From January 1995 to December 2004, 860 rainwater samples were collected in the humid savannah of Lamto. Using the Henry’s law, we determined the content of formic and acetic acids in the air based on their concentrations in rainwater. The annual partial pressure of both formic and acetic acids over the decade is variable. It covers a range of 0.003 (1998) to 0.21 ppbv (1996) and 0.27 (1999) to 0.47 ppbv (1996) for formic and acetic acids respectively. Also, the partial pressure in the dry season is higher than that in the wet season. This difference is related to the enrichment of the organic acid content in the air by the various sources that produce these acids. One of the main sources of increment in organic acid is biomass burning. This biomass burning contributes between 21% and 51% to the formation of the two acids in the humid savannah of Lamto. Ultimately the average annual organic acidity varies from 40% to 60% over the ten years period.
Numerous field investigations of organic acids in the boundary layer and through the free troposphere have been undertaken in situ and remotely during isolated, time limited local as well as regional studies for more than three decades. The results indicate that formic and acetic acids are ubiquitous components of the atmosphere [
Carboxylic acids measurement methods are numerous and various (e.g., Mist chamber technique [
Lamto geophysical station is located in West Africa precisely in Côte d’Ivoire. Since 1994 it has became IDAF (IGAC/DEBITS/Africa: International Global Atmospheric Chemistry/Deposition of Biogeochemically Important Trace Species/AFrica) network experimental site, where it characterises the humid savannah ecosystem. This station is located between the Guinean savannah and tropical forest (6˚31'N latitude and 5˚02'W longitude),
For this study, we used two types of data. The meteorological data produced by Ivory Coast Meteorological Agency (SODEXAM). They have been used to determine the seasonal distribution of rainfall at Lamto during the period of study and to calculate the dryness index. Physicochemical data of rain waters have been supplied
by IDAF network and used to determine the partial pressures of formic and acetic acids.
The seasonal distribution of rainfall was defined based on Gaussen and Bagnouls [
Beyond this seasonal distribution, the yearly seasonal drought is also an important parameter which should be considered in the biomass burning impact on air quality. The amount of biomass burning is dependent on the type of vegetation but also on meteorological factors such intense drought [
where t is the monthly mean temperature (˚C) and p is monthly mean rainfall (mm).
Since 1994, the chemical signature of the Lamto savannah has been monitored through rainfall, gas and aerosols samples collection. A total of 860 samples were collected and analysed during the study period. The main dissolved chemical substances (H+,
During cloud formation, the partial pressure of organic monoacid (HCOOH or CH3COOH) in the air is balanced
1994 | 1995 | 1996 | 1997 | 1998 | 1999 | 2000 | 2001 | 2002 | 2003 | 2004 | |
---|---|---|---|---|---|---|---|---|---|---|---|
January | |||||||||||
February | |||||||||||
March | |||||||||||
April | |||||||||||
May | |||||||||||
June | |||||||||||
July | |||||||||||
August | |||||||||||
September | |||||||||||
October | |||||||||||
November | |||||||||||
December |
Years | 1994 | 1995 | 1996 | 1997 | 1998 | 1999 | 2000 | 2001 | 2002 | 2003 | 2004 |
---|---|---|---|---|---|---|---|---|---|---|---|
Dry season | 0.38 | 0.12 | 0.20 | 0.34 | 0.20 | 0.43 | 0.45 | 0.33 | 0.38 | 0.40 | 0.45 |
Wet season | 1.43 | 1.73 | 1.67 | 1.56 | 1.23 | 1.72 | 1.57 | 1.55 | 1.46 | 1.76 | 1.22 |
with the acid concentration on gaseous form present in the rainfall. The gas solubility balance in the aqueous phase of cloud droplet is governs by Henry law [
where Pi is the gas partial pressure (atm), CS is maximum concentration at saturation (mole/L) and H is the Henry’s law constant (mole/Latm).
Considering the acid (AH):
The partial pressure Pgas of the each carboxylic monoacid in equilibrium with the water cotent of cloud is calculated by:
where Pgas is Gas partial pressure in the air (atm), X is the total concentration of the acid measured in the rainfall water (mole/L), H is the Henry’s law constant (mole/L.atm), pH is the acidity of the rainfall sample and pKa is the pH at which half of the acid is dissociated.
Over the decade (1995-2004), the annual mean partial pressures of organic monoacid (formic and acetic) have been determined. Pgas values vary from 0.003 (1998) ppbv to 0.21 ppbv (1996) for formic acid and from 0.27 (1999) ppbv to 0.45 ppbv (1996) for acetic acid.
similar interannual variations. However, the acetic acid appears to be less than the formic over the decade. The mean partial pressure in the air is of 0.12 ± 0.06 ppbv for a variation coefficient of 50% against 0.38 ± 0.06 ppbv with a variation coefficient of 16% for acetic acid. The decadal mean value of Pgas for the acetic acid is 3.2 times greater than the formic acid. The interannual variability over the decade is related to organic acid sources. The organic acid in the air is dependent on biomass burning, isoprene gaseous phase photochemical, aldehydes oxidation in liquid phase and the direct emission of acid by vegetation [
Pgas observed in the atmosphere of the humid savannah Lamto is within the range of values generally observed in the atmosphere of rural areas (
Pgas seasonal values for the formic and acetic acid vary from 0.002 (1998) to 0.61 ppbv (1995) and 0.28 (1999) to 0.98 ppbv (1995) respectively during dry season. During wet season, these values vary between 0.003 (1998) and 0.17 ppbv (1996) for formic acid and 0.23 (1999) and 0.44 ppbv (2002) for acetic. Dry season Pgas values are higher than those of the wet season despite the considered acid and year. However, in 1998, the formic acid partial pressure values during the wet season were close to those of the dry season (
Excluding the 1998, which is a particular year, the mean ration of Pgas for the organic acid during dry season over wet season was 2.5 ± 0.9 ppbv and 1.8 ± 0.6 ppbv respectively. The partial pressure of organic acid in the atmosphere of humid savannahs of Lamto increased from factor 2 from wet season to dry season. This seasonal gain is not only explained by rainfall difference between dry and wet season since there is no linearity factors between the dilution factor (24.4 ± 4.82) and Pgas variation of organic acid. Therefore, the seasonal variation of organic acid involves several sources with a predominance related to climatologically and meteorological conditions. However, an intensification of organic acids during dry season or additional contribution related to fires may be considered. Biomass burning fires intensification and the additional contribution of fires located far
Location | Formic acid | Acetic Acid | References |
---|---|---|---|
Equatorial forest of Mayombe (Congo) | 0.08 | 0.13 | Servant et al. [ |
Amazonia | 0.20 | 0.20 | Talbot et al. [ |
Germany | 0.20 | 0.70 | Hartmann et al. [ |
Venezuela | 0.80 | 0.50 | Sanhueza et al. [ |
Hawaii | 0.50 | 0.40 | Norton [ |
Congo | 0.50 | 0.60 | Helas et al. [ |
Humid savannah of Lamto (Côte d’Ivoire) | 0.12 | 0.38 | Present study |
away may explain this organic acid concentration increasing during dry season. Indeed, during this period, from October of the previous year to March of the following year, biomass fires are prevalent in West Africa [
The decade was not homogeneous. It is interesting to notice that maximum ratios are 3.8 and 2.7 for formic and acetic acids respectively, in 1995, while the minimum values of 1.3 and 1.1 were recorded in 2002. The low ratio obtained in 2002 could find an explanation in reducing the contributions of local fires. Indeed, from September to December 2002, the migration of the central and northern populations to the cities of the south because of the war has allowed a reduction of agriculture (slash and burn agriculture, hunting, charcoal production) and the resulting forced fallow.
In this part of work, to determine the contribution of organic acids to total free acidity (TFA), it is important to introduce oxalic acid in the calculations and the analysis. Because it is the third abundant carboxylic acids measured at Lamto [
So to determine organic acidity contribution to the rainfall acidity, the total acidity (TFA) was defined as:
where
concentration,
The most common method used to evaluate the contribution of maritime salt to ions concentration in precipitation is the comparison of the Cl− to Na+ ratio measured in the rainfall and the sea waters. It is 1.14 close to the value found by Yoboué et al. [
The concentration of sulfate ions from maritime source was determined at Lamto. This lead to total acidity calculation.
The total acidity values (TFA) over 1998-2004 were determined. The TFA varied between 24.84 µeq∙L−1 in 1999 and 40.28 µeq∙L−1 in 2004 during wet season and between 42.47 µeq∙L−1 in 1998 and 91.78 µeq∙L−1 in 2004 during dry season. TFA mean values over the study period are 35.02 ± 5.16 and 67.32 ± 23.25 µeq∙L−1 respectively for wet and dry seasons. TFA variation was weak during wet season but strong during dry season, where they are two times more than the wet season ones. This may be explained by the weighted concentrations during dry season which are higher than the wet season one. The mean contribution of organic acids (
Comparing to studies of the tropical regions of America and Africa such as Brazil (Amazonia), Congo (Dimonika), Cameroon (Zoétélé) and in less polluted regions of North America and Australia [
In the purpose to analyse biomass fires impact, their acid formic and acetic concentration, nitrate ion have been used as chemical tracer. This choice is justified by the existence of strong correlation of this ion and organic acids of rainfall over Lamto and is considered as the best indicator of biomass fires impact on rainfall chemical composition [
Years | Wet season | Dry season | |||||||
---|---|---|---|---|---|---|---|---|---|
%OAa | %FAb | %AAc | %AOXd | %AOa | %FAb | %AAc | %AOXd | ||
1998 | 46.29 | 23.78 | 14.13 | 8.37 | 33.04 | 12.23 | 1.48 | 19.32 | |
1999 | 37.72 | 20.80 | 12.40 | 4.52 | 36.07 | 25.91 | 5.88 | 4.28 | |
2000 | 45.86 | 26.07 | 12.91 | 6.89 | 51.24 | 32.62 | 12.35 | 6.26 | |
2001 | 57.18 | 33.24 | 14.51 | 9.43 | 44.36 | 27.63 | 8.48 | 8.25 | |
2002 | 57.48 | 34.40 | 15.93 | 7.15 | 53.51 | 32.31 | 14.80 | 6.40 | |
2003 | 51.11 | 29.82 | 12.45 | 8.84 | 45.26 | 26.84 | 11.20 | 7.23 | |
2004 | 47.27 | 25.43 | 15.03 | 6.81 | 54.26 | 31.97 | 15.41 | 6.89 | |
Mean | 48.99 | 27.65 | 13.91 | 7.43 | 45.36 | 27.07 | 9.94 | 8.38 | |
Standard deviation | 6.96 | 5.02 | 1.36 | 1.64 | 8.36 | 7.12 | 5.02 | 4.98 | |
aOrganic acids; bFormic acid; cAcetic acid; dOxalic acid.
Ions | Amazonia (pulme) [X], ppt | Lamto (rainfall) [X], µéq/L | Amazonia (plume) [X]/[Nitrate] | Lamto (rainfall) [X]/[Nitrate] | ||
---|---|---|---|---|---|---|
Dry | Dry | Wet | Dry | Dry | Wet | |
Formate | 2.80 | 23.50 | 13.40 | 0.23 | 0.91 | 1.24 |
Acetate | 2.00 | 8.50 | 5.70 | 0.16 | 0.33 | 0.53 |
Nitrate | 302.00 | 25.80 | 10.80 | 1.00 | 1.00 | 1.00 |
Oxalate | 97.00 | 2.80 | 1.70 | 0.32 | 0.11 | 0.16 |
Years | Wet season | Dry season | |||
---|---|---|---|---|---|
Formate [%] | Acetate [%] | Formate [%] | Acetate [%] | ||
1995 | 16.43 | 40.00 | 28.75 | 80.00 | |
1996 | 17.69 | 40.00 | 28.75 | 80.00 | |
1997 | 16.43 | 26.67 | 28.75 | 76.67 | |
1998 | 23.00 | 40.00 | 25.56 | 76.67 | |
1999 | 28.75 | 32.00 | 38.33 | 80.00 | |
2000 | 20.90 | 32.00 | 17.69 | 32.00 | |
2001 | 13.52 | 22.86 | 28.75 | 76.67 | |
2002 | 15.33 | 22.86 | 19.17 | 32.00 | |
2003 | 38.33 | 26.67 | 28.75 | 76.67 | |
2004 | 32.86 | 26.67 | 17.69 | 22.86 | |
1995-2004 | 18.56 | 30.32 | 25.30 | 48.56 | |
West African monsoon is well established over the continent, south-western may transport plumes from southern hemisphere and central part of Africa towards West Africa [
Fires contribution to increase in organic acidity during the dry season compare to the wet season is about 7% and 18% for formic and acetic acid respectively. This increment may be attributed to rainfall deficit at this period of the year, which is important and leads to very weak drought indexes (0.12 to 0.45). Thus, vegetation particularly grass wither due to dryness in savannah region is favourable to biomass fires.
Dry and wet season’s frequency, as well as the drought of the dry season and its length has important impact on biomass fires and organic acidity since its play an important role in biomass availability. Thus, assuming that each year with a long drought and preceded by a relative wet year, meaning with wetter days than the current year, the risk of having more and intense fire is higher than another year in the same conditions but with longer sweet dry season. As illustration, the beginning of dry season and biomass fires contribution in 1999. Generally, in Lamto, the big dry season start in November/December and end in March/April of the following year. However, in 1999, November and December 1998 were particularly wet with 1.2 as drought index, favouring vegetation growth, in opposition to January-February-March 1999 where the drought index is close to 0 (severe drought), associated more contribution of biomass fires to formic (from 23% to 26%) and acetic (from 40% to 80%) acids.
Finally, the presents results are similar to those obtained in Dominka forest in northern Congo [
The present study results show that formic and acetic acids in the atmosphere of Lamto from January 1995 to December 2004 confirmed the important role of two acids in the acid rainfall with a contribution estimated between 45% and 50% of the total acidity. Moreover, it is important to underline that biomass fires contribute from 18.9% to 30.3% to acid emission for formic and acetic acids respectively during wet season. During the dry season, it is estimated to be 25.3% for formic acid and 48.6% for acetic acid emission contribution from biomass fires. This aspect therefore puts a significant emphasis on the role of biomass burning in the atmospheric chemistry of African tropical regions. Ultimately, it would be interesting to carry out in situ measurements of organic acids and volatile organic compounds such as terpenes, isoprene and aldehydes in order of better understanding of the contributions of others sources such as vegetation and photochemical reactions.
Pêlèmayo Raoul Touré,Georges Kouame Kouadio,Urbain Kouakou Koffi,Charles Romaric Beugré, (2016) Study of Formic and Acetic Acids in the Air of Humid Savannah Case of Lamto (Côte d’Ivoire). Atmospheric and Climate Sciences,06,254-266. doi: 10.4236/acs.2016.62021