Ammonia and ethene are pollutants which are arousing concern as regards their environmental impact, e.g. as greenhouse gases. Road traffic is an increasingly important emission source for these gases. As part of an atmospheric pollution measurement campaign, we performed in situ measurements of NH_3, C_2H_4 and CO_2 concentrations at the mouth of a freeway tunnel with a time resolution of 1 min using a mobile laser-based photoacoustic system. Measurements were performed over a period of 5 weeks. In good temporal correlation with traffic counts, we observed peak concentrations of >600 ppb (> 1200 ppb for Friday afternoon peaks) for NH_3, of up to 400 ppb for C_2H_4 and > 2000 ppm for CO_2. Preliminary comparisons with previous measurements at the same location indicate a considerable increase in ammonia and CO_2 peak concentrations (greater than the increase in traffic over the same period) and a less pronounced increase in C_2H_4 concentrations.
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