Abstract. Free tropospheric aerosol was sampled at the Pico Mountain Observatory located at 2225 m above mean sea level on Pico Island of the Azores archipelago in the North Atlantic. The observatory is located ~ 3900 km east and downwind of North America, which enables studies of free tropospheric air transported over long distances. Aerosol samples collected on filters from June to October 2012 were analyzed to characterize organic carbon, elemental carbon, and inorganic ions. The average ambient concentration of aerosol was 0.9 ± 0.7 μg m−3. On average, organic aerosol components represent the largest mass fraction of the total measured aerosol (60 ± 51%), followed by sulfate (23 ± 28%), nitrate (13 ± 10%), chloride (2 ± 3%), and elemental carbon (2 ± 2%). Water-soluble organic matter (WSOM) extracted from two aerosol samples (9/24 and 9/25) collected consecutively during a pollution event were analyzed using ultrahigh-resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. 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 had unsaturated structures with CHO and CHNO elemental compositions. FLEXPART retroplume analyses showed the sampled air masses were very aged (average plume age > 12 days). These aged aerosol WSOM compounds had an average O/C ratio of ~ 0.45, which is relatively low compared to O/C ratios of other aged aerosol. 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 the WSOM 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 phenolic species suggests that the aerosol collected at the Pico Mountain Observatory had undergone cloud processing before reaching the site. Finally, the air masses of 9/25 were more aged and influenced by marine emissions, as indicated by the presence of organosulfates and other species characteristic of marine aerosol. The change in the air masses for the two samples was corroborated by the changes in ethane, propane, and ozone, morphology of particles, as well as by the FLEXPART retroplume simulations. This paper presents the first detailed molecular characterization of free tropospheric aged aerosol intercepted at a lower free troposphere remote location and provides evidence of low oxygenation after long-range transport. We hypothesize this is a result of the selective removal of highly aged and polar species during long-range transport, because the aerosol underwent a combination of atmospheric processes during transport facilitating aqueous-phase removal (e.g., clouds processing) and fragmentation (e.g., photolysis) of components.

Ten-year observations of trace gases at Pico Mountain Observatory (PMO), a free troposphere site in the central North Atlantic, were classified by transport patterns using the Lagrangian particle dispersion model, FLEXPART. The classification enabled identifying trace gas mixing ratios associated with background air and long- range transport of continental emissions, which were defined as chemical signatures. Comparison between the chemical signatures revealed the impacts of natural and anthropogenic sources, as well as chemical and physical processes during long transport,on air composition in the remote North Atlantic. Transport of North American anthropogenic emissions(NA-Anthro) and summertime wildfire plumes (Fire) significantly enhanced CO and O3 at PMO. Summertime CO enhancements caused by NA-Anthro were found to have been decreasing by a rate of 0.67 ± 0.60 ppbv/year in the ten-year period, due possibly to reduction of emissions in North America. Downward mixing from the upper troposphere and stratosphere due to the persistent Azores-Bermuda anticyclone causes enhanced O3 and nitrogen oxides. The d[O3]/d [CO] value was used to investigate O3 sources and chemistry in different transport patterns. The transport pattern affected by Fire had the lowest d [O3]/d [CO], which was likely due to intense CO production and depressed O3 production in wildfire plumes. Slightly enhanced O3 and d [O3]/d [CO] were found in the background air, suggesting that weak downward mixing from the upper troposphere is common at PMO. Enhancements of both butane isomers were found during upslope flow periods, indicating contributions from local sources. The consistent ratio of butane isomers associated with the background air and NA-anthro implies no clear difference in the oxidation rates of the butane isomers during long transport. Based on observed relationships between non-methane hydrocarbons, the averaged photochemical age of the air masses at PMO was estimated to be 11 ± 4 days.