Carboxylic monoacids in the air of mayombe forest (Congo): Role of the forest as a source or sink

Abstract

In the tropical rain forests of the Congo during the dry season, from June to September 1987, carboxylic acid partial pressures (Pga~) in the air above the canopy, at ground level, and at the boundary layer, were estimated from water samples such as fog and rainwater. The concentrations of these acids were also measured in the sap of tree leaves. Tree leaves act as a sink or as a source if the acid Pga~ is greater of lower than the acid concentrations in molecular form in sap. For each of these soluble gases, there is a value of Pga~ where the exchange is nul. This is called the compensation point. Values of the compensation point for some tree leaves were evaluated according to Henry's law. Henry's law coefficients at ppm levels were redetermined for formic (HCOOH), acetic (CH3COOH), propionic (CH3CH2COOH), and isobutyric (CH3CH(CH3)COOH) acids.By comparison of Pg~ and compensation points, it is concluded that the forest was a potential source for these acids. The soil -or the litter -acts as a significant source of a carboxylic acid of C 3 or C 4 atoms in the aliphatic chain. This carboxylic acid, not yet fully characterized, could play an important role in the rain acidity in forested zones of the equatorial areas.The direct emission of these carboxylic acids by vegetation was the main source in the boundary layer above the forest. The average emissions were 3.1 × 109, 7.8 × 109, and 8.4 x 109 molecules cm -2 s i for HCOOH, CH3COOH, and CH3CH2COOH, respectively. The savanna is an exogenous source of HCOOH and CH3CH2COOH during moderately rainy days for 30% of the time. The ozonolysis of isoprene seems to be a small source of HCOOH.

Natural emissions of non-methane volatile organic compounds, carbon monoxide, and oxides of nitrogen from North America

“…Reported emission rates for acetaldehyde, formaldehyde, acetic and formic acid, acetone, ethanol and butanone from apparently undamaged plant tissues range from less than 0.01 to about 0.5 gC g\ h\ (Talbot et al, 1990;Servant et al, 1991;MacDonald and Fall, 1993a;Konig et al, 1995;Kesselmeier et al, 1997;Kirstine et al, 1998). Considerably higher emissions, 0.1 to '10 gC g\ h\ have been reported for methanol (MacDonald and Fall, 1993b;Kirstine et al, 1998).…”

“…Each compound is assigned one of "ve emission capacities: 0.02, 0.04, 0.09, 0.16 or 0.6 g-C g\ h\. These emission capacities are based on measurements reported in the literature (Talbot et al, 1990;Servant et al, 1991;Arey et al, 1991a,b;MacDonald and Fall, 1993a,b;Konig et al, 1995;Goldstein et al, 1996;Kesselmeier et al, 1997;Kirstine et al, 1998;Helmig et al, 1999). The emission capacities for each compound emitted by processes other than DST and CHL are shown in Table 5 and sum to a total of 2 g-C g\ h\ which is 33% lower than the value assigned by Guenther et al (1994).…”

“…Subsequent studies have found that aromatic and alkane compounds appear to be a very minor component of total NMVOC emissions. In addition to the compounds included in Table 5, VOC that have been reported as vegetation emissions include alkanes (C to C ), alkenes (C to C ), ketones (pentanone, octanone, methyl heptanone), alcohols (methyl propanol, butanol, octanol), aldehydes (propanal, butanal and butenal), organic acids (pyruvic and propionic acid) and organic sulfur compounds (dimethyl sul"de, dimethyl disul"de, dipropenyldisul"de and methylpropenyldisul"de) (Isidorov et al, 1985;Lamb et al, 1987b;Talbot et al, 1990;Servant et al, 1991;Winer et al, 1992;Harbourne, 1993;Konig et al, 1995;Guenther et al, 1996b;Puxbaum and Konig, 1997). The pathways for biosynthesis of some of these compounds, e.g., pentane (Pattee et al, 1974) and heptane (Savage et al, 1996), have been described, but the processes leading to the emission of many of these compounds are unknown.…”

“…This can easily be incorporated into our modeling procedure, since stomatal conductance is calculated in order to estimate leaf temperature. Another factor to be considered is that the ambient and cellular concentrations of some of these compounds may be of a similar magnitude and the magnitude and direction of #ux may be dependent on ambient concentrations (Servant et al, 1991;Kesselmeier et al, 1997). This may require an emission activity algorithm that includes a compensation point where emission and deposition are in equilibrium.…”

Urban and rural observations of carboxylic acids in rainwater in Southwest of China: the impact of urbanization

“…On the contrary, the gas phase F/A ratio is more than 1 in Guiyang which may be caused by secondary sources in the atmosphere (Jordan et al 2009). Biomass burning 0.1∼0.5 (Talbot et al 1988) 0.2∼0.4 (Hartmann et al 1991) Vehicular emissions 0.4∼0.6 (Talbot et al 1988) 0.3∼0.5 (Grosjean 1992) Vegetation (tropical forests) 0.6 (Talbot et al 1990) Vegetation emissions 0.4 (Servant et al 1991) Secondary sources >1 (Talbot et al 1988) The unsaturated hydrocarbons (olefins, isoprene) are important factor to control the concentrations of carboxylic acids in the atmosphere which may originate from biogenic or anthropogenic sources (Lee et al 2006). …”