Molecular insights on aging and aqueous-phase processing from ambient biomass burning emissions-influenced Po Valley fog and aerosol

Matthew Brege,Stefano Decesari,Maria Cristina Facchini,Lynn R Mazzoleni,Marco Paglione,Stefania Gilardoni

doi:10.5194/acp-18-13197-2018

Abstract

Abstract. To study the influence of regional biomass burning emissions and secondary processes, ambient samples of fog and aerosol were collected in the Po Valley (Italy) during the 2013 Supersito field campaign. After the extent of fresh vs. aged biomass burning influence was estimated from proton nuclear magnetic resonance (1H NMR) and high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS), two samples of fog water and two samples of PM1 aerosol were selected for ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis. Molecular compositions indicated that the water-soluble organic matter was largely non-polymeric without clearly repeating units. The selected samples had an atypically large frequency of molecular formulas containing nitrogen and sulfur (not evident in the NMR composition) attributed to multifunctional organonitrates and organosulfates. Higher numbers of organonitrates were observed in aerosol, and higher numbers of organosulfates were observed in fog water. Consistent with the observation of an enhanced aromatic proton signature in the 1H-NMR analysis, the average molecular formula double-bond equivalents and carbon numbers were higher in the fresh biomass-burning-influenced samples. The average O : C and H : C values from FT-ICR MS were higher in the samples with an aged influence (O : C = 0.50–0.58, and H : C = 1.31–1.37) compared to those with fresh influence (O : C = 0.43–0.48, and H : C = 1.13–1.30). The aged fog had a large set of unique highly oxygenated CHO fragments in the HR-ToF-AMS, which reflects an enrichment of carboxylic acids and other compounds carrying acyl groups, highlighted by the NMR analysis. Fog compositions were more oxidized and SOA (secondary organic aerosol)-like than aerosols as indicated by their NMR measured acyl-to-alkoxyl ratios and the observed molecular formula similarity between the aged aerosol and fresh fog, implying that fog nuclei must be somewhat aged. Overall, functionalization with nitrate and sulfate moieties, in addition to aqueous oxidation, triggers an increase in the molecular complexity in this environment, which is apparent in the FT-ICR MS results. This study demonstrates the significance of the aqueous phase in transforming the molecular chemistry of atmospheric organic matter and contributing to secondary organic aerosol.

Highlights

Atmospheric organic aerosol particles are comprised of a complex mixture of numerous individual organic compounds, produced by direct emissions and secondary processes, of which a significant impact is from transformations in the aqueous phase
Aerosol samples were selected based on Positive matrix factorization (PMF) source apportionment of “fresh” and “aged” wood burning emissions using HRToF-AMS and 1H-NMR data, as described in Gilardoni et al (2016)
On 13 February 2013, a high concentration of secondary organic aerosol (SOA) was observed, where the ratio of SOA to POA was ∼ 4, and the aqueous SOA from biomass burning accounted for about 55 % of total SOA

Summary

Introduction

Atmospheric organic aerosol particles are comprised of a complex mixture of numerous individual organic compounds, produced by direct emissions and secondary processes, of which a significant impact is from transformations in the aqueous phase. Biomass burning products include simple organic acids, sugars and anhydrosugars, substituted phenols, polycyclic aromatic hydrocarbons, and other compounds, depending on the type of fuel and burn conditions (Mazzoleni et al, 2007; Pietrogrande et al, 2014a, b; Gilardoni et al, 2016) These water-soluble emissions can serve as precursors for SOA once dissolved in the aqueous phase (Chang and Thompson, 2010; Yu et al, 2014, 2016), and upwards of 50 % of organic matter in fog and cloud droplets remains unidentified (Herckes et al, 2013). Laboratory studies indicate that, in addition to hydrophilic species, even refractory “tar balls” emitted from smoldering biomass burning begin to absorb water at high relative humidity (Hand et al, 2005; Laskin et al, 2015)

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