Geochemical cycles of atmospheric organic acids and aldehydes

PUJA KHARE*, BP BARUAH

Coal Chemistry Division, North-East Institute of Science and Technology (NEIST),

Council of Scientific and Industrial Research (CSIR), Jorhat- INDIA

* Email: kharepuja@rediffmail.com

Abstract:- Organic acids (formic and acetic acids) and formaldehyde are ubiquitous key components of

the atmosphere. They are the major constituents of both marine and continental rain. One of the most

significant aspects of organic acids and aldehydes geochemistry in the troposphere is its role in the

removal of incompletely oxidized carbon from the atmosphere via wet deposition. In addition to their

presence in the atmosphere, organic acids contribute significantly to the acidity of precipitation and cloud

water, especially in remote regions. The aldehydes are the one of the major sinks for OH radicals in the

troposphere. The photolysis of formaldehyde produces radicals that, in the presence of sufficient amount of

nitrogen oxides, results in the formation of tropospheric ozone. Thus, formaldehyde is a key component to

understanding the oxidizing capacity of the atmospheric and formation of tropospheric O3.

Organic acids in the atmosphere show high concentration in highly polluted areas and low at marine

locations. They are scavenged by wet deposition and dry deposition. Their sources varied with different

geographical locations i.e. mid-latitude continental, tropical continental and marine sites. Major sources are

anthropogenic i.e. vehicular emissions and natural sources i.e. ants, plants and soils. Direct and secondary

emissions from vegetation and biomass burning are also important contributor to both acids.

Levels of formaldehyde ranged from 0.1 ppbv in the clean marine environment upto 150 ppbv in the urban

areas. In rainwater, formaldehyde concentration was uniform at different geographical locations and the

average value was same for rural, continental and coastal sites. Particulate phase concentration of

formaldehyde was several orders of magnitude lower than that of the corresponding gas-phase level and it

contribute little to the overall budget of formaldehyde in the atmosphere. Formaldehyde may be emitted by a

variety of incomplete combination processes and is also known to be formed from several radical reactions.

In this papers, the work on concentration, distribution of organic acids (formic and acetic acids) and

formaldehyde in multiple phases, their atmospheric reactions and their sources is reviewed in view of the

uncertainty of the geochemical cycles of these acids in the environment.

Key words: HCHO, HCOOH, CH3COOH, geochemical cycles

1. Introduction

Carbon-containing species are the important

constituents of atmosphere. Significant part of the

organic species is soluble in water [1]. Organic

carbon fraction may account for 80% of the global

flux of rainwater carbon [2]. These water soluble

compounds have been found at significant

concentrations in wet precipitations from various

locations [3]. The removal process of the atmospheric

organic carbon by rain prior to its oxidation to carbon

dioxide is of great importance in evaluating the

global carbon biogeochemical cycle. Hence,

precipitation act as a globally important removal

mechanism for atmospheric dissolved organic carbon

(DOC) with a carbon flux (0.3 Gt yr-1

) equal to

approximately 6% of the fossil fuel flux influx (5.5

Gt yr_1

) to the atmosphere. Dissolved organic carbon

can be of greater bioavailability to the oceanic biota

than that derived from river water [4]. The rainwater

removal of atmospheric organic carbon plays a

significant role in global carbon cycling and hence

must be considered along with other parameters in

global warming models.

Recent evidence suggests that carboxylic acids

are one of the dominant classes of organic

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compounds found in the atmosphere in a variety of

phases and contribute a large fraction to the rainwater

organic carbon and also non-methane hydrocarbon

atmospheric mixture [5]. Organic acids (acetic,

formic, oxalate, pyruvate and methansulfonate)

contributed 18.5% of the TOC or 21.5% of the DOC.

The individual contribution of acetate, formate,

oxalate, pyruvate and methane sulfonate was 6.9%,

2.8%, 5.25, 0.35 and 0.15, respectively. The organic

acids found in the gas-phase, aerosol and in rain are

from both developed and remote areas of the world

[6]. Sanhueza et al. [7] established that, in the gas-

phase, the organic acids are essentially formic and

acetic acids and that both can represent 25% of

atmospheric hydrocarbons, methane excluded. In

rainwater, Keene et al. [8]estimated the organic acid

contribution to the acidity in North America to be

between 16% and 35% of the total acidity, with the

remainder coming from sulphuric and nitric acids.

The contribution of organic acids to the total

rainwater acidity was higher in remote areas, Likens

et al. [9] determined it to be 64% for an isolated area

in Australia and Andreae et al. [10] estimated that

formic and acetic acids account for as much as 89–

90% of the acidity in remote areas of the world.

Aldehydes particularly formaldehyde is also a

ubiquitous component of both the remote atmosphere

and polluted urban atmospheres. It is unique in that it

acts as a ‘funnel’ through which much of the carbon

flux passes in the course of oxidation to CO and CO2

[11] Formaldehyde is a key component in our

understanding of the oxidising capacity of the

atmosphere and formation of tropospheric O3.

Oxidation of formaldehyde by OH in fog and cloud

droplets has been postulated as a source of HCOOH

[12,13]. S (IV) adducts with formaldehyde make

significant contributions to the total dissolved S(IV)

[14]. Atmospheric deposition is a significant source of

formaldehyde [15] since concentration in rainwater is

approximately three order of magnitude higher than in

surface waters [16]. Hence, it may contribute

significantly to atmospheric carbon budget.

Besides the impotence of trios in the atmosphere,

their geo chemical cycles are unknown. In present

study, we compared concentration of HCHO, HCOOH

and CH3COOH, their source and removal processes in

the atmosphere at various geographical locations. Data

reported in peer-reviewed publication is used for

investigation. The comparison provides global

geochemical cycles of HCHO, HCOOH and

CH3COOH and their contribution to total atmospheric

carbon budgets.

2 Atmospheric Concentration

2.1 Vapour Phase Fig 1 shows the reported minimum and maximum

levels of HCHO, HCOOH and CH3COOH in different

location. Khare et al., [5] reviewed and reported that

the levels of gaseous formic and acetic acids are at least

2 orders lower in remote locations i.e. typically 0.02-1

ppbv than in highly polluted areas, where the values

range between 1 and 16 ppbv. Arlander et al., [17]

reported values as low as 0.02 ppb in remote regions of

the Pacific and Indian Oceans, with lowest values of

both acids occurring in the southern hemisphere.

Dawson and Farmer [11] reported mean concentrations

of formic and acetic acids in the southwest United

States that varied between 0.7 and 0.6 ppbv at rural site

and between 3 and 4 ppbv at an urban site.

Vapour phase formaldehyde has been studied in

different geographical regions of world. Very few

formaldehyde measurements have been reported for

tropical latitudes. Table 1 shows mixing ratios

observed in various locations of the world. The highest

values are associated with urban air, especially under

conditions of photochemical smog. Mixing ratios in

rural areas are of the order of a few ppbv and still lower

values are found in marine air masses. Formaldehyde

ranges from 0.1 ppbv in the clean marine environment

[18] upto 150 ppbv in urban areas of Los Angeles [19].

Formaldehyde levels at rural site, vary between 0.03

ppbv [18] and 9.0 ppbv [20] at Eifel region Germany

and North Carolina, respectively. In marine locations

minimum levels (0.1-0.4 ppbv) were observed at Irish

west coast [18] and maximum (0.8-11 ppbv) over the

eastern Indian ocean [21]. Greadel [22] in his model

calculations, predicted formaldehyde mixing ratios

between 0.06 and 2.0 ppbv above the marine surface.

Zafiriou et al. [23]reported formaldehyde levels

between 0.11 ppbv and 0.57 ppbv over the remote

Pacific Ocean. Lowe and Schmidt [24] have measured

formaldehyde levels over Atlantic ocean as a function

of latitude as shown in Fig 1. In the region 33ºS-40ºN

the data scatter around a mean mixing ratio of 0.22

ppbv, further northward they decline. During a trans

equatorial ship cruise from Kamchatka to Wellington,

NZ [17] HCHO concentration in the range of 0.5-0.8

ppbv were observed between 15ºN-15ºS with lower

concentration towards higher latitudes. The

concentration observed (0.1 ppbv at 30ºS) in the

southernmost region over Indian ocean were among the

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lowest observed anywhere on the cruise. Their

increased moving northward, however no clear pattern

was discernable in the southern hemisphere. Values

obtained in the northern hemisphere (0.3-0.8 ppbv)

over Indian ocean were still on average much lower

than those obtained by Fushimi and Miyaki, [21] over

the eastern Indian Ocean (0.8 to 11.0 ppbv).

2.2 Aerosol

Concentrations of formate and acetate in

particulate matter (atmospheric aerosols) have been

generally found to be low, ranging i.e. 0.5 and 40 ppt

[25]. However, in India, the mean concentration of

formate and acetate in particulate matter was 290±205

and 320±90 ng m-3

in winter and 240±160 and

440±100 ng m-3

in summer [26] .

Khwaja [17] reported that a U.S. suburban site,

>93% of the total measured formic acids and >91% of

the total measured acid was in gas phase, while

Andreae et al., [10] showed that over 98% in the

wintertime, compared with 99.2% during summertime.

However, Khare et al., [26] reported higher percentage

of these acids in particulate phase (25-29% and 29-

43% for formic and acetic acids, respectively). These

higher percentages in particulate phase may be due to

the basic nature of aerosol in reported sites of India.

The aerosol formaldehyde ranged from 5.36 ng

m-3

(marine site) to 40.9 ng m-3

(continental site) at

Deuselbach [28]. The formaldehyde fractions on total

aerosol and found that their magnitude was similar for

continental and maritime aerosol. Subsequently,

Klippel and Warneck, [28], made measurements of

aerosol formaldehyde in urban, continental rural and

coastal regions of Europe. For the rural continental

sites the average aerosol formaldehyde concentration

was » 40 ng m-3

. In urban air at Mainz, formaldehyde

was found to be 65 ng m-3

while the average

concentration in the coastal region of West Ireland was

5 ng m-3

. De Andrade et al. [29]measured

formaldehyde concentration in aerosol at Brazil and

reported formaldehyde levels to be 28 ngm-3

in aerosol.

Particulate phase concentration of formaldehyde are

several orders of magnitude lower than that of the

corresponding gas-phase levels and hence aerosol

formaldehyde contributes little to the overall budget of

formaldehyde in the atmosphere [28] and particle

scavenging is expected to make a negligible

contribution to hydrometeor formaldehyde budgets.

3.0 Sources

Formic and acetic acids have different sources at

different geographical locations. Generally,

photochemical reactions i.e. ozone–olefin reaction,

isoprene oxidation, gas phase reaction of formaldehyde

with HO2 and aqueous phase oxidation of

formaldehyde are important sources of these acids. In

mid latitude continental regions, possible sources of

formic and acetic acids, other than photochemical

reactive vehicular emissions are direct emissions from

vegetation and biomass burning. In tropical continental

sites, direct emission from vehicles and soil, vegetation

and biomass bruning are important source of these

species. The probable sources at marine locations are

photochemical reactions, biogenic emissions and long

range transport from continental site [5]. The probable

photochemical reactions are

a. O3-Olefin reaction

>C=C< + O3 >C-O-O-O-C< >C

.-O-O

.

+ O=C<

Criegee intermediate

Rearrangement of Criegee intermediate

>C.-O-O RCOOH

.

b. Isoprene oxidation

Iroprene + OH +

NO

+ O2 MVK +

HCHO + 2NO2 + OH + MA + .CH2OO

.

Criegee intermediate

Oxidation of Methyl vinyl ketone (MVK) and

Methacrolein (MA) provides the addition sources for

both acids.

c. Gas-phase reaction of formaldehyde with HO2

CH2O + HO2 . O2CH2OH

O2CH2OH +

HO2 HCOOH + H2O + O2

O2CH2OH

NO+ O2

HCOO

H

+ NO2 + HO2

O2CH2OH + O2CH2OH O2

HCOOH + 2 HO2

.

The predominant source of urban formaldehyde is

believed to be the photo oxidation of the many

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hydrocarbons that are present in the atmosphere

(Altshuller,1993). RH + .OH R. + H2O

R. + O2 + M RO2. + M

RO2. + NO RO. + NO2

RO. + O2 RCHO + H2O

In rural areas of dense vegetation, biogenic sources

are often the dominant precursor. For example,

isoprene oxidation initiated by reactions with either OH

or O3 efficiently forms formaldehyde with several other

key atmospheric species [30]. In urban air, direct

emissions from vehicular exhaust and incomplete

combustion processes are the major source of

formaldehyde [31]. In marine air, oxidation of olefins,

emitted from biogenic sources, may be a source of

formaldehyde [32]. In unpolluted marine regions the

main source of formaldehyde is the oxidation of

methane by the hydroxyl radical. In rural continental

area, oxidation of natural and anthropogenic non-

methane hydrocarbons (NMHC) contributes

significantly to the formation of formaldehyde. Among

the biogenically emitted NMHCs, isoprene and

terpenes are the most abundant [33] and the dominant

source of formaldehyde in rural areas [34]. Gas phase

oxidation of alkane (mainly ethane) and alkenes

(ethene, propene) also contribute as a significant source

of formaldehyde [35]. Non-photochemical reactions of

different atmospheric hydrocarbons with O3 or NO3

radicals [36] are additional sources of formaldehyde,

which may be important during the night or in polar

regions in the dark [37].

4.0 Removal

4.1 Wet deposition

The high solubility of two acids in water makes

rain an ideal scavenging medium. Therefore, rain

may wash out formic and acetic acids in both the

vapour and particulate phases. They contribute

significantly (up to 64%) to the free acidity of rain in

remote regions of the world [8]. organic acids

(including oxalic acid) accounted for 33% of the total

free acidity (on average) in Los Angles. Fig 2 shows

the minimum and maximum levels of HCHO,

HCOOH and CH3COOH reported in marine, remote,

semiurban and urban rainwater. It can be seen

clearly that both acid contribute significantly to total

dissolved organic carbon in remote locations.

Khare, [38] reported that formaldehyde

concentration ranges between 0.3 and 8.2 µM at

different geographical locations of the world.

Maximum levels of formaldehyde in rainwater were

found at rural site (Fig 2). It indicates that not only

sources strength but other phase partitioning and

aqueous phase reactions also are the controlling

processes its levels in rainwater.

Removal of HCHO, HCOOH and CH3COOH

by dew and fog water also affect their diurnal and

seasonal cycles in the atmosphere. The reported

concentration of formic and acetic acids in dew water

and fog water ranged between 8.1µM to 69.5 µM at

different geographical location [5]. In most of the

cases concentrations of fomate were found higher

than the acetate levels in dew and fog [39]. A

probable reason for this is that HCOOH is

incorporated to the liquid phase than CH3COOH

[38]. Aqueous phase concentration of these acids is

pH dependent. Khare et al., [39] reported that they

deviate from the Henry law for phase partitioning.

These deviations are of several order of magnitude

and attributed to droplet size of liquid and organic

film formation on surface.

Formaldehyde levels in fog and dew ranged

between 150 to 400 µM. HCHO dissolution in dew

and fog water follows Henry law. However, Khare,

1998 reported two order variations in measured and

predicted value by applying Henry law phase

partitioning. These deviation may be due to

hydrolysis of HOCH2OOH in dew water. The

concentration of HCHO, HCOOH and CH3COOH

were found higher in dew water than in winter rain

[39] indicating significant contribution of dew to

organic carbon flux in the atmosphere.

4.2 Dry deposition

Only few studies concerning fomate and acetate

levels in dry deposition are reported [26] even though

their rates of dry deposition are regarded as area of

uncertainity [32]. The dry deposition of particle acetate

and formate to Teflon filters was measured by Talbot et

al., [25]. They obtained fluxes of 5-10 n mol cm-2

Y-1

.

The presence of both these species in dry deposition

has been attributed to settling of large particles of these

acids. In India, the fluxes of formate in winter and

summer were 132±114 and 155±55 µg m-2

day-1

respectively, while corresponding acetate fluxes were

177±166 and 80±54 µg m-2

day-1

[26]. Higher fluxes

in India were attributed to higher atmospheric loading

of particulate matter dominated by the soil derived

elements.

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4.3. Atmospheric reactions

Photolysis and oxidation by OH radicals are the

major removal process of atmospheric formaldehyde.

The photolysis of formaldehyde produces radicals that,

in the presence of sufficient amounts of nitrogen

oxides, result in the formation of tropospheric ozone.

An important degradation product of formaldehyde is

CO.

Daytime

HCHO CO + H2

HCHO CHO + HO2. CO + HO2

HCHO + OH HCO. + H2O CO

Night time RCHO + NO3

. RCO. + HNO3

Oxidation of formaldehyde by OH in fog and cloud

droplets has been postulated as a source of HCOOH

[13,14].

CH2O + H2O CH2(OH)2

CH2(OH)2 + .OH CH(OH)2 + H2O

Abstraction of H atom

CH(OH)2 + O2 HCOOH(aq) + HO2.

Figures 3 and 4 show geochemical cycles of both acids

and HCHO, respectively. Anthropogenic, natural and

biogenic emissions, in-situ reactions and long range

transport are the major sources of formic and acetic

acids in the atmosphere. Strength of these sources may

vary at different geographical locations and control the

levels of acids in the atmospheric. However,

atmospheric concentration of formaldehyde is

controlled by anthropogenic, natural and biogenic

emissions and in-situ reactions. Wet and dry

deposition processes are significant removal processes

for trios. Photolysis and oxidation reaction of

formaldehyde also act as significant degradation

process.

Acknowledgement

The authors are grateful to Dr P.G. Rao, Director,

North East Institute of Science and Technology

(NEIST), Jorhat for his keen interest and support.

This study was financially supported by the

Department of Science and Technology, New Delhi,

Government of India (NO: SR/FTP/ES-58/2006) to

one of the authors (PK).

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Fig: 1 Vapour phase concentration distribution of HCH, HCOOH

and CH3COOH at different locations.

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Fig 2. Concentration of HCHO, HCOOH and CH3COOH in

rainwater at different locations

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Fig 3. Geo-chemical cycle of atmospheric HCOOH and CH3COOH.

Fig 4. Geo-chemical cycle of atmospheric HCHO.

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