Oxalic acid in clear and cloudy atmospheres: Analysis of data from International Consortium for Atmospheric Research on Transport and Transformation 2004

Oxalic acid in clear and cloudy atmospheres: Analysis of data from

International Consortium for Atmospheric Research on Transport

and Transformation 2004

Armin Sorooshian,

Varuntida Varutbangkul,

Fred J. Brechtel,

1,2

Barbara Ervens,

3,4

Graham Feingold,

Roya Bahreini,

1,6

Shane M. Murphy,

John S. Holloway,

Elliot L. Atlas,

Gintas Buzorius,

Haflidi Jonsson,

Richard C. Flagan,

and John H. Seinfeld

Received 10 November 2005; revised 11 February 2006; accepted 22 March 2006; published 26 August 2006.

[

]

Oxalic acid is often the leading contributor to the total dicarboxylic acid mass in

ambient organic aerosol particles. During the 2004 International Consortium for

Atmospheric Research on Transport and Transformation (ICARTT) field campaign, nine

inorganic ions (including SO

) and five organic acid ions (including oxalate) were

measured on board the Center for Interdisciplinary Remotely Piloted Aircraft Studies

(CIRPAS) Twin Otter research aircraft by a particle-into-liquid sampler (PILS) during

flights over Ohio and surrounding areas. Five local atmospheric conditions were

studied: (1) cloud-free air, (2) power plant plume in cloud-free air with precipitation from

scattered clouds overhead, (3) power plant plume in cloud-free air, (4) power plant plume

in cloud, and (5) clouds uninfluenced by local pollution sources. The aircraft sampled

from two inlets: a counterflow virtual impactor (CVI) to isolate droplet residuals in clouds

and a second inlet for sampling total aerosol. A strong correlation was observed between

oxalate and SO

when sampling through both inlets in clouds. Predictions from a

chemical cloud parcel model considering the aqueous-phase production of dicarboxylic

acids and SO

show good agreement for the relative magnitude of SO

and oxalate

growth for two scenarios: power plant plume in clouds and clouds uninfluenced by

local pollution sources. The relative contributions of the two aqueous-phase routes

responsible for oxalic acid formation were examined; the oxidation of glyoxylic acid was

predicted to dominate over the decay of longer-chain dicarboxylic acids. Clear evidence is

presented for aqueous-phase oxalic acid production as the primary mechanism for oxalic

acid formation in ambient aerosols.

Citation:

Sorooshian, A., et al. (2006), Oxalic acid in clear and cloudy atmospheres: Analysis of data from International Consortium

for Atmospheric Research on Transport and Transformation 2004,

J. Geophys. Res.

111

, D23S45,

doi:10.1029/2005JD006880.

1. Introduction

[

] Dicarboxylic acids are ubiquitous in atmospheric

aerosols [

Norton et al.

, 1983;

Kawamura and Kaplan

1987;

Kawamura and Ikushima

, 1993;

Khwaja et al.

1995;

Kawamura et al.

, 1995, 1996, 2003, 2005;

Liu et

al.

, 1996;

Kawamura and Sakaguchi

, 1999;

Limbeck and

Puxbaum

, 1999;

Kerminen et al.

, 2000;

Poore

, 2000;

Puxbaum et al.

, 2000;

Pakkanen et al.

, 2001;

Rohrl and

Lammel

, 2001;

Yao et al.

, 2002a, 2004;

Salam et al.

, 2003a,

2003b;

Crahan et al.

, 2004;

Shantz et al.

, 2004;

Yu et al.

2005] and cloud droplets [

Weathers et al.

, 1988;

Limbeck

and Puxbaum

, 2000;

Hegg et al.

, 2002;

Loflund et al.

, 2002;

Crahan et al.

, 2004]; however, the physical and chemical

routes by which these compounds form and are sequestered

in particulate matter are not fully understood. The presence

of dicarboxylic acids in pure and mixed aerosols may

affect both deliquescence relative humidity and hygroscopic

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, D23S45, doi:10.1029/2005JD006880, 2006

Departments of Environmental Science and Engineering and Chemical

Engineering, California Institute of Technology, Pasadena, California, USA.

Also Brechtel Manufacturing Inc., Hayward, California, USA.

Department of Atmospheric Science, Colorado State University, Fort

Collins, Colorado, USA.

Also at Earth System Research Laboratory/Chemical Sciences

Division, NOAA, Boulder, Colorado, USA.

Earth System Research Laboratory/Chemical Sciences Division,

NOAA, Boulder, Colorado, USA.

Now at Earth System Research Laboratory/Chemical Sciences

Division, NOAA, Boulder, Colorado, USA.

Division of Marine and Atmospheric Chemistry, Rosenstiel School of

Marine and Atmospheric Science, University of Miami, Miami, Florida,

USA.

Center for Interdisciplinary Remotely Piloted Aircraft Studies, Naval

Postgraduate School, Marina, California, USA.

D23S45

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behavior [

Cruz and Pandis

, 1998;

Brooks et al.

, 2002;

Kumar et al.

, 2003]; the radiative impact of particles

containing dicarboxylic acids depends on their effectiveness

in absorbing water as a function of relative humidity.

[

] Oxalic acid, (COOH)

, a by-product of fossil fuel

combustion, biomass burning, and biogenic activity, has

been shown in many studies to be the most abundant

dicarboxylic acid in tropospheric aerosols [

Kawamura and

Kaplan

, 1987;

Kawamura and Ikushima

, 1993;

Kawamura

et al.

, 1995, 1996, 2003, 2005;

Liu et al.

, 1996;

Kawamura

and Sakaguchi

, 1999;

Kerminen et al.

, 2000;

Poore

, 2000;

Loflund et al.

, 2002;

Yao et al.

, 2002a, 2004;

Salam et al.

2003a, 2003b;

Crahan et al.

, 2004;

Yu et al.

, 2005]. Studies

have suggested, however, that primary sources of particulate

oxalic acid cannot account for ambient levels that have been

measured globally [

Poore

, 2000;

Warneck

, 2003;

Yao et al.

2004;

Yu et al.

, 2005].

[

] Oxalate, the anion of oxalic acid, is typically detected by

analytical techniques such as ion chromatography (IC). In

some observations, oxalate-containing particles show two

distinct accumulation modes at 0.2 ± 0.1

m and 0.7 ±

0.2

m, with the larger mode being associated with SO

[

Kerminen et al.

, 1999, 2000;

Kalberer et al.

, 2000;

Yao et al.

2002b, 2003;

Crahan et al.

, 2004]. Suggested oxalate forma-

tion mechanisms in the fine particulate mode include in-

cloud processing, oxidation of gaseous species followed by

condensation, and aerosol surface reactions [

Kawamura and

Ikushima

, 1993;

Faust

, 1994;

Chebbi and Carlier

, 1996;

Blando and Turpin

, 2000;

Yao et al.

, 2002b, 2003;

Mochida

et al.

,2003;

Turekian et al.

, 2003].

Crahan et al.

[2004]

measured in a coastal marine environment the air-equivalent

concentrations of SO

and oxalate in cloudwater as approx-

imately two and three times greater, respectively, than concen-

trations measured below cloud. They also detected glyoxylic

acid, an intermediate in aqueous-phase oxalate production

[

Leitner and Dore

, 1997], in cloudwater samples. On the basis

of data from the literature and results from their own study,

et al.

[2005] argue that a dominant in-cloud pathway can

explain the close correlation between SO

and oxalate.

[

] During the 2004 International Consortium for Atmo-

spheric Research on Transport and Transformation

(ICARTT) study, measurements made by surface, airborne,

and satellite platforms focused on examining the nature of

air masses in the northeastern United States, the western

Atlantic Ocean, and the Maritime Provinces of Canada. In

August 2004, the Center for Interdisciplinary Remotely

Piloted Aircraft Studies (CIRPAS) Twin Otter (TO), based

at Hopkins International Airport in Cleveland, Ohio, par-

ticipated in the ICARTT field campaign. The focus of the

CIRPAS participation was to study aerosol and cloud

condensation nuclei (CCN) physics and chemistry.

[

] The goal of the present work is to elucidate oxalic

acid formation in the atmosphere. We analyze airborne

measurements of SO

and oxalate in different conditions

in the atmosphere around Ohio during ICARTT. The

majority of the oxalate measurements were made while

sampling either droplet residuals (the particle remaining

after a cloud droplet has evaporated) or the total aerosol

in clouds (the sum of interstitial aerosol and droplet resid-

uals). We investigate the correlation between SO

and

oxalate, and the role of other organic acids in aqueous-phase

production of oxalate. The relative contributions of different

aqueous-phase reactions are examined, as well as the

magnitude of SO

and oxalate mass production rates in

cloud-processed air. Finally, we assess the extent to which

these rates agree with those predicted by a cloud parcel

model containing a state-of-the-art mechanism for aqueous-

phase production of SO

and oxalate.

2. Twin Otter (TO) Research Aircraft

[

] Table 1 summarizes the instruments that were oper-

ated on the TO. Chemical composition data analyzed here

were obtained by a particle-into-liquid sampler (PILS),

which will be described below. A quadropole Aerodyne

Aerosol Mass Spectrometer (AMS) [

Jayne et al.

, 2000]

provided chemical composition data for nonrefractory aero-

sol species. Aerosol size distribution data (10–800 nm

diameter) were obtained by a Caltech dual differential

mobility analyzer (DMA) system [

Wang et al.

, 2003], also

called the Dual Automated Classified Aerosol Detector

(DACAD). An external Passive Cavity Aerosol Spectrom-

eter Probe (PCASP, PMS, modified by DMT Inc.) and an

external Forward Scattering Spectrometer Probe (FSSP,

PMS, modified by DMT Inc.) provided cloud and aerosol

size distributions for the following particle diameter ranges,

respectively: 0.1 to 2.0

m and 2.0 to 40.0

[

] The main inlet was the primary inlet supplying

sample flow to all instruments anytime the TO was outside

of clouds and sometimes when it was inside clouds. The

main inlet of the TO uses two diffusers to decelerate air by a

factor of ten before it is sent to the sampling instruments.

Hegg et al.

[2005] characterized the behavior of the TO

inlet; they report that the transmission efficiency of the inlet

under standard flight conditions for particle diameters less

than 3.5

m is near unity, decreasing for larger particles

until 5.5

m and above, where the transmission efficiency

remains slightly in excess of 60%. This transmission effi-

ciency persists for particle diameters up to 9

m, the upper

limit of the characterization tests. When used in clouds, the

main inlet sampled the total aerosol, which included inter-

stitial aerosol and residual particles from evaporated cloud

droplets. Evidence that droplet residuals were being sam-

pled through the main inlet during ICARTT included the

detection of aqueous-phase precursors to oxalic acid.

[

] A counterflow virtual impactor (CVI) inlet was

employed only in clouds. The CVI selectively samples cloud

droplets larger than a cutoff diameter of 10 (±20%)

mby

isolating them from the interstitial aerosol, by means of

inertial impaction. During the time the CVI was used, sample

flow was diluted and divided for supply only to the PILS,

AMS, a particle soot absorption photometer (PSAP, Radiance

Research Inc.), and a condensation particle counter (TSI CPC

3010), during which time, all other instruments still sampled

air entering through the main inlet. The instruments behind

the CVI sampled residual particles from evaporated cloud

droplets. PILS concentrations reported during CVI sampling

should be considered as the lower limit to their true value

because of uncertainties in the CVI transmission efficiency.

3. PILS-IC Measurements

[

] The PILS-IC is a quantitative technique for measur-

ing water-soluble ions, including inorganic and organic acid

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D23S45

ions in aerosol particles. The PILS developed and used in

this study (Brechtel Manufacturing Inc., www.brechtel.com)

is based on the prototype design [

Weber et al.

, 2001] with

key modifications, including integration of a liquid sample

fraction collector and real-time control of the steam injec-

tion tip temperature [

Sorooshian et al.

, 2006]. Ambient air

is sampled through a 1-micrometer cut-size impactor and a

set of three denuders (URG and Sunset Laboratories) to

remove inorganic and organic gases that may bias aerosol

measurements. The two annular glass denuders (URG-

2000-30

242-3CSS) used for removal of inorganic gases

are coated with solutions of either 2% sodium carbonate or

2% phosphoric acid (for removal of acidic and basic gases,

respectively) in a solution of 100 mL of Milli Q water,

80 mL of methanol, and 2 g of glycerol. The third denuder

(Sunset Laboratory Inc.), composed of 15 thin carbon filter

paper sheets (3.15 cm

20.32 cm

0.04 cm thick) with

0.2 cm gaps between them, removes organic gases. Sample

air mixes with steam in a condensation chamber where rapid

adiabatic mixing produces a high water supersaturation.

Droplets grow sufficiently large to be collected by inertial

impaction before being delivered to vials held on a rotating

carousel. The contents of the vials are subsequently ana-

lyzed off-line using a dual IC system (ICS-2000 with 25

sample loop, Dionex Inc.) for simultaneous anion and cation

analysis. A fraction of each liquid sample was frozen for

future reanalysis with a longer IC program capable of better

detection of organic acid ions.

[

] Nine inorganic ions (Na

,NH

,Mg

,Ca

,NO

, and SO

) and five organic acid ions

(oxalate, malonate, glutarate, pyruvate, and glyoxylate)

were measured during the field campaign. The limit of

detection (LOD) for each ion was determined by running

the lowest concentration standard detectable by the IC and

using the average concentration plus three times the stan-

dard deviation (

= 50) to calculate the air-equivalent

concentration of each ion. The LODs for the ions measured

using the PILS-IC technique for this study are all below

0.1

g/m

, with the organic anions and SO

exhibiting

LODs below 0.03

g/m

[

] Measurements presented for glyoxylate, pyruvate,

and glutarate are derived from reanalyzing the stored liquid

volumes (they were frozen for 13 months). It should be

noted that all vials were spiked with 5

L of dichloro-

methane prior to storage to prevent biological processing.

The mass concentrations of SO

,NH

, and oxalate from

the original and reanalyzed vials were compared for flight 5

on 9 August 2004 to determine the magnitude of variability

between the two batches (Table 2). The concentrations

measured in the original vials for the three ions exceeded

the reanalyzed concentrations by factors between 1.54 and

1.83, indicating that there was degradation of ion levels

during storage; potential explanations for degradation in-

clude the freezing and thawing process, the interaction of

the sample with the vial surface, and the effect of dichloro-

methane. The reported concentrations of glyoxylate, pyru-

vate, and glutarate should be viewed as a lower limit,

assuming that their degradation rates were similar to those

measured for SO

,NH

, and oxalate.

4. Field Measurements

4.1. Field Data Summary

[

] The TO flight tracks for all twelve flights between 1

and 21 August 2004 are shown in Figure 1. Several flights

focused on areas downwind of the Monroe Power Plant

(Monroe County, Michigan) and the Conesville Power Plant

(Coshocton County, Ohio), both of which are coal-burning

plants. The following types of atmospheric conditions were

encountered over the entire 12-flight mission: (1) cloud-free

air, (2) power plant plume in cloud-free air with precipita-

tion from scattered clouds overhead, (3) power plant plume

in cloud-free air, (4) power plant plume in cloud, and

(5) clouds uninfluenced by local pollution sources. Figure 2

illustrates the sampling strategy used by the TO during

flights when atmospheric condition types 2 and 4 were

encountered; the TO flew a number of horizontal transect

legs perpendicular to the studied plume. Figure 2b repre-

sents the case of type 3 as well, but in the absence of

precipitation. Data grouped in the two cloud cases were

identified by elevated relative humidities (RH > 100%) and

high liquid water content (LWC > 0.1 g/m

). Plumes were

Table 1.

Twin Otter Payload Description

Instrument

Data

Condensation particle counter (CPC)

aerosol number concentration

Cloud condensation nucleus counter (CCN)

cloud condensation nucleus number concentration at 3 supersaturations

Counterflow virtual impactor (CVI)

virtual impactor for isolating cloud droplets

Dual automated classified aerosol detector (DACAD)

submicrometer aerosol size distribution (10–800 nm) at low and high RH

Aerosol spectrometer probe (PCASP), aerodynamic particle

sizer (APS), forward scattering spectrometer probe (FSSP),

cloud and aerosol particle spectrometer (CAPS)

aerosol/cloud droplet size distribution (120 nm to 1.6 mm)

Aerodyne aerosol mass spectrometer (AMS)

nonrefractory aerosol chemistry

Particle-into-liquid sampler (PILS)

submicrometer aerosol chemistry (IC: inorganic and some organic acid ions)

Filters

bulk aerosol chemistry (FTIR: functional group analysis)

PSAP,

photoacoustic, SP2

soot absorption (multiwavelength/incandescence)

Miscellaneous navigational and meteorology probes

navigational data, temperature, dew point, RH, pressure, liquid water content,

wind direction/speed, updraft velocity, etc.

These are the only instruments that sampled air coming through the CVI inlet.

Table 2.

Comparison of PILS Original (Old) and Reanalyzed

(New) Vials From Flight 5 on 9 August 2004

[Old]/[New]

(

= 42) slope (R

)

1.66 (0.84)

(

= 38) slope (R

)

1.54 (0.50)

Oxalate (

= 34) slope (R

)

1.83 (0.50)

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SOROOSHIAN ET AL.: OXALIC ACID DATA ANALYSIS FROM ICARTT

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D23S45

identified by a dramatic increase in aerosol number con-

centration. It should be noted that the clouds uninfluenced

by local pollution sources may have still been influenced by

long-range transport of urban pollution.

[

] Characteristics of the clouds observed during flights

5 and 12 (9 and 21 August 2004, respectively), which are

flights that will be examined in detail subsequently, are

representative of the clouds encountered over the entire

mission. The clouds were typically cumulus, which broadly

encompasses stratocumulus, fair-weather cumulus, and cu-

mulus congestus clouds, with thicknesses between 500 to

700 m. Cloud bases stayed on the order of 1500 m, and as

the day proceeded, the tops grew from around 2000 m to

between 2300 and 2700 m. Typical LWC vertical profiles

indicate that there were slightly variable cloud base alti-

tudes, and some entrainment drying. The LWC in clouds

typically ranged from 0.1 to 1.0 g/m

. The lower tropo-

sphere in the sampling region was marked by turbulence

and convective instability. Soundings obtained on the

ascents out of Cleveland show ambient and dew point

temperatures tracing nearly straight lines to cloud base

altitudes, reflecting instability and vigorous mixing. On

average, vertical velocities near the cloud bases tended to

be 1 m/s, increasing to 2 m/s during passes at higher levels

in cloud.

[

] Figure 3 displays the vertical distribution of specific

ions and the total mass measured through both inlets for the

entire 12-flight mission. The mass concentrations of oxalate

and SO

were evenly scattered at all altitudes up to about

2250 m, where they start to decrease, indicating the begin-

Figure 1.

Twin Otter flight tracks during the ICARTT field study. RF refers to research flight.

Figure 2.

Simplified illustration of how the Twin Otter flew when it encountered two specific

atmospheric conditions: (a) power plant plume in cloud and (b) power plant plume in cloud-free air with

precipitation from scattered clouds overhead.

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D23S45

ning of the free troposphere. Oxalate constituted up to 6.3%

of the total water-soluble ionic mass as measured by the

PILS. Table 3 shows the variation of the molar ratio of

oxalate relative to three of the major inorganic ions for each

atmospheric condition encountered and each inlet used.

Overall, oxalate exhibits the strongest correlation with

, followed by NH

and then NO

. Ammonium is

correlated with SO

, since acidic sulfate-containing par-

ticles absorb ammonia. Oxalate and NO

show a poor

correlation, suggesting that these ions arise from different

processes.

[

] The strongest correlations between SO

and oxalate

were seen for the power plant plume in cloud category when

droplet residual particles (R

= 0.97,

= 11) and the total

aerosol (R

= 0.77,

= 44) were being individually sampled

through the CVI and the main inlet, respectively. The next

strongest correlation (R

= 0.75,

= 54) was observed when

the total aerosol was measured in clouds uninfluenced by

local pollution sources. During ICARTT, the highest single

oxalate measurement (0.94

g/m

) occurred in a droplet

residual sample from a power plant plume in cloud. Aerosol

samples in that category represented the largest average

oxalate (0.28

g/m

)andSO

(6.51

g/m

) loadings.

Flight 5, the flight with the majority of the data points

representing power plant plume in cloud, will be addressed

in the next section.

[

] The lowest SO

and oxalate loadings occurred in

cloud-free air (Figure 4); these samples showed no corre-

lation between SO

and oxalate. Oxalate exceeded detec-

tion limits in only 24 out of 196 samples collected in

cloud-free air. For total aerosol samples collected in power

plant plume in cloud-free air, a moderate correlation

between SO

and oxalate (R

= 0.56,

= 18) was

observed only for the precipitation case, where oxalate

levels exceeded 0.1

g/m

. In the absence of precipitation,

the correlation was much lower (R

= 0.21,

= 10) and

Figure 3.

Vertical distribution of ions measured by the PILS during ICARTT.

Table 3.

Molar Ratio of Oxalate Relative to Sulfate, Nitrate, and Ammonium

Atmospheric Condition Type and Inlet (Number of Vials With

Oxalate Above Detection Limits/Total Number of Vials)

Slope (R

)NO

Slope (R

)NH

Slope (R

)

Clouds uninfluenced by local pollution sources

CVI (34/131)

0.014 (0.37)

0.010 (0.15)

0.006 (0.24)

Main (54/92)

0.024 (0.75)

0.007 (0.01)

0.015 (0.59)

Cumulative (88/223)

0.023 (0.71)

0.000 (0.00)

0.013 (0.55)

Power plant plume in cloud

CVI (11/32)

0.083 (0.97)

0.009 (0.00)

0.073 (0.45)

Main (44/79)

0.028 (0.77)

0.110 (0.03)

0.026 (0.81)

Cumulative (55/111)

0.028 (0.82)

0.031 (0.04)

0.026 (0.84)

Cloud-free air

Main (24/196)

0.000 (0.00)

0.010 (0.08)

0.002 (0.17)

Power plant plume in cloud-free air

Main (10/24)

0.015 (0.21)

0.011 (0.10)

0.011 (0.28)

Power plant plume in cloud-free air with precipitation from scattered clouds overhead

Main (18/21)

0.051 (0.56)

0.031 (0.09)

0.024 (0.52)

CVI samples correspond to cloud droplet residual particles; main inlet samples correspond to total aerosol, which includes a mixture of interstitia

aerosol and droplet residuals in clouds.

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D23S45

oxalate levels were less than 0.1

g/m

. All samples

representing power plant plume in cloud-free air with

precipitation from scattered clouds overhead are from flight

4 on 8 August 2004, where oxalate constituted up to 5.1%

of the water-soluble ionic mass, which corresponded to the

vial with the maximum oxalate loading (0.33

g/m

Table 3 shows that oxalate was measured in the greatest

amount relative to SO

during this flight compared to all

total aerosol samples from other atmospheric condition

categories; however, no key aqueous-phase intermediates

in oxalic acid formation were detected. Therefore we

cannot preclude sources of oxalic acid other than cloud

processing, such as gas-phase oxidation of parent organic

species followed by condensation.

[

] As noted in the Introduction, previous studies have

ruled out primary sources as the main formation mechanism

for oxalic acid based on spatial and temporal characteristics

[

Poore

, 2000;

Yao et al.

, 2004;

Yu et al.

, 2005]. If primary

sources were responsible for observed oxalate levels, then

comparable levels should have been measured in all of the

aforementioned atmospheric conditions encountered. That

the oxalate-to-sulfate molar ratios in the two cloud catego-

ries (power plant plume in cloud and clouds uninfluenced

by local pollution sources) show consistency and that the

oxalate mass concentrations in cloud-free air were signifi-

cantly lower is consistent with the hypothesis of an in-cloud

production pathway for oxalate. Although SO

and oxalate

are not directly linked chemically, the correlation between

their levels is a result of the fact that both formation

mechanisms require the aqueous medium. While informa-

tion regarding the size distribution of oxalate and SO

not available from the PILS measurements, past studies

have noted similar size distributions of these two species,

suggesting a common source [

Yao et al.

, 2003;

Crahan et

al.

, 2004]. Further evidence for aqueous oxalate production

is provided by the detection of aqueous intermediates in

oxalate formation, including glyoxylate, which was mea-

sured in 6 CVI samples (clouds uninfluenced by local

pollution sources) and 13 total aerosol samples (9 in clouds

uninfluenced by local pollution sources and 4 in clouds

influenced by power plant plumes).

4.2. Case Study: Power Plant Plume in Cloud

(Flight 5 on 9 August 2004)

[

] On 9 August 2004 (1709–2216 UT), the TO sam-

pled the Conesville Power Plant plume in cloud (Figure 5a).

Located south of Cleveland, this plant emits SO

and NO

at rates of 1.31

kg/yr and 2.16

kg/yr,

respectively, and VOCs at 1.21

kg/yr (http://www.

emissionsonline.org/nei99v3/plant/pl44374x.htm, 1999).

The plume was transported northeast (

230

)atwind

speeds between 6 and 12 m/s. Figure 5b shows the TO

altitude, LWC, and aerosol number concentration. The TO

flew a stair-step pattern between altitudes of 1500 and

2200 m downwind of the plant between 1815 and 2140 UT.

LWC measurements reached values as high as 0.94 g/m

[

] Typical FSSP cloud droplet size distributions showed

a number concentration mode ranging from 7.7 to 9.0

with a geometric standard deviation ranging between 1.2

and 1.5. All instruments were sampling the total aerosol

through the main inlet, which when used in cloud, as noted

earlier, contained a mixture of interstitial aerosol, particles

that contain significant amounts of water but are not

activated (usually <2

m), and droplet residuals. The

DACAD measured a consistent bimodal size distribution

with increasing distance downwind of the power plant in

cloud, with a smaller mode that grew from 25 to 42 nm

(Figure 6). The aerosol number concentration outside of

cloud and close to the power plant was dominated by

particles with

< 40 nm. A cloud-processing mode in

the range between 100 and 200 nm was measured through-

out the flight. The hygroscopic growth factor for the larger

mode ranged between 1.16 and 1.20 at the relative humid-

ities shown in Figure 6. The growth factors are low relative

to that for pure ammonium sulfate particles (1.17 compared

to 1.44 for pure ammonium sulfate at 77% RH), suggesting

that the sampled aerosol was composed of less hygroscopic

components.

Figure 4.

Oxalate and SO

molar concentrations for each atmospheric condition encountered. The

total aerosol samples collected in cloud consist of a mixture of interstitial aerosol and droplet residuals.

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SOROOSHIAN ET AL.: OXALIC ACID DATA ANALYSIS FROM ICARTT

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D23S45

[

] Figure5cshowsatimeseriesofSO

,various

organic ions, and the volume concentration, from the

DACAD, of the total aerosol sampled (

m). There

is significant growth in SO

, oxalate, and the aerosol

volume concentration in the plume proceeding downwind

of the power plant. The aerosol volume concentration and

the SO

loadings are correlated and close in magnitude,

suggesting that most of the volume growth was a result of

the conversion of SO

to SO

. The ammonium-to-sulfate

molar ratio was between 1.1 and 1.6 for most of the flight

downwind of the plant, indicating that there was insufficient

ammonia to neutralize the relatively high level of sulfuric

acid. This high level of acidity can also explain why

virtually no NO

was detected (<0.3

g/m

). The aerosol

volume concentration reached a maximum close to 40 km

downwind of the plant (2043 UT), the point at which the

highest SO

and oxalate loadings occurred (18.90 and

0.61

g/m

, respectively). Oxalate grew with increasing

distance from the plant as well, representing between 0.5

and 3.1% of the total water-soluble mass for the majority of

Figure 5.

Flight 5 (9 August 2004): (a) flight tracks, (b) time series of altitude, LWC, and aerosol

number concentration; and (c) time series of SO

, organic ions, and the aerosol volume concentration.

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the flight and 2.6% in the vial with its maximum mass. Out-

of-plume, oxalate concentrations dropped below 0.1

g/m

and SO

dropped to between 1 and 5

g/m

[

] Aqueous-phase precursors to oxalate were only iden-

tified in cloud, offering support to the hypothesis of an

aqueous formation pathway. Glyoxylate, the anion of glyox-

ylic acid, was measured in two successive vials, at a level

on the order of 0.06

g/m

, while oxalate was measured as

0.26

g/m

and 0.39

g/m

in these two vials. Two other

vials contained 0.03

g/m

glyoxylate, while oxalate was at

0.19

g/m

and 0.36

g/m

. Pyruvate, the anion of pyruvic

acid, which is thought to be a precursor of glyoxylic acid

[

Lim et al.

, 2005], was measured at a level between 0.09 and

0.10

g/m

. Glutarate, the anion of the glutaric acid (C

dicarboxylic acid), reached a maximum level of 0.24

g/m

One vial contained 0.05

g/m

of malonate, the anion

of malonic acid (C

dicarboxylic acid). Longer-chain

dicarboxylic acids such as glutaric acid are oxidized in

the aqueous phase leading to smaller dicarboxylic acids,

ultimately reaching oxalic acid [

Kawamura and Sakaguchi

1999]. The aforementioned data alone do not reveal the

relative contribution from the decay of longer-chain

acids such as glutarate and the oxidation of glyoxylate to

the production of oxalate mass; this will be explored

subsequently.

[

] Since the concentrations of SO

and oxalate are

correlated and they are both typically found in the same

mode in the submicrometer aerosol size range, it is hypoth-

esized that oxalate is formed in the aqueous phase, for

which the presence of key aqueous intermediates to oxalic

acid provides evidence. The mass concentrations of SO

and oxalate were higher in the flight 5 total aerosol samples

as compared to the total aerosol and droplet residual

measurements in clouds uninfluenced by local pollution

sources from other flights. This is hypothesized to be a

result of longer cloud processing times for sampled particles

during flight 5 since the clouds were more abundant and

closely packed. Also, some of the highest LWC values from

the entire mission were observed, leading to increased

partitioning of gases (specifically SO

, organic precursors

to oxalic acid, and oxidants) into the droplets, yielding

higher mass production rates of SO

and oxalate. The

highly turbulent nature of the lower atmosphere on

9 August 2004 may have also promoted the reactivation

of evaporated droplets, via cycling in and out-of-cloud, to

produce increasing amounts of SO

and oxalate. We note

that although the SO

concentrations were measured to be

higher in plume compared to nonplume conditions, the

concentrations of volatile organic carbon (VOC) species

were essentially similar, which highlights the importance of

non-VOC factors in oxalic acid production.

5. Cloud Parcel Model

5.1. Description and Modifications

[

] The overall goal of this study is to understand the

mechanism of occurrence of oxalic acid in atmospheric

aerosols. The hypothesis is that aqueous-phase chemistry

provides the dominant route for oxalic acid formation. To

evaluate the extent to which the data support this hypoth-

esis, we compare ambient oxalate measurements to predic-

tions of a state-of-the-art microphysical/chemical cloud

parcel model [

Ervens et al.

, 2004] that simulates the

activation of a population of aerosol particles. The model

also simulates cloud cycles that are intended to represent the

trajectory of a typical air parcel in a cloudy atmosphere,

including gas and aqueous-phase chemical reactions.

Four gas-phase VOC precursors of dicarboxylic acids are

included; toluene and ethene represent anthropogenic

emissions, isoprene represents biogenic emissions, and

Figure 6.

Evolution of the DACAD aerosol size distribution (dry: <20%) for flight 5 on 9 August 2004.

GF refers to the aerosol growth factor at the RH value given.

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cyclohexene serves as a model compound for symmetrical

alkenes similar to monoterpenes emitted by biogenic sour-

ces. The oxidation products of these species in the gas phase

(glyoxal, glyoxylic acid, glycolic acid, hydroxyacetalde-

hyde, methylglyoxal, pyruvic acid, acetic acid, adipic acid,

and glutaric acid) can transfer to the aqueous phase. Key

multiphase organic reactions leading to the formation and

depletion of oxalic acid are shown in Figure 7, whereas

production in the model is simply governed by SO

oxidation with H

and O

[

Seinfeld and Pandis

, 1998].

The model does not consider the volatilization of particulate

dicarboxylic acids as their vapor pressures are sufficiently

low (<10

mm Hg) [

Saxena and Hildemann

, 1996;

Tong et

al.

, 2004].

[

] Oxalic acid has two sources and one sink in the

aqueous phase; it is consumed by OH oxidation to yield

, and it is formed by OH oxidation of longer-chain

dicarboxylic acids and glyoxylic acid (including its anion,

glyoxylate). The formation route arising from the decay of

longer dicarboxylic acids begins with adipic acid, which is

formed primarily by the aqueous uptake of gas-phase

products from cyclohexene oxidation by ozone and OH.

Glutaric acid is formed by the aqueous oxidation of adipic

acid and by the aqueous uptake of gas-phase products from

the ozonolysis of cyclohexene. A series of oxidation steps

leads, in order, from glutaric acid to succinic, malonic, and

oxalic acids. The second formation route considers the

oxidation of glyoxylic acid, which is formed by the OH

oxidation of glyoxal, glycolate, methylglyoxal, and acetic

acid. These intermediates are formed in the mechanisms

associated with toluene, isoprene, and ethene.

Lim et al.

[2005] have proposed that methylglyoxal yields low vola-

tility organic acids through oxidation to glyoxylic acid, via

intermediate steps involving pyruvic and acetic acids, and

finally to oxalic acid. The

Ervens et al.

[2004] aqueous-

phase mechanism for isoprene oxidation has been modified

to account for these reactions (Figure 7).

[

] The model was applied to match the conditions

encountered in the ICARTT flights. To estimate the soluble

aerosol fraction (by mass), the total PILS water-soluble

mass (inorganic and organic acid species) was divided by

the sum of the PILS inorganic water-soluble mass and the

organic mass measured by the AMS. The AMS organic

measurement includes both water-soluble and nonwater

soluble species. The model was also modified to incorporate

measured ambient particle size distributions.

[

] Since measurements were carried out in plumes,

it is necessary to account for plume dispersion. Using

measurements and estimates for various properties in the

background air (including concentrations of SO

gas-phase organic precursors, and the mixing ratio of

water), the following parameterized factor, which will be

called the entrainment rate, is applied to simultaneously

simulate open system conditions and plume dilution,

@

@

ðÞ

where

and

are the values of a property in the moving

parcels and in the entrained air, respectively. As will be

discussed later, a constant measured value representative of

background air in the ambient Ohio atmosphere was used

for properties represented by

, while

was initialized

with measured values at the source of the ambient process

being simulated (one simulation to be addressed in a later

section begins near a power plant stack) and subsequently

calculated after each time step by the model. The equation

above is analogous to the plume entrainment rate presented

Squires and Turner

[1962], where a value of

= 0.2 is

based on their laboratory experiments. The parameter

Figure 7.

Multiphase organic chemistry (shaded area indicates aqueous phase). References are

Ervens

et al.

[2004] and

Lim et al.

[2005].

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defined as the characteristic length scale for the entrainment

process at time

, and taken here as 500 m, the typical depth

of the clouds sampled. It is emphasized that the entrainment

rate represents a simplified ad hoc factor.

[

] The model simulates 1-hour cloud cycles that are

intended to represent the trajectory of a typical air parcel in

a cloudy atmosphere. Twelve cloud cycles, limited by the

prescribed trajectory used, are simulated for each experi-

ment. One cloud cycle begins with a parcel of air ascending

from near the ground up through the mixed layer until

activation in cloud. The parcel continues to rise and then

descends through the cloud back to the mixed layer. During

each cycle, a parcel is in-cloud for approximately 800 s; the

same trajectory is then repeated with the number concen-

tration remaining the same, but with particle sizes altered

because of mass addition after each cycle. The LWC time

evolution in the prescribed thermodynamic trajectory rep-

resents mixing and drying by entrained air, based on the

host model from which it is derived. A rough estimate of

the in-cloud residence time can be estimated by calculating

the volume fraction of the boundary layer (BL) occupied by

cloud [

Feingold et al.

, 1998]; typical volume fractions

observed were between 20 and 25% suggesting that the

hourly cloud contact time was between 720 and 900 s for a

well-mixed BL, a range that is consistent with the cloud

contact time of 800 s assumed in the cloud cycles. Photol-

ysis rates are calculated for 40

N on 21 June at a height of

1 km for 12 hours starting at 6 am. The photolysis rates are

time-dependent and uninfluenced by the presence of clouds.

Gas uptake and chemical aqueous-phase processes during

each cycle occur only if the LWC exceeds 1 mg/kg and for

each particle size bin if the sum of the ammonium and

sulfate concentrations does not exceed 1 M. The model

neglects aqueous-phase chemical processes when the ionic

strength exceeds 1 M. This assumption is the result of a lack

of data for the estimation of rate coefficients and because

the solubility of organic gases typically decreases with

increasing ionic strength. The model also does not consider

aerosol loss by wet and dry deposition. Predictions for SO

and oxalate levels are presented here at the end of each

cloud cycle. The comparison between field data and pre-

dictions should be viewed as semiquantitative; for instance,

the actual trajectories of air parcels are not known. Our goal

is to determine the extent to which the levels of SO

and

oxalate measured are generally consistent with those pre-

dicted by a model that is based on cloud processing and

aqueous-phase chemistry.

5.2. Sensitivity Analysis

[

] Prior to actually simulating the field data, we explore

the sensitivity of the model to key parameters and initial

conditions related to the initial particle population and the

ambient atmosphere which include meteorological condi-

tions and concentrations of gaseous species.

Ervens et al.

[2004] simulated generic ‘‘clean continental’’ and ‘‘polluted’’

cases, in which for a closed system (without entrainment of

background air) SO

reached its ultimate level by the end of

the third cloud cycle. With continuous replenishment of SO

on the other hand, SO

is predicted to form continuously.

Oxidation of all organic species in the aqueous phase yields

which replenishes the H

supply, also promoting

production. Sulfate production is controlled mainly by

the initial concentrations of SO

and H

; a set of simu-

lations show that the percentage change in SO

mass

depends almost linearly on the percentage change in the

initial SO

concentration, while an increase in H

yields a

less than proportional increase in SO

. The initial value of

is governed by the size distribution and soluble fraction

of the initial particle population, where the initial soluble

portion is assumed to be pure ammonium sulfate.

[

]

Ervens et al.

[2004] showed that approximately eight

cloud cycles are required for aqueous-phase organic oxida-

tion to be completed in ‘‘clean continental’’ and ‘‘polluted’’

cases (since organics are involved in more oxidation steps

than SO

). As with sulfur, with continued replenishment of

organic precursor gases, organics continue reacting in the

aqueous phase. Oxalic acid production does not exhibit the

same degree of sensitivity to its precursor concentrations as

does SO

to SO

. In some cases, even less oxalic acid was

produced when its gaseous precursor concentrations were

increased. One explanation is that oxalic acid does not form

in direct proportion to its parent precursor VOC (toluene,

ethene, isoprene, cyclohexene) levels because of subsequent

steps required in its formation and competing reactions that

deplete OH, the primary oxidant in the aqueous-phase

organic reactions. Examples of such competing reactions

include the OH oxidation of hydrogen peroxide, formic

acid, formate, and hydrated formaldehyde. In addition,

several volatile organic species, including HCHO and

, are produced as side-products in the aqueous-phase

mechanism and do not contribute to oxalic acid production.

[

] The base value of 500 m used for

in the entrain-

ment rate was doubled and halved to explore the sensitivity

of the production rates of SO

and oxalic acid to this

parameterized factor. For these sensitivity tests, the concen-

trations of gas-phase species in the moving parcels (

) and

in the entrained air (

) were identical and equal to the

initial conditions for the case of clouds uninfluenced by

local pollution sources in Table 4. As will be discussed later,

these conditions are taken from field measurements made at

1000 m in the ambient Ohio atmosphere. With higher

entrainment rates, the simulated particles are more effec-

tively replenished with oxidants, SO

, and organic precur-

sors. Sulfate steadily changed to reach a final mass 28.3%

greater and 27.2% lower when the entrainment rate was

doubled and halved, respectively. Doubling and halving the

base entrainment rate yielded 11% more and 3% less oxalic

acid, respectively, at the end of the simulation. Although

less oxalic acid was formed in the case with the lowest

replenishment rate of oxidants and precursors, oxalic acid

was produced more efficiently relative to the total organic

mass compared to simulations with higher entrainment

rates; the oxalic acid predicted contribution to the total

organic aerosol mass in the case with the lowest entrainment

rate exceeded that of the other two cases by a minimum of

40% after the second cloud cycle. Concentrations of the

first-generation aqueous-phase organic products, like adipic

acid, showed the most proportional response to changes in

the entrainment rate (they grew with increasing entrainment

rate) followed by-products in subsequent generations. Thus

the nonlinear response of oxalic acid to the entrainment rate

compared to SO

can be explained by the multiple steps

required in oxalic acid formation and the complex feed-

backs of reaction chains in the chemical mechanism, spe-

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cifically with the oxidant cycles (including HO

,NO

and H

). Also, the production of oxalic acid from

glyoxylic acid oxidation is less efficient as the pH

decreases, with the lowest pH values predicted in the

simulation with the highest entrainment rate, because the

rate constant of the oxidation of glyoxylic acid is an order of

magnitude lower than that of its anion, glyoxylate.

[

] The results of these sensitivity tests emphasize the

critical nature of allowing for an open system with entrain-

ment as opposed to a closed system when simulating the

production rates of SO

and oxalic acid. Sulfate was

shown to be the most sensitive to its precursor concen-

trations (SO

). Although the total organic aerosol mass and

are comparably sensitive to their precursor concen-

trations, oxalic acid is not. This is because the total organic

mass is dominated by first-generation products in the

aqueous phase, mainly adipic acid. Several oxidation steps

are required to form oxalic acid, which allows for the

depletion of OH, the primary oxidant for organic com-

pounds, and the production of volatile organic side-products

that do not contribute to the production of oxalic acid.

6. Comparison of Model Predictions to Field

Measurements

[

] Measured ambient conditions from two specific

flights (Table 4) are used here to initialize individual

simulations to represent the two types of clouds that were

studied; flights 5 (9 August 2004) and 12 (21 August 2004)

represent power plant plume in cloud and clouds uninflu-

enced by local pollution sources, respectively. The repre-

sentative measurements from flight 5 were made directly

downwind of the Conesville Power Plant outside of cloud in

its plume at 1000 m, whereas the flight 12 measurements

were made at the same altitude at a location in the Ohio

atmosphere unaffected by local pollution sources. Gas-

phase measurements were not carried out on the TO;

estimates of the gaseous precursor concentrations were

obtained from the WP-3D and Convair, operated by NOAA

and the Meteorological Service of Canada (MSC), respec-

tively, which flew in the same areas as the TO, but at

different times. In both simulations, concentrations of gas-

phase species in the entrained air were set equal to those in

the case of clouds uninfluenced by local pollution sources

(Table 4), which represent background air conditions. Two

key differences in the plume case are the higher SO

and

aerosol number concentrations. Although the ethene and

toluene levels are each below 0.15 ppb for both cases, they

are nearly twice as large in the plume case suggesting that

anthropogenic emissions were stronger directly downwind

of the Conesville plant compared to a nearby location in

the Ohio atmosphere unaffected by local pollution sources.

The soluble fraction of the initial particle population in the

plume case is calculated to be 0.45 as opposed to 0.60 for

clouds uninfluenced by local pollution sources. These two

values agree with an alternate calculation of the soluble

fraction by dividing the total PILS water-soluble mass by

the DACAD-derived mass concentration assuming a total

aerosol density of 1.3 g/cm

in both cases. Measurements

show that the sampled particles from the plume flight were

composed of less hygroscopic components; the accumula-

tion mode (100 to 200 nm) growth factors for the represen-

tative measurements during flight 5 and 12 were 1.15 (77%

RH) and 1.20 (75% RH), respectively. For the aerosol

microphysics, it is assumed that the soluble fraction is

entirely ammonium sulfate. The measured contribution of

ammonium and sulfate to the total water-soluble mass

measured by the PILS was on the order of 90% for the

representative data for both flights; thus the predicted SO

concentrations are adjusted accounting for the 10% deficit

in the initial water-soluble ammonium sulfate mass.

6.1. Case Study: Power Plant Plume in Cloud

[

] Figure 8 shows predictions for the growth rate of

and various organic ions in the particle phase imme-

Table 4.

Initial Conditions for Two Simulations

Power Plant Plume

in Cloud, ppb

Unless Otherwise

Stated

Clouds Uninfluenced

by Local Pollution

Sources, ppb Unless

Otherwise Stated

69.5

0.5

40.0

1.0

0.25

HNO

2.0

360 ppm

0.02

HCHO

1.5

OHCHO

1.0

(CHO)

1.0

C(O)CHO

1.0

HCOOH

0.5

COOH

1.0

CHO

1.0

C(O)CH

1.0

(toluene)

0.11

0.06

(ethene)

0.13

0.05

(cyclohexene)

0.01

(isoprene)

0.04

3.27

0.07

9.42

0.50

118.0

114.1

1842.0

1825.0

Initial D

,nm

Power Plant Plume

in Cloud: N, cm

(Sum = 4890 cm

)

Clouds Uninfluenced

by Local Pollution

Sources: N, cm

(Sum = 1755 cm

)

828.2

574.4

2.34

0.435

340.6

9.8

212.8

471

52.3

140.1

711

146.6

113.6

502

90.7

97.2

549

129.6

51.9

1219

486

36.0

343

351

17.3

767

489

9.7

238

0.426

Power Plant

Plume in Cloud

Clouds Uninfluenced

by Local Pollution

Sources

Soluble fraction by mass, %

T, C

P, bar

900

Altitude, m

1000

RH, %

Conditions for clouds uninfluenced by local pollution sources are also

those representing the background air in the entrainment rate for both

simulations.

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diately downwind of the Conesville Power Plant. Sulfate is

predicted to continuously increase, reaching a total submi-

crometer mass of 31.4

g/m

, with its sharpest increase

predicted during cloud cycles 4 and 5. The predicted total

organic mass, which grows to 0.82

g/m

by the end of the

simulation, is dominated by oxalic and adipic acids (the

model treats the oxidation of cyclohexene as a one-step

process leading immediately to adipic acid).

Ervens et al.

[2004] state that the cyclohexene mechanism provides an

upper estimate for its contribution to the predicted dicar-

boxylic acid mass due to the simplified (one-step) chemistry

in the model. The background concentration of cyclohexene

is assumed to be 0.01 ppb, which was chosen to stay

consistent with previous measurements showing that its

emission rate is an order of magnitude less than that of

toluene [

Grosjean and Fung

, 1984]. Even though the

background concentration of cyclohexene, which was not

measured by the WP-3D or Convair, in the atmosphere

above Ohio may have been lower since adipate was not

measured by the PILS, cyclohexene can be considered to

serve as a surrogate compound for all symmetrical alkenes

similar to monoterpenes.

[

] Adipic and glutaric acids are oxidized, forming the

shorter-chain acids starting with succinic acid, which then

fuels malonic acid production. Succinic and malonic acids

are each predicted to remain below 0.01

g/m

and 0.3% of

the total organic mass throughout the simulation. The

oxidation of malonic acid leads to oxalic acid, which is

predicted to grow to levels comparable to those of adipic

acid (0.27

g/m

and 0.24

g/m

, respectively) after three

cloud cycles since it is also produced by glyoxylic acid

oxidation. Oxalic acid reaches a peak mass of 0.38

g/m

after the sixth cloud cycle before steadily decreasing to

0.17

g/m

by the end of the simulation. It grows the most

during the third and fourth cloud cycles, which corresponds

to the two cycles with the highest average droplet pH

values (2.61 and 2.46, respectively) and when glyoxylic

acid and its precursors reached their highest concentrations

(droplet pH values from the simulations will be displayed

subsequently).

[

] Figure 9 compares predictions to field measurements

from flight 5, where first-order plume age is based on the

distance away from the power plant and the wind speed.

Selected measurements from the field data were taken to

represent different distances downwind of the Conesville

Power Plant where transects were made. The field measure-

ments for SO

show continuous growth until the last

transect where plume dilution may have dominated. The

initial SO

mass predicted prior to the first cloud cycle

(cloud cycle 0 in Figure 9) is highly dependent on the

assumed particle size distribution and soluble fraction of the

particles used to initialize the simulation. Using the mea-

sured values for these two parameters, the initial SO

predicted (7.37

g/m

) is already more than 2

g/m

greater

than that measured in the first transect downwind of the

Conesville Power Plant (5.00

g/m

at a plume age of

0.6 hours). The model predicted that 3.46

g/m

of SO

grew

after the first cloud cycle, while the most growth occurs in the

fourth and fifth cloud cycles (4.83

g/m

and 5.42

g/m

respectively). Sulfate grows between 0.5 and 2

g/m

in each

of the other cloud cycles. It is hypothesized that SO

increased the most during cloud cycles 4 and 5 because the

average cloud cycle droplet pH value experienced its largest

decline in the simulation from 2.61 in the third cycle to 1.95 in

the fifth cycle, which enhanced SO

production.

[

] Oxalic acid first increases and eventually decreases

slightly with downwind distance in both the field data and

the predictions. The decrease in oxalic acid is predicted to

be due to its oxidation in the particle phase to CO

. The

predicted oxalate-to-sulfate molar ratio agrees with the field

data points (between 0.01 and 0.05) from the third cloud

cycle until the end of the eighth cloud cycle.

Figure 8.

Model predictions for the power plant plume in cloud case (simulation of flight 5 on 9 August

2004).

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