesday samples receive from

the site operator. Low temperatures and low sulfate concentrations favor the stability of ammonium nitrate particles,

so nitrate concentrations in most of the country peak during

the winter. The Nylon filters used for IMPROVE’s nitrate

measurement retain any nitric acid gas released after collection by thermal dissociation of ammonium nitrate. Warming

of winter-Tuesday samples during the brief handling period,

when any volatilized nitric acid would not be drawn back

into the filter, must for the moment be considered as a possible explanation for the nitrate minimum on winter Tuesdays.

3.4 Lead and other metals

Lead is the only species studied with a weekend maximum

(Fig. 8). The largest single source of airborne Pb, accounting for about 25% of US emissions, is leaded gasoline in

small piston aircraft (Murphy et al., 2007). Many of these

small planes are recreational and emissions probably peak on

weekends. The next largest sources of airborne Pb are coalfired power plants and heavy steel industry. Those emissions

probably go down slightly on weekends as shown earlier for

SO2.

Zinc and Pb are highly correlated in atmospheric samples, probably because of similar electric utility and industrial sources and similar condensation in combustion exhaust

Atmos. Chem. Phys., 8, 2729–2739, 2008 www.atmos-chem-phys.net/8/2729/2008/
D. M. Murphy et al.: Weekly patterns of aerosol 2737

(Murphy et al., 2007). Their ratio is less variable than their

absolute concentrations with a very consistent Sunday maximum (Fig. 8). Most locations in the United States have

Pb/(Pb+Zn) in a narrow range between 0.3 and 0.33. The

ratio Pb/(Pb+Zn) is computed instead of Pb/Zn because the

latter diverges when Zn concentrations are small.

By itself, Zn had a small weekly cycle with a Sunday minimum. Neither vanadium nor selenium had a statistically significant weekly cycle. The data for copper are noisy.

3.5 Optical properties of aerosols

Changes in the optical properties of aerosols at two sites

in the central United States are consistent with the weekly

cycles from the IMPROVE network. These NOAA sites

use completely different instrumentation than the IMPROVE

sites and operate continuously rather than sampling for 24 h

every third day. At both NOAA sites, there was a weekly

minimum in aerosol light absorption on Monday (Fig. 9). Although the national minimum for IMPROVE elemental carbon was on Sunday, the clusters of sites close to Bondville

and Southern Great Plains had minima on Monday (Fig. 3).

With samples every third day, there were not enough data to

do the direct comparison of just the Bondville IMPROVE site

with the NOAA Bondville data. There were no statistically

significant weekly cycles in light scattering at either NOAA

site. This is consistent with the lack of a statistically significant weekly cycle in sulfate and the small cycle in organic

carbon.

4 Implications

The weekly cycles shown here are unlikely to be from local sources at the sampling sites for two reasons. First, the

data show regional rather than local patterns. Second, with

the urban sites excluded many of the remaining IMPROVE

sites are at national parks and monuments. One would expect weekend maxima rather than minima if local traffic were

generating the weekly cycles.

Large-scale weekly cycles in aerosol have implications for

emissions, health effects, and climate studies. The weekly

cycles in emissions must be much stronger than the up to

20% cycles observed in aerosol species. At any site and

time, some sources will be very close, some one day upwind,

and others further upwind. Multiple sources and variations in

winds will both tend to smear out the weekly cycle in emissions.

Weekly cycles of traffic emissions have been studied in

California. There, passenger vehicle travel is similar on

weekdays and weekends but heavy truck emissions decrease

by 70 to 80% on weekends (Marr et al., 2002; Chinkin et al.,

2003; Motallebi et al., 2003; Harley et al., 2005). For passenger vehicles, there are diurnal differences between weekday

and weekend. The peak emissions are at rush hours during







	





 	
 
 


 

  
  
 









	





	
 	  




   !

Fig. 9. Aerosol light absorption and scattering from the NOAA

monitoring sites at Bondville, Illinois, and Southern Great Plains,

Oklahoma.

the week but around midday on weekends. The dominant effect for elemental carbon emissions is the decrease of diesel

emissions on weekends. Elemental carbon concentrations on

all days should decrease as new controls on heavy truck emissions penetrate the fleet.

The weekly cycle in crustal elements implies a large anthropogenic source in the United States besides wind-blown

dust. The crustal elements have an even larger weekly cycle

than elemental carbon, which is affected by the 70% decrease

in heavy truck emissions on weekends. That means that the

anthropogenic source for crustal elements must be both large

and have decrease very significantly on weekends. The larger

weekly cycle for crustal elements and PM10 compared to

PM2.5 is consistent with the analysis by Bell et al. (2008).

Light vehicle traffic on dirt or dusty roads probably does

not have a sufficiently large weekly cycle. We do not know

what fractions of plowing and other agricultural operations

are conducted on weekends, and these may vary with region

and season. Construction and heavy truck traffic (Venkatram

et al., 1999; Muleski et al., 2005) are possible sources of dust

with large weekly cycles. For Pb, the IMPROVE data support a significant source from leaded aviation fuel.

www.atmos-chem-phys.net/8/2729/2008/ Atmos. Chem. Phys., 8, 2729–2739, 2008
2738 D. M. Murphy et al.: Weekly patterns of aerosol

Weekly cycles may have implications for epidemiological studies of the health effects of aerosols. First, personal

exposure to certain species will be affected if people spend

more time outdoors on Sundays when mineral dust and elemental carbon concentrations are lower. Second, there might

be some effects on the statistics in time series studies. Such

studies often include day-of-week in the statistical analysis

(Pope and Dockery, 2006). This should take out the biggest

effects but it is hard to exclude subtle interactions in the time

series. On the other hand, the consistency of weekly cycles

throughout the United States means that studies comparing

different cities over the same time period should compensate very well for any weekly cycles. Third, health care can

be better on weekdays than weekends, resulting in different

mortality rates for the same conditions (Bell and Redelmeier,

2001). This could conceivably cause aerosol-health correlations to be underestimated if aerosol health effects maximize

on weekdays when treatment is better. Finally, it is possible that the differing weekly cycles of elemental carbon and

sulfate could be used in a statistical sense to examine the difficult question of what aerosol components are most responsible for the health effects.

These results are important for studies of weekly and other

short-term climate effects of aerosols. The direct climate

effects of elemental carbon and non-absorbing fine particle mass have opposite signs. It will not be possible to relate weekly cycles in temperature or precipitation to aerosols

without considering elemental carbon separately from other

species. The differing weekly cycles of various aerosol

species could provide a powerful check on aerosol transport

models. The species with the biggest weekly cycles, crustal

material and elemental carbon, are insoluble. The most common soluble species is sulfate, which shows no significant

weekly cycle. Therefore, weekly cycles in cloud condensation nuclei are likely to be small unless there are systematic

changes in the size distribution. Finally, mineral dust particles can be potent ice nuclei so the strong weekly cycle in

crustal elements could affect ice clouds and precipitation.

Acknowledgements. S. Capps was supported by the National

Oceanic and Atmospheric Administration’s Ernest F. Hollings

Undergraduate Scholarship Program administered by the Oak

Ridge Institute for Science and Education.

Edited by: A. Laaksonen

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