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

Atmospheric organic matter is a complex mixture of thousands of individual organic compounds originating from a combination of primary emissions and secondary processes. 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) observations, 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. Over 4300 distinct molecular formulas were assigned to electrospray ionization FT-ICR MS anions and were sorted into four elemental groups (CHO, CHNO, CHOS and CHNOS) and 64 subclasses. Molecular weight distributions indicated that the water-soluble organic matter was largely non-polymeric without clearly repeating units, although some evidence of dimerization was observed for C10 compounds and especially for C8–9 CHNO species in the aged aerosol. 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. While higher numbers of organonitrates were observed in aerosol (dry or deliquesced particles), higher numbers of organosulfates were mostly found in fog water and so chemical reactions promoted by liquid water must be postulated for their formation. 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, whereas the average O : C and H : C values from FT-ICR MS were higher in the samples with an aged influence (O : C > 0.6 and H : C > 1.2). The aged fog had a large set of unique highly oxygenated CHO fragments in HR-TOF-AMS mass spectra, which reflects an enrichment of carboxylic acids and other compounds carrying acyl groups as highlighted by the NMR analysis. Fog compositions were more SOA-like than aerosols as indicated by the observed 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 trigger 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 to transform the molecular chemistry of atmospheric organic matter and contribute to secondary organic aerosol.

Atmospheric organic matter in clouds: exact masses and molecular formula identification using ultrahigh-resolution FT-ICR mass spectrometry

Clouds alter the composition of atmospheric aerosol by acting as a medium for interactions between gas- and particulate-phase substances. To determine the cloud water atmospheric organic matter (AOM) composition and study the cloud processing of aerosols, two samples of supercooled clouds were collected at the Storm Peak Laboratory near Steamboat Springs, Colorado (3220 m a.s.l.). Approximately 3000 molecular formulas were assigned to ultrahigh-resolution mass spectra of the samples after using a reversed-phase extraction procedure to isolate the AOM components from the cloud water. Nitrogen-containing compounds (CHNO compounds), sulfur-containing compounds (CHOS and CHNOS compounds) and other oxygen-containing compounds (CHO compounds) with molecular weights up to 700 Da were observed. Average oxygen-to-carbon ratios of ∼0.6 indicate a slightly more oxidized composition than most water-soluble organic carbon identified in aerosol studies, which may result from aqueous oxidation in the clouds. The AOM composition indicates significant influences from biogenic secondary organic aerosol (SOA) and residential wood combustion. We observed 60% of the cloud water CHO molecular formulas to be identical to SOA samples of α-pinene, β-pinene, d-limonene, and β-caryophyllene ozonolysis. CHNO compounds had the highest number frequency and relative abundances and are associated with residential wood combustion and NOx oxidation. Multiple nitrogen atoms in the assigned molecular formulas for the nighttime cloud sample composite were observed, indicating the significance of nitrate radical reactions on the AOM composition. Several CHOS and CHNOS compounds with reduced sulfur (in addition to the commonly observed oxidized sulfur-containing compounds) were also observed; however further investigation is needed to determine the origin of the reduced sulfur-containing compounds. Overall, the molecular composition determined using ultrahigh-resolution Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry provides an unambiguous identification of the cloud water organic anion composition in the Rocky Mountain area that could help to improve the understanding of aqueous-phase processes.

Atmospheric organic matter in clouds: exact masses and molecular formula identification using ultrahigh resolution FT-ICR mass spectrometry

Clouds alter the composition of atmospheric aerosol by acting as a medium for interactions between gaseous and particulate phase substances. To determine the cloud water atmospheric organic matter (AOM) composition and study the cloud processing of aerosols, two samples of supercooled clouds were collected at Storm Peak Laboratory near Steamboat Spring, Colorado (3220 m a.s.l.). Approximately 3000 molecular formulas were assigned to ultrahigh resolution mass spectra of the samples after using a reverse phase extraction procedure to isolate the AOM components from the cloud water. Nitrogen containing compounds (CHNO compounds), sulfur containing compounds (CHOS and CHNOS compounds) and other oxygen containing compounds (CHO compounds) with molecular weights up to 700 Da were observed. Average oxygen-to-carbon ratios of ~0.6 indicate a slightly more oxidized composition than most water-soluble organic carbon identified in aerosol studies, which may result from aqueous oxidation in the clouds. The AOM composition indicates significant influences from biogenic secondary organic aerosol (SOA) and residential wood combustion. We observed 60% of the cloud water CHO molecular formulas to be identical to SOA samples of α-pinene, β-pinene, d-limonene, and β-caryophyllene ozonolysis. CHNO compounds had the highest number frequency and relative abundances and are associated with residential wood combustion and NOx oxidation. We observed multiple nitrogen atoms in the assigned molecular formulas for the nighttime cloud sample composite indicating the significance of nighttime emissions or NOx oxidation on the AOM composition. Several CHOS and CHNOS compounds with reduced sulfur (in addition to the commonly observed oxidized sulfur containing compounds) were also observed, however further investigation is needed to determine the origin of the reduced sulfur containing compounds. Overall, the molecular composition determined using ultrahigh resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry provides an unambiguous identification of the cloud water organic composition in the Rocky Mountain area which could help to improve the understanding of aqueous phase processes.