Occurrence and formation of chloroform at Danish forest sites

Ambient air and soil air of spruce forest, beech forest and grassland from Zealand, Denmark, were investigated for volatile chlorinated compounds by a dsorbent tube sampling, thermodesorption, cryo-trapping and analysis by hig h-resolution gas chromatography with electron capture detection. The mean c oncentration of chloroform in the top soil-air was in the range of 0.4-2.3 ng 1(-1). Compared to ambient air, the chloroform concentration in the tops oil was 6.7, 4.3 and 4.6 times higher for spruce forest, beech forest and g rassland, respectively. For tetrachloromethane, 1,1,1-trichloroethane, tric hloroethene and tetrachloroethene, the concentrations were in the same orde r of magnitude (0.04-1.2 ng 1(-1)). However, the concentration ratios betwe en soil air and ambient air were close to unity. Release studies in the top soil of the spruce forest showed an increase of the chloroform concentratio n of approximately 4 times the initial soil air concentration after 38 h, w hile the concentrations of the other volatile chlorinated compounds investi gated remained fairly constant. The observed chloroform concentration profi les and release rates may indicate a biogenic formation of chloroform in th e upper soil layer of spruce forests, whereas an anthropogenic origin is su ggested for the other chlorinated compounds investigated. From the release study and concentration gradient measurements in the spruce forest soil, ch loroform release to the atmosphere was calculated for northern temperate re gions. The release was in the range of the annual anthropogenic chloroform emissions, and, therefore, the terrestrial environment can be considered as an important contributor to the atmospheric chloroform input. (C) 1999 Els evier Science Ltd. All rights reserved.