Hydrogen sulfide (H2S) in urban ambient air

Despite indications of high hydrogen sulfide levels in some urban environments, only sparse measurements have been reported in the literature. Here we present one full year of hydrogen sulfide measurements in an urban traffic site in the city of Thessaloniki, Greece. In this 1-million-population city the H2S concentrations were surprisingly high, with a mean annual concentration of 8 μg m−3 and wintertime mean monthly concentrations up to 20 μg m−3 (12.9 ppb). Daily mean concentrations in the winter were up to 30 μg m−3 (19.3 ppb), while hourly concentrations were up to 54 μg m−3 (34.8 ppb). During calm (wind velocity < 0.5 m s−1) conditions, mainly encountered during night-time hours, hourly values of H2S were highly correlated with those of CO (r2 = 0.75) and SO2 (r2 = 0.70), pointing to a common traffic source from catalytic converters. Annual mean concentrations are above the WHO recommendation for odor annoyance; hence, H2S might play a role to the malodorous episodes that the city occasionally experiences. The high ambient H2S levels might also be relevant to the implementation of preservation efforts for outdoor marble and limestone historical monuments that have been targeting SO2 emissions as an atmospheric acidity source, since the measurements presented here suggest that about 19% of the annual sulfur (SO2 + H2S) emissions in Thessaloniki are in the form of H2S.