Black carbon and molecular characterization of free tropospheric aerosol in the North Atlantic at the Pico mountain observatory

Free tropospheric aerosol in the North Atlantic was studied at the Pico Mountain Observatory located on the Pico volcano in the Azores, Portugal (2225 m asl; 38.47°N, 28.40°W). Overall the aerosol chemical, morphological and physical properties indicate a predominance of biomass burning emissions during polluted North American outflows. Black carbon (BC) equivalent mass concentrations have been measured at the station since 2001 with a 7-wavelength aethalometer and were analysed to study seasonal and diurnal variations. There was a large day-to-day variability in the BC values due to varied meteorological conditions that resulted in different diurnal patterns for different months. The daily mean BC at this location ranged between 0 and ~430 ngm-3, with the most frequent values occurring in the range 0-100 ngm-3. The BC values exhibited a consistent annual trend being lower in winter months and higher in summer months, barring year to year variations. Recently, additional aerosol properties were studied using a set of four high-volume samplers for the chemical analysis of aerosol, a 3-wavelength nephelometer to measure aerosol light scattering and backscattering fraction, a two channel optical particle counter and a sequential sampler to collect aerosol for electron microscopy analysis. Summertime daily concentrations of free tropospheric organic carbon, elemental carbon, water-soluble organic carbon (WSOC), anions and cations were measured from the high-volume filter samples. Selected WSOC samples from September 2012 were further analysed using ultrahigh resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). In this analysis, an increase in aerosol loading was linked to biomass burning emissions using detailed molecular properties and markers associated with the aerosol despite the long-range transport from North America consistent with previous observations