EXCHANGE OF ATMOSPHERIC FORMIC AND ACETIC-ACIDS WITH TREES AND CROP PLANTS UNDER CONTROLLED CHAMBER AND PURIFIED AIR CONDITIONS

We investigated the exchange of formic and acetic acids between the at mosphere and various tree species such as beech (Fagus sylvatica L.), ash (Fraxinus excelsior L.), spruce (Picea abies L.) Karst, helm oak ( Quercus ilex L.), and birch (Betula pendula L.). and some crop-plant s pecies such as corn (Zea mays, var. Banjo), pea (Pisum sativum, var. S olara), barley (Hordeum vulgare, var. Igri) and oat (Avena sativa, var . Wiesel). All experiments were done with dynamic enclosures flushed w ith purified oxidant-free air, containing only low or controlled amoun ts of the two acids. Significant and light-triggered emission of both acids from all tree species was observed. For one tree species (ash) a seasonal large increase in fall due to early leaf decomposition was f ound. The standard emission Factors (30 degrees C and PAR = 1000 mu mo l m(2) s(-1)) given as (nmol m(-2) min(-1)) for acetic and formic acid s, respectively, were 8.1 and 29.7 (ash, autumn), 1.0 and 3.3 (ash, su mmer), 0.9 and 1.4 (beech), 0.7 and 1.45 (spruce), 1.9 and 2.4 (Holm o ak) and 1.7 and 6.7 (birch). Rough estimation of global annual emissio ns range between 20 and 130 Gmol formic acid and 10 and 33 Gmol acetic acid. These numbers reflect a 15-30% contribution by forest emissions to the continental organic acid budget. As compared to the global tot al NMHC emissions low molecular weight organic acids are of minor impo rtance. In contrast to the trees, none of the crop-plant species inves tigated showed an emission, but always a clear deposition of both acid s. Both emission from trees as well as uptake by the agricultural plan ts could be related to transpiration rates and leaf conductances. (C) 1998 Elsevier Science Ltd. All rights reserved.