Organic molecular tracers in atmospheric PM1 at urban intensive traffic and background sites in two high-insolation European cities

Atmospheric PM1 has an impact on health and the environment, and is highly variable. PM1 filter samples were collected in urban roof and street sites in two Mediterranean cities in Europe (Barcelona and Madrid) to study the spatial and temporal trends of primary and secondary organic tracer compounds. Both cities are exposed to modern urban emission sources with intensive traffic in their centers. Moreover, their geographical situation in the Mediterranean region favors photo-chemical reactions and accumulation of secondary aerosols, due to sunny anti-cyclonic weather conditions. Barcelona is located along the coast with high relative humidity and an active sea-mountain breeze system, while the relative humidity in Madrid is low in summer, due to its location in the center of the Iberian Peninsula. These differences may influence the accumulation of primary air pollutants and the formation of secondary aerosols. In street site samples, the levels of primary organics, such as polycyclic aromatic hydrocarbons, hopanes and nicotine, were about two times higher than those collected simultaneously at roof sites. Primary emitted levoglucosan and isomers from biomass burning were observed in higher concentrations in winter in both urban areas, but without significant difference between the road and roof sites samples, indicating regional transport of biomass smoke to the cities. Similar roof and street site sample concentrations were also observed for secondary organic aerosol (SOA) compounds, i.e. dicarboxylic acids and oxidation products of isoprene and alpha-pinene. The SOA compound concentrations were ten times higher in summer compared to winter samples, emphasizing the importance of SOA formation processes in summer. More evidence was obtained that linked the presence of cis-pinonic acid in the filter samples to new particle formation during nucleation. The urban organic PM1 is dominated by SOA, while the primary organic aerosol (POA) tracer compounds from combustion sources, including traffic and biomass burning, dominates in winter.