Molecular formula characterization of biogenic secondary organic aerosol: Descriptive statistical evaluation

The detailed molecular composition of approximately 20 laboratory generated terpene ozonolysis secondary organic aerosol (SOA) samples was studied using ultrahigh resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Individual experiments were conducted with one of four terpene SOA precursors (α-pinene, β-pinene, limonene or β-caryophyllene), varied relative humidity (RH) conditions (0%, 4%, or 30%) and the presence or absence of cyclohexane (serving as a radical scavenger). In this work, we focus on the molecular composition of the SOA experiments conducted at 4% and 30% RH without cyclohexane. In each of the experimental SOA samples, the oxygen number and the DBE values increase with increasing carbon number and three or four distinct groups (aka oligomer groups) were observed in the mass spectra. The overall bulk properties, such as the elemental ratios and the average number of double bond equivalents (DBE), of the SOA were highly similar. Despite the high number of identified species (N ≥ 1000) in each SOA sample, compounds unique to the SOA formed at either 4% or 30% RH conditions were comparatively low (\u3c 200). An exception to this was observed for the D-limonene ozonolysis SOA formed at 4% RH conditions where over 450 unique molecular formulas were observed. Due to the similarity in the bulk properties and composition of the SOA from the experiments, multivariate statistics were used to distinguish the experiments from each other. Hierarchical cluster analysis and principal component analysis was performed using the molecular formulas and their relative abundances for all of the identified species. Slight compositional differences between the experiments showed that experiments with the same terpene SOA precursor were most closely related regardless of the RH or the presence/absence of cyclohexane. Furthermore, SOA experiments with D-limonene and β-caryophyllene as precursors were clearly distinguished from β-pinene and α-pinene. When the experimental SOA composition was compared with ambient samples, we observed a high number of common monoisotopic molecular formulas for summer aerosol [63%; Mazzoleni et. al., Env. Chem. 2012] and winter cloudwater samples [60%; Zhao et. al., ACPD 2013]. However the molecular formulas identified as significant using principal components analysis, were not found consistently in both samples indicating variable SOA contributions to summer and winter ambient samples. Mazzoleni, L.R., P. Saranjampour, M.M. Dalbec, V. Samburova, B. Zielinska, A.G. Hallar, D. Lowenthal, and S. Kohl, Identification of Water-Soluble Organic Carbon in Nonurban Organic Aerosols using Ultrahigh-Resolution FT-ICR Mass Spectrometry: Organic Anions, Environmental Chemistry, Vol. 9(3) 285-297, 2012. Zhao, Y., A.G. Hallar, and L.R. Mazzoleni, Atmospheric Organic Matter in Clouds: Exact Masses and Molecular Formula Identification using Ultrahigh Resolution FT-ICR Mass Spectrometry, Atmospheric Chemistry and Physics Discussion, In Press, 2013

Identification of water-soluble organic carbon in non-urban aerosols using ultrahigh-resolution FT-ICR mass spectrometry: organic anions

Water-soluble organic carbon (WSOC) is a complex mixture of thousands of organic compounds which may have significant influence on the climate-relevant properties of atmospheric aerosols. An improved understanding of the molecular composition of WSOC is needed to evaluate the effect of aerosol composition upon aerosol physical properties. In this work, ultrahigh-resolution Fourier transform–ion cyclotron resonance mass spectrometry (FT-ICR MS) was used to characterise aerosol WSOC collected during the summer of 2010 at the Storm Peak Laboratory (3210 m ASL) near Steamboat Springs, CO. Approximately 4000 molecular formulas were assigned in the mass range of 100–800 Da after negative-ion electrospray ionisation and more than 50 % of them contained nitrogen or sulfur. The double bond equivalents (DBEs) of the molecular formulas were inversely proportional to the O : C ratio, despite a relatively constant H : C ratio of ~1.5. Despite the range of DBE values, the elemental ratios and the high number of oxygen atoms per formula indicate that a majority of the compounds are aliphatic to olefinic in nature. These trends indicate significant non-oxidative accretion reaction pathways for the formation of high molecular weight WSOC components. In addition, a significant number of molecular formulas assigned in this work matched those previously identified as secondary organic aerosol components of monoterpene and sesquiterpene ozonolysis

Functional groups and structural insights of water-soluble organic carbon using ultrahigh resolution FT-ICR tandem mass spectrometry

Water-soluble organic carbon (WSOC) is a complex mixture of thousands of organic compounds which may have significant influence on the climate-relevant properties of atmospheric aerosols. An improved understanding of the molecular composition of WSOC is needed to evaluate the effect of aerosol composition upon aerosol physical properties. Products of gas phase, aqueous phase and particle phase reactions contribute to pre-existing aerosol organic mass or nucleate new aerosol particles. Thus, ambient aerosols carry a complex array of WSOC components with variable chemical signatures depending upon its origin and aerosol life-cycle processes. In this work, ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was used to characterize aerosol WSOC collected during the summer of 2010 at the Storm Peak Laboratory (3210 m a.s.l.) near Steamboat Springs, CO. Approximately 4000 molecular formulas were assigned in the mass range of m/z 100-800 after negative-ion electrospray ionization. The observed trends indicate significant non-oxidative accretion reaction pathways for the formation of high molecular weight WSOC components closely associated with terpene ozonolysis secondary organic aerosol (SOA). The aerosol WSOC was further characterized using ultrahigh resolution tandem MS analysis with infrared multiphoton dissociation to determine the functional groups and structural properties of 1700 WSOC species up to m/z 600. Due to the complex nature of the WSOC, multiple precursor ions were simultaneously fragmented. The exact mass measurements of the precursor and product ions facilitated molecular formula assignments and matching of neutral losses. The most important neutral losses are CO2, H2O, CH3OH, HNO3, CH3NO3, SO3 and SO4. The presence and frequency of these losses indicate the type of functional groups contained in the precursor structures. Consistent with the acidic nature of WSOC compounds, the most frequently observed losses were CO2 (~65%), H2O (~60%) and CH3OH (~40%). Several of the studied precursors had two or more losses associated with them and combinations of neutral losses such as, H4O2, CH2O3, C2H4O3 and C2O4. These neutral losses clearly indicate a multifunctional nature of the studied aerosol WSOC. Analysis of the fragment ions which were not associated with typical neutral losses indicates an overall aliphatic SOA-like structure with regular differences of 14 Da and 18 Da between low molecular weight fragment ions. Many of the fragment ions were observed in 85% or more of the MS2 spectra. The patterns observed in the low molecular weight fragment ions were very consistent over all of the mass spectra providing evidence for the significance of the non-oxidative accretion formation pathways

Functional groups and structural insights of water-soluble organic carbon using ultrahigh resolution FT-ICR tandem mass spectrometry

Water-soluble organic carbon (WSOC) is a complex mixture of thousands of organic compounds which may have significant influence on the climate-relevant properties of atmospheric aerosols. An improved understanding of the molecular composition of WSOC is needed to evaluate the effect of aerosol composition upon aerosol physical properties. Products of gas phase, aqueous phase and particle phase reactions contribute to pre-existing aerosol organic mass or nucleate new aerosol particles. Thus, ambient aerosols carry a complex array of WSOC components with variable chemical signatures depending upon its origin and aerosol life-cycle processes. In this work, ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was used to characterize aerosol WSOC collected during the summer of 2010 at the Storm Peak Laboratory (3210 m a.s.l.) near Steamboat Springs, CO. Approximately 4000 molecular formulas were assigned in the mass range of m/z 100-800 after negative-ion electrospray ionization. The observed trends indicate significant non-oxidative accretion reaction pathways for the formation of high molecular weight WSOC components closely associated with terpene ozonolysis secondary organic aerosol (SOA). The aerosol WSOC was further characterized using ultrahigh resolution tandem MS analysis with infrared multiphoton dissociation to determine the functional groups and structural properties of 1700 WSOC species up to m/z 600. Due to the complex nature of the WSOC, multiple precursor ions were simultaneously fragmented. The exact mass measurements of the precursor and product ions facilitated molecular formula assignments and matching of neutral losses. The most important neutral losses are CO2, H2O, CH3OH, HNO3, CH3NO3, SO3 and SO4. The presence and frequency of these losses indicate the type of functional groups contained in the precursor structures. Consistent with the acidic nature of WSOC compounds, the most frequently observed losses were CO2 (~65%), H2O (~60%) and CH3OH (~40%). Several of the studied precursors had two or more losses associated with them and combinations of neutral losses such as, H4O2, CH2O3, C2H4O3 and C2O4. These neutral losses clearly indicate a multifunctional nature of the studied aerosol WSOC. Analysis of the fragment ions which were not associated with typical neutral losses indicates an overall aliphatic SOA-like structure with regular differences of 14 Da and 18 Da between low molecular weight fragment ions. Many of the fragment ions were observed in 85% or more of the MS2 spectra. The patterns observed in the low molecular weight fragment ions were very consistent over all of the mass spectra providing evidence for the significance of the non-oxidative accretion formation pathways