Origin, transport and processing of organic aerosols at different altitudes in coastal Mediterranean urban areas

Abstract. Organic molecular markers in atmospheric PM₁₀ were analysed by off-line GC-MS techniques in an urban background site (81 m above sea level (asl)) and in a nearby elevated sub-urban background site (415 m asl), in cold and warm periods in Barcelona; situated in the Western Mediterranean Basin. Previous studies reported similar PM concentrations and substantial organic matter contributions in both sites but did not analyze the organic molecular composition, which is expected to vary within the city's vertical airshed due to a weakening influence of local emission sources and enhanced influence of regional air masses. Multi-variant analysis of organic molecular marker concentrations, together with major air quality parameters (NO, NO₂, O₃, PM₁₀), resolved six components that represented primary emissions sources and secondary organic aerosol formation processes: 1) diurnal traffic 2) nocturnal traffic, 3) biomass burning, 4) biogenic with primary and secondary organic markers, 5) fresh secondary, and 6) regional secondary. Urban traffic emissions reached the elevated site during daytime through the sea-mountain breeze, while nocturnal traffic emissions accumulated in the nighttime urban atmosphere, when the two sites were often disconnected by temperature inversions. Biomass burning, dominant in the cold period, was the main contributor to toxic PAHs in these two background sites. Regional secondary organic aerosol contribution was more abundant in the elevated background site. Several SOA formation mechanisms were identified such as the oxidation of traffic emissions by NO_x, the aqueous-phase oxidation under high relative humidity, and formation of fresh SOA under conditions of low relative humidity.