Atmospheric Environment 35 (2001) 4917–4927 Aromatic hydrocarbons at urban, sub-urban, rural (8152 0 N; 67119 0 W) and remote sites in Venezuela R. Holzinger a, *, B. Kleiss a , L. Donoso b , E. Sanhueza a,b a MPI f . ur Chemie, Luftchemie, Postfach 3060, 55020 Mainz, Germany b IVIC, Atmospheric Chemistry Laboratory, Apartado 21827, Caracas 1020A, Venezuela Received 5 December 2000; received in revised form 3 May 2001; accepted 21 May 2001 Abstract Using the novel on-line proton transfer reaction mass spectrometry (PTR-MS) technique, atmospheric concentrations of benzene, toluene, xylenes, and C 9 -benzenes were measured in Caracas (urban), Altos de Pipe (sub- urban), Calabozo (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 C 9 -benzenes. At the sub-urban site, located only few km from Caracas, relatively low levels (B20% 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 aged urban plume, whereas biomass burning emissions dominate during the dry season. From rural and urban [benzene]/[toluene] ratios a mean HO concentration of 2.6  10 6 molecules/cm 3 was estimated during the wet season. This value must be considered an overestimate because it does not account for background concentrations which are likely for benzene and toluene. At the remote ‘‘La Gran Sabana’’ region (Parupa) very low mixing ratios (0.031 and 0.015 nmol/mol for benzene and toluene) are showing the pristine region to be unaffected by local sources. From the [benzene]/[toluene] ratio we deduced, that ‘‘urban’’ air arriving from the coastline (350 km) is likely mixed with air containing some background of benzene and toluene. Urban emissions (automobiles) should be the major source of aromatic compounds, however, during the dry season biomass burning seems to make an important contribution. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Toluene; Benzene; Proton transfer reaction mass spectrometry; Traffic emissions; Biomass burning; HO radical 1. Introduction Aromatic hydrocarbons (AHs) are important consti- tuents of polluted urban atmospheres (e.g., Singh et al., 1985) and, represent a significant fraction of the non- methane hydrocarbons (NMHC) found in the tropo- sphere. Due to their reactivity with the HO radical they play an important role in tropospheric photochemistry and contribute to the formation of photochemical ozone in cities. All known important sources are anthropo- genic and mainly related with combustion processes. A significant fraction (10–25%) of gasoline and other fuel oils consist of aromatic hydrocarbons, which are also present in automobile exhaust (e.g., Bailey et al., 1990). Moreover, biomass burning is a large global source of AHs (Lobert et al., 1991; Holzinger, 1998). The AHs are mainly removed from the atmosphere by HO radicals and the age of the pollutants in an air mass coming from a pollution source (e.g., automobiles) can be estimated *Corresponding author. Tel.: +49-6131-305-464; fax: +49- 6131-305-436. E-mail address: holzing@mpch-mainz.mpg.de (R. Holzinger). 1352-2310/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII:S1352-2310(01)00286-2