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 adsorbent tube sampling, thermodesorption, cryo-trapping and analysis by high-resolution gas chromatography with electron capture detection. The mean concentration of chloroform in the top soil–air was in the range of 0.4–2.3 ng l−1. Compared to ambient air, the chloroform concentration in the topsoil was 6.7, 4.3 and 4.6 times higher for spruce forest, beech forest and grassland, respectively. For tetrachloromethane, 1,1,1-trichloroethane, trichloroethene and tetrachloroethene, the concentrations were in the same order of magnitude (0.04–1.2 ng l−1). However, the concentration ratios between soil air and ambient air were close to unity. Release studies in the topsoil of the spruce forest showed an increase of the chloroform concentration of approximately 4 times the initial soil air concentration after 38 h, while the concentrations of the other volatile chlorinated compounds investigated remained fairly constant. The observed chloroform concentration profiles and release rates may indicate a biogenic formation of chloroform in the upper soil layer of spruce forests, whereas an anthropogenic origin is suggested for the other chlorinated compounds investigated. From the release study and concentration gradient measurements in the spruce forest soil, chloroform release to the atmosphere was calculated for northern temperate regions. 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.