Aromatic hydrocarbons at urban, sub-urban, rural (8 degrees 52 ' N; 67 degrees 19 ' W) and remote sites in Venezuela

Using the novel on-line proton transfer reaction mass spectrometry (PTR-MS) technique, atmospheric concentrations of benzene, toluene, xylenes, and Cg -benzenes were measured in Caracas (urban), Altos de Pipe (suburban), Calab ozo, (rural) and Parupa (remote), during various campaigns in 1999 and 2000 . Average daytime mixing ratios measured in Caracas are 1.1, 3.2, 3.7, and 2. 7 nmol/mol for benzene, toluene, xylenes, and Cg-benzenes. At the sub-urban site, located only few km from Caracas, relatively low levels (similar to 20% of the levels measured in Caracas) of these aromatic hydrocarbons were observed. At the rural site during the dry season, higher concentrations of benzene ( 0.15 nmol/mol) were recorded, whereas those of toluene (0.08 nmol/mol) were lower during that time. The aromatic hydrocarbon ratios in the wet season (benzene: 0.08 nmol/mol; toluene: 0.09 nmol/mol) are consistent with an age d urban plume, whereas biomass burning emissions dominate during the dry se ason. From rural and urban [benzene]/[toluene] ratios a mean HO concentrati on of 2.6 x 10(6) molecules/cm(3) was estimated during the wet season. This value must be considered an overestimate because it does not account for b ackground concentrations which are likely for benzene and toluene. At the remote "La Gran Sabana" region (Parupa) very low mixing ratios (0.03 1 and 0.015 nmol/mol for benzene and toluene) are showing the pristine regi on to be unaffected by local sources. From the [benzene]/[toluene] ratio we deduced, that "urban" air arriving from the coastline (350 km) is likely m ixed with air containing some background of benzene and toluene. Urban emissions (automobiles) should be the major source of aromatic compou nds, however, during the dry season biomass burning seems to make an import ant contribution. (C) 2001 Elsevier Science Ltd. All rights reserved.