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. Aerosolproperties are transformed by atmospheric processes during long-rangetransport and play a key role in the Earth's radiative balance. To understandthe molecular and physical characteristics of free tropospheric aerosol, westudied samples collected at the Pico Mountain Observatory in the NorthAtlantic. The observatory is located in the marine free troposphere at2225 m above sea level, on Pico Island in the Azores archipelago. The siteis ideal for the study of long-range-transported free tropospheric aerosolwith minimal local influence. Three aerosol samples with elevated organiccarbon concentrations were selected for detailed analysis. FLEXPARTretroplumes indicated that two of the samples were influenced by NorthAmerican wildfire emissions transported in the free troposphere and one byNorth American outflow mainly transported within the marine boundary layer.Ultrahigh-resolution Fourier transform ion cyclotron resonance massspectrometry was used to determine the detailed molecular composition of thesamples. Thousands of molecular formulas were assigned to each of theindividual samples. On average ∼60  % of the molecular formulascontained only carbon, hydrogen, and oxygen atoms (CHO), ∼30  %contained nitrogen (CHNO), and ∼10  % contained sulfur (CHOS). Themolecular formula compositions of the two wildfire-influenced aerosolsamples transported mainly in the free troposphere had relatively low average O∕C ratios ( 0.48±0.13 and 0.45±0.11 ) despite the 7–10days of transport time according to FLEXPART. In contrast, the molecularcomposition of the North American outflow transported mainly in the boundarylayer had a higher average O∕C ratio ( 0.57±0.17 ) with 3 days oftransport time. To better understand the difference between free tropospherictransport and boundary layer transport, the meteorological conditions alongthe FLEXPART simulated transport pathways were extracted from the GlobalForecast System analysis for the model grids. We used the extractedmeteorological conditions and the observed molecular chemistry to predict therelative-humidity-dependent glass transition temperatures ( Tg ) ofthe aerosol components. Comparisons of the Tg to the ambienttemperature indicated that a majority of the organic aerosol componentstransported in the free troposphere were more viscous and therefore lesssusceptible to oxidation than the organic aerosol components transported inthe boundary layer. Although the number of observations is limited, theresults suggest that biomass burning organic aerosol injected into the freetroposphere is more persistent than organic aerosol in the boundary layerhaving broader implications for aerosol aging.

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