Summer ammonia measurements in a densely populated Mediterranean city
Real-time measurements of ambient concentrations of gas-phase ammonia (NH 3) were performed in Barcelona (NE Spain) in summer between May and September 2011. Two measurement sites were selected: one in an urban background traffic-influenced area (UB) and the other in the historical city centre (CC). Levels of NH 3 were higher at CC (5.6 ± 2.1 μg m −3 or 7.5 ± 2.8 ppbv) compared with UB (2.2 ± 1.0 μg m −3 or 2.9 ± 1.3 ppbv). This difference is attributed to the contribution from non-traffic sources such as waste containers, sewage systems, humans and open markets more dense in the densely populated historical city centre. Under high temperatures in summer these sources had the potential to increase the ambient levels of NH 3 well above the urban-background-traffic-influenced UB measurement station. Measurements were used to assess major local emissions, sinks and diurnal evolution of NH 3. The measured levels of NH 3, especially high in the old city, may contribute to the high mean annual concentrations of secondary sulfate and nitrate measured in Barcelona compared with other cities in Spain affected by high traffic intensity. Ancillary measurements, including PM 10, PM 2.5, PM 1 levels (Particulate Matter with aerodynamic diameter smaller than 10 μm, 2.5 μm, and 1 μm), gases and black carbon concentrations and meteorological data, were performed during the measurement campaign. The analysis of specific periods (3 special cases) during the campaign revealed that road traffic was a significant source of NH 3. However, its effect was more evident at UB compared with CC where it was masked given the high levels of NH 3 from non-traffic sources measured in the old city. The relationship between SO 42− daily concentrations and gas-fraction ammonia (NH 3/(NH 3 + NH 4+)) revealed that the gas-to-particle phase partitioning (volatilization or ammonium salts formation) also played an important role in the evolution of NH 3 concentration in summer in Barcelona.