Abstract. Aerosol properties are transformed by atmospheric processes during long-range transport and play a key role in the Earth's radiative balance. To understand the molecular and physical characteristics of free tropospheric aerosol, we studied samples collected at the Pico Mountain Observatory in the North Atlantic. The observatory is located in the marine free troposphere at 2225 m above sea level, on Pico Island in the Azores archipelago. The site is ideal for the study of long-range transported free tropospheric aerosol with minimal local influence. Three aerosol samples with elevated organic carbon concentrations were selected for detailed analysis. FLEXPART retroplumes indicated that two of the samples were influenced by North American wildfire emissions transported in the free troposphere and one by North American outflow mainly transported within the marine boundary layer. To determine the molecular composition of the samples, we used ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry. Thousands of molecular formulas were assigned to each of the individual samples. On average ~ 60 % of the molecular formulas contained only carbon, hydrogen, and oxygen atoms (CHO), ~ 30 % contained nitrogen (CHNO), and ~ 10 % contained sulfur (CHOS). The molecular formula composition of the two wildfire influenced aerosol samples transported mainly in the free troposphere had relatively low average O/C ratios (0.46 ± 0.12 and 0.47 ± 0.15) despite 7–10 days of transport time according to FLEXPART. However, the molecular composition of the North American outflow transported mainly in the boundary layer had a higher average O/C ratio (0.55 ± 0.19) with 3 days of transport time. Thus, aerosol oxidation appears to be related to environmental factors during transport and not simply aging time. Meteorological conditions extracted from GFS analysis for model grids along the FLEXPART simulated transport pathways and the observed molecular chemistry were used to predict the phase state of the aerosol samples, indicating that the aerosol transported in the free troposphere was more likely to be solid and therefore less oxidized than the aerosol transported in the boundary layer. This suggests that biomass burning emissions injected into the free troposphere are longer-lived than emissions in the boundary layer.
THE APLACOPHORAN FAMILY PROCHAETODERMATIDAE IN THE NORTH AMERICAN BASIN, INCLUDING CHEVRODERMA N.G. AND SPATHODERMA N.G. (MOLLUSCA; CHAETODERMOMORPHA)