Continuous atmospheric boundary layer observations in the coastal urban area of Barcelona, Spain

Continuous measurements of Surface Mixed Layer (SML), Decoupled Residual/Convective Layer (DRCL) and aerosol backscatter coefficient were performed within the Barcelona (NE Spain) boundary layer from September to October 2010 (30 days) in the framework of the SAPUSS (Solving Aerosol Problems Using Synergistic Strategies) field campaign. Two near-infrared ceilometers (Jenoptik CHM15K) vertically and horizontally-probing (only vertical profiles are discussed) were deployed during SAPUSS and compared with potential temperature profiles measured by daily radiosounding (midnight and midday) to interpret the boundary layer structure in the urban area of Barcelona. Ceilometer-based DRCL (1761±363 m a.g.l.) averaged over the campaign duration were twice as high as the mean SML (904±273 m a.g.l.) with a marked SML diurnal cycle. The overall agreement between the ceilometer-retrieved and radiosounding-based SML heights (<i>R</i>²=0.8) revealed overestimation of the SML by the ceilometer (Δ<i>h</i>=145±145 m). After separating the data in accordance with different atmospheric scenarios, the lowest SML (736±183 m) and DRCL (1573±428 m) were recorded during warm North African (NAF) advected air mass. By contrast, higher SML and DRCL were observed during stagnant regional (REG) (911±234 m and 1769±314 m, respectively) and cold Atlantic (ATL) (965±222 m and 1878±290 m, respectively) air masses. The SML during the NAF scenario frequently showed a flat upper boundary throughout the day because of strong winds from the Mediterranean Sea that limit the midday SML convective growth observed during ATL and REG scenarios. The mean backscatter coefficients were calculated at two selected heights as representative of middle and top SML portions, i.e. β₅₀₀=0.59±0.45 M m⁻¹ sr⁻¹ and β₈₀₀=0.87±0.68 M m⁻¹ sr⁻¹ at 500 m and 800 m a.g.l., respectively. The highest backscatter coefficients were observed during NAF (β₅₀₀=0.77±0.57 M m⁻¹ sr⁻¹) when compared with ATL (β₅₀₀= 0.51±0.44 M m⁻¹ sr⁻¹) and REG (β₅₀₀= 0.64±0.39 M m⁻¹ sr⁻¹). <br><br> The relationship between the vertical change in backscatter coefficient and atmospheric stability (∂θ/∂<i>z</i>) was investigated in the first 3000 m a.g.l., demonstrating a positive correlation between unstable conditions and enhanced backscatter and vice versa.