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

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. 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 ∼ 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 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 analysis for the model grids. We used the extracted meteorological conditions and the observed molecular chemistry to predict the relative-humidity-dependent glass transition temperatures (T g ) of the aerosol components. Comparisons of the T g to the ambient temperature indicated that a majority of the organic aerosol components transported in the free troposphere were more viscous and therefore less susceptible to oxidation than the organic aerosol components transported in the boundary layer. Although the number of observations is limited, the results suggest that biomass burning organic aerosol injected into the free troposphere is more persistent than organic aerosol in the boundary layer having broader implications for aerosol aging.

Atmospheric Research Over the Western North Atlantic Ocean Region and North American East Coast: A Review of Past Work and Challenges Ahead

“…Understanding the role of clouds and humidity in impacting SOA formation and evolution has received scarce attention over the WNAO, in contrast to other regions such as the northeastern Pacific Ocean (Sorooshian et al, ; Sorooshian et al, ). Schum et al () showed that the oxygen‐to‐carbon (O:C) ratio for wildfire‐influenced aerosols (~0.45–0.48) transported over 7–10 days in the free troposphere to the PICO Mountain Observatory in the North Atlantic was lower compared to that of North American outflow aerosols transported mainly through the MBL over only three days (~0.57). While the O:C ratio difference was attributed to the free tropospheric particles being more viscous and thus less vulnerable to oxidation than the MBL aerosols, it is worth considering in future model‐measurement studies how enhanced humidity and cloudiness in the WNAO MBL could enhance the O:C ratio via aqueous oxidation pathways (Ervens et al, ; Ervens et al, ).…”

Relative-humidity-dependent organic aerosol thermodynamics via an efficient reduced-complexity model

“…The relatively large variability among the miscibility limits in terms of O : C ratio emphasizes the importance of distinguishing among different types of oxygen-bearing functional groups. In the case of ambient and laboratory-generated aerosol mixtures containing inorganic salts, the transition from LLPS to completely miscible (at any composition) spans a O : C ratio range from 0.4 to 0.8 based on experimental data (Song et al, 2012;You et al, 2014;You and Bertram, 2015). That O : C range is comparable to the difference between a hydroperoxide molecule with a molar mass of 100 g mol −1 vs. 400 g mol −1 .…”

Quantifying organic matter and functional groups in particulate matter filter samples from the southeastern United States – Part 1: Methods

“…The residual OM was calculated by subtracting the weighted sum of the major inorganic chemical constituents and elemental carbon from PM 2.5 mass for each SEARCH sample using the equation described by Hand et al (2012). A particle water correction was made (Dabek-Zlotorzynska et al, 2011;Simon et al, 2011). Metrics used between measured and reference OM or OC were coefficient of determination R 2 , bias-corrected error (also known as the median absolute deviation, as previously described by Weakley et al, 2016), and orthogonal least-squares regression slope.…”

“…Other methods with involved chemical analyses of discrete filter samples have been used to estimate OM concentration. These include multiple linear regression of aerosol constituents using various analytical techniques (Hand et al, 2019;Malm and Hand, 2007;Simon et al, 2011), extrapolation from gas chromatography-mass spectrometry of extracts (Turpin and Lim, 2001), infrared absorption spectrometry of extracts (Polidori et al, 2008), or thermal-optical and gravimetric analyses of extracts (El-Zanan et al, 2009). However, each of these methods is subject to specific limitations.…”