Winter and summer characterization of biogenic enantiomeric monoterpenes and anthropogenic BTEX compounds at a Mediterranean Stone Pine forest site

Measurements of biogenic gases including enantiomeric monoterpenes and isoprene, and anthropogenic gases such as benzene, toluene, ethylbenzene, ortho-, meta- and para- xylene (BTEX) compounds were made by GC-MS in November and December 2008 within a stone pine (Pinus pinea L) forest located on the Southwest coast of Spain (37.10°N, 6.70°W). Mixing ratios of the biogenic species were found to be low (mean circa 10 pptv) consistent with previously observed low wintertime regional forest emission rates. In contrast, anthropogenic species were significantly higher (mean 10–156 pptv), the dominant emissions originating from the city of Huelva and associated petrochemical activities, located 25 km north west of the measurement site. In wintertime the monoterpene (?)-α-pinene was found to be in slight enantiomeric excess over (+)-α-pinene at night but by day the measured ratio was closer to one i.e. racemic. Samples taken the following summer in the same location showed much higher monoterpene mixing ratios and revealed a strong enantiomeric excess of (?)-α-pinene. This indicates a strong seasonal variance in the enantiomeric emission ratio which is not manifested in the day/night temperature cycles in wintertime. Mixing ratios of the xylene isomers (meta- and para-) and ethylbenzene, which are all well resolved on the beta-cyclodextrin column, were exploited to estimate average OH radical exposures to VOCs from the Huelva industrial area. These were compared to empirical estimates of OH based on JNO₂ measured at the site. The deficiencies of each estimation method are discussed.