Molecular and physical characteristics of aerosol at a remote free troposphere site: implications for atmospheric aging

摘要

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. Ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry was used to determine the detailed molecular composition of the samples. Thousands of molecular formulas were assigned to each of the individual samples. On average similar to 60 % of the molecular formulas contained only carbon, hydrogen, and oxygen atoms (CHO), similar to 30 % contained nitrogen (CHNO), and similar to 10 % contained sulfur (CHOS). The molecular formula compositions of the two wildfire-influenced aerosol samples transported mainly in the free troposphere had relatively low average O/C ratios (0.48 +/- 0.13 and 0.45 +/- 0.11) despite the 7-10 days of transport time according to FLEXPART. In contrast, the molecular composition of the North American out- flow transported mainly in the boundary layer had a higher average O/C ratio (0.57 +/- 0.17) with 3 days of transport time. To better understand the difference between free tropospheric transport and boundary layer transport, the meteorological conditions along the FLEXPART simulated transport pathways were extracted from the Global Forecast System a