Molecular characterization of free tropospheric aerosol collected at the Pico Mountain Observatory: a case study with long range transported biomass burning plumes
Authors: Dzepina K. , Mazzoleni C. , Fialho P. , China S. , Zhang B. , Owen R. C. , Helmig D. , Hueber J. , Kumar S. , Perlinger J. A. , Kramer L. , Dziobak M. P. , Ampadu M. T. , Olsen S. , Wuebbles D. J. , Mazzoleni L. R. 
Published: September 2014

Abstract. Free tropospheric aerosol was sampled at the Pico Mountain Observatory located at 2225 m a.m.s.l. on Pico Island of the Azores archipelago in the North Atlantic. The observatory (38°28'15'' N; 28°24'14'' W) is located ∼3900 km east and downwind of North America, which enables studies of free tropospheric air transported over long distances, mainly from North America. Aerosol samples collected on filters from June to October 2012 were analyzed to characterize organic carbon, elemental carbon and inorganic ion species. The average ambient concentration of aerosol was 0.9 μg m−3; on average organic aerosol contributes the majority of mass (57%), followed by sulfate (21%) and nitrate (17%). Filter-collected aerosol measurements were positively correlated (with an r2 ≥ 0.80) with continuous aerosol measurements of black carbon, aerosol light scattering and number concentration. Water-soluble organic carbon (WSOC) species extracted from two aerosol samples (9/24 and 9/25) collected consecutively during a pollution event were analyzed using ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry. FLEXPART retroplume analysis shows the sampled air masses were very aged (average plume age > 12 days). Approximately 4000 molecular formulas were assigned to each of the mass spectra in the range of m/z 100–1000. The majority of the assigned molecular formulas have unsaturated structures with CHO and CHNO elemental compositions. These aged WSOC compounds have an average O / C ratio of ∼0.45, which is relatively low compared to O / C ratios of other aged aerosol and might be the result of evaporation and increased fragmentation during long-range transport. The increase in aerosol loading during the measurement period of 9/24 was linked to biomass burning emissions from North America by FLEXPART retroplume analysis and Moderate Resolution Imaging Spectroradiometer (MODIS) fire counts. This was confirmed with biomass burning markers detected in WSOC species and with the morphology and mixing state of particles as determined by scanning electron microscopy. The presence of markers characteristic of aqueous-phase reactions of biomass burning phenolic species suggests that the aerosol collected at the Pico Mountain Observatory had undergone cloud processing before reaching the site. Finally, the air masses on 9/25 were more aged (∼15 days) and influenced by marine emissions, as indicated by organosulfates and other species characteristic for marine aerosol such as fatty acids. The change in air masses for the two samples was corroborated by the changes in ozone and the non-methane hydrocarbons ethane and propane, morphology of particles, as well as by the FLEXPART retroplume simulations. This manuscript presents the first detailed molecular characterization of free tropospheric aged aerosol intercepted at a lower free troposphere remote location in the North Atlantic.