Long-term measurements of light hydrocarbons (C-2-C-5) at Schauinsland (Black Forest)

Long-term measurements of light hydrocarbons (C-2-C-5 HC's) were performed in the course of the EUROTRAC sub-project TOR (Tropospheric Ozone Research) in the southern part of the Black Forest in southwest Germany. The measurements cover a time period of five years (January 1989-January 1994) and the air samples were analyzed onsite by an automated GC-system. Pronounced annual cycles with maxima in late winter and minima in late summer were observed in the case of the slowly reacting hydrocarbons ethane, propane and acetylene, reflecting the fact that the seasonal variation of these species is photochemically driven. For the shorter lived compounds the seasonal variations are considerably weaker, connected with a stronger scatter of the individual measurements, which is caused by different distances to their main sources for different wind vectors as well as by varying source strengths. From a detailed characterization of the hydrocarbon patterns the influence of two different sources could be distinguished. An extrapolation to photochemical age of zero and completion of our data with those from a speciated VOC inventory yields an estimated [VOC]/NOx source ratio for Schauinsland of approximate to 5 [ppbC/ppb]. Comparable [VOC]/NOx ratios are observed in automobile exhaust gases under low speed conditions, which points to the important role of traffic under conditions, when freshly polluted air masses from a near-by city are advected to the site. From an investigation of the photochemical age of the advected air masses it becomes clear that there must exist biogenic sources of light olefins in the vicinity of the observatory during the vegetation period. For propene and the butenes we are able to estimate a lower limit of their contributions in terms of reactivity to the total reactivity (Sigma[HC](i).k(OH)(i), i = C-2-C-5) of the measured hydrocarbons. For lowest pollution levels in summer (acetylene <300 ppt, about 40% of the summer values) this source is found to be responsible for 15-20% of the total C-2-C-5 reactivity observed at Schauinsland. On the average, this source accounts for 5-10% of the total C-2-C-5 reactivity.