Varying chiral ratio of Pinic acid enantiomers above the Amazon rainforest

Chiral chemodiversity plays a crucial role in biochemical processes such as insect and plant communication. However, the vast majority of organic aerosol studies do not distinguish between enantiomeric compounds in the particle phase. Here we report chirally specified measurements of secondary organic aerosol (SOA) at the Amazon Tall Tower Observatory (ATTO) at different altitudes during three measurement campaigns at different seasons. Analysis of filter samples by liquid chromatography coupled to mass spectrometry (LC-MS) has shown that the chiral ratio of pinic acid (C9H14O4) varies with increasing height above the canopy. A similar trend was recently observed for the gas-phase precursor α-pinene, but more pronounced. Nevertheless, the measurements indicate that neither the oxidation of (+/−)-α-pinene nor the incorporation of the products into the particulate phase proceeds with stereo preference and that the chiral information of the precursor molecule is merely transferred to the low-volatility product. The observation of the weaker height gradient of the present enantiomers in the particle phase at the observation site can be explained by the significant differences in the atmospheric lifetimes of reactant and product. Therefore, it is suggested that the chiral ratio of pinic acid is mainly determined by large-scale emission processes of the two precursors, while meteorological, chemical, or physicochemical processes do not play a particular role. Characteristic emissions of the chiral aerosol precursors from different forest ecosystems, in some cases even with contributions from forest related fauna, could thus provide large-scale information on the different contributions to biogenic secondary aerosols via the analytics of the chiral particle-bound degradation products.

Photo-Oxidation of cis-Pinic Acid in the Aqueous Phase: A Mechanistic Investigation Under Acidic and Basic pH Conditions

Atmospheric aqueous phases (cloud and fog droplets, aerosol liquid water) are important reaction media for the processing of water-soluble organic acids (OAs). The photochemistry of these species is known to...

Structural Characterisation of Dimeric Esters in α-Pinene Secondary Organic Aerosol Using N2 and CO2 Ion Mobility Mass Spectrometry

The atmospheric oxidation of monoterpenes leads to the formation of secondary organic aerosol (SOA). While numerous works have been carried out in the past to characterise SOA at a molecular level, the structural elucidation of SOA compounds remains challenging owing to the lack of authentic standard compounds. In this work, the structures of α-pinene originating dimeric esters in SOA with m/z 357 (C17H25O8-) and m/z 367 (C19H27O7-) were characterised using UPLC/ESI(-)IMS-TOFMS2 (ultra-performance liquid chromatography coupled to ion mobility spectrometry tandem time-of-flight mass spectrometry). The measured collision cross-section (ΩN2) values were compared to theoretically calculated ΩN2 values. Selected product ions of dimeric compounds and the authentic standard compounds of product ions were subjected to CO2-IMS-TOFMS for more detailed structural characterisation. Our results were consistent with previously reported subunits of the m/z 357 (terpenylic acid and cis-pinic acid), and the m/z 367 (10-hydroxy-cis-pinonic acid and cis-pinic acid) ions. The measured and calculated ΩN2 values of m/z 367 ions further support the conclusion of earlier structural characterisation; however, the structure of the m/z 357 ion remains vague and requires further characterisation studies with a synthesised reference compound.

Real-time Detection and Tandem Mass Spectrometry of Secondary Organic Aerosols with a Quadrupole Ion Trap

An aerosol quadrupole ion trap mass spectrometer is reported that is sensitive, has unique capabilities to perform chemical ionization, is operated in real-time, and is able to perform tandem mass spectrometry. The instrument samples particles with an aerodynamic lens and volatilizes them within the heated ion trap electrode assembly. Analyte molecules are ionized within the ion trap by proton transfer from reagent ions, and resultant fragmentation is reduced compared to vacuum UV photoionization. Particle concentrations can be detected linearly over two orders of magnitude and as low as 5 μg/m3. To demonstrate the real-time analysis capability of the instrument, secondary organic aerosol particles were produced by reaction of 100 ppb α-pinene and 200 ppb ozone in an aerosol bag and observed in real-time to monitor the progress of the reaction. Pinic acid and pinonic acid are two of the many components of the secondary aerosol mixture that form and gradually decrease in concentration. Individual concentrations are calculated using pinic acid as an internal standard and vary from 4-36 ppb. The identities of analyte ions from both compounds are confirmed by tandem mass spectrometry in real-time.

Real-time Detection and Tandem Mass Spectrometry of Secondary Organic Aerosols with a Quadrupole Ion Trap

An aerosol quadrupole ion trap mass spectrometer is reported that is sensitive, has unique capabilities to perform chemical ionization, is operated in real-time, and is able to perform tandem mass spectrometry. The instrument samples particles with an aerodynamic lens and volatilizes them within the heated ion trap electrode assembly. Analyte molecules are ionized within the ion trap by proton transfer from reagent ions, and resultant fragmentation is reduced compared to vacuum UV photoionization. Particle concentrations can be detected linearly over two orders of magnitude and as low as 5 μg/m3. To demonstrate the real-time analysis capability of the instrument, secondary organic aerosol particles were produced by reaction of 100 ppb α-pinene and 200 ppb ozone in an aerosol bag and observed in real-time to monitor the progress of the reaction. Pinic acid and pinonic acid are two of the many components of the secondary aerosol mixture that form and gradually decrease in concentration. Individual concentrations are calculated using pinic acid as an internal standard and vary from 4-36 ppb. The identities of analyte ions from both compounds are confirmed by tandem mass spectrometry in real-time.

High-molecular-weight esters in <i>α</i>-pinene ozonolysis secondary organic aerosol: structural characterization and mechanistic proposal for their formation from highly oxygenated molecules

Stable high-molecular-weight esters are present in α-pinene ozonolysis secondary organic aerosol (SOA) with the two most abundant ones corresponding to a hydroxypinonyl ester of cis-pinic acid with a molecular weight (MW) of 368 (C19H28O7) and a diaterpenylic ester of cis-pinic acid with a MW of 358 (C17H26O8). However, their molecular structures are not completely elucidated and their relationship with highly oxygenated molecules (HOMs) in the gas phase is still unclear. In this study, liquid chromatography in combination with positive ion electrospray ionization mass spectrometry has been performed on high-molecular-weight esters present in α-pinene ozonolysis SOA with and without derivatization into methyl esters. Unambiguous evidence could be obtained for the molecular structure of the MW 368 ester in that it corresponds to an ester of cis-pinic acid where the carboxyl substituent of the dimethylcyclobutane ring and not the methylcarboxyl substituent is esterified with 7-hydroxypinonic acid. The same linkage was already proposed in previous work for the MW 358 ester (Yasmeen et al., 2010), but could be supported in the present study. Guided by the molecular structures of these stable esters, we propose a formation mechanism from gas-phase HOMs that takes into account the formation of an unstable C19H28O11 product, which is detected as a major species in α-pinene ozonolysis experiments as well as in the pristine forest atmosphere by chemical ionization–atmospheric pressure ionization–time-of-flight mass spectrometry with nitrate clustering (Ehn et al., 2012, 2014). It is suggested that an acyl peroxy radical related to cis-pinic acid (RO2⚫) and an alkoxy radical related to 7- or 5-hydroxypinonic acid (R′O⚫) serve as key gas-phase radicals and combine according to a RO2 + R′O⚫ → RO3R′ radical termination reaction. Subsequently, the unstable C19H28O11 HOM species decompose through the loss of oxygen or ketene from the inner part containing a labile trioxide function and the conversion of the unstable acyl hydroperoxide groups to carboxyl groups, resulting in stable esters with a molecular composition of C19H28O7 (MW 368) and C17H26O8 (MW 358), respectively. The proposed mechanism is supported by several observations reported in the literature. On the basis of the indirect evidence presented in this study, we hypothesize that RO2 + R′O⚫ → RO3R′ chemistry is at the underlying molecular basis of high-molecular-weight ester formation upon α-pinene ozonolysis and may thus be of importance for new particle formation and growth in pristine forested environments.

High-molecular-weight esters in α-pinene ozonolysis secondary organic aerosol: Structural characterization and mechanistic proposal for their formation from highly oxygenated molecules

Stable high-molecular-weight esters are present in α-pinene ozonolysis secondary organic aerosol (SOA) with the two most abundant ones corresponding to a diaterpenylic ester of cis-pinic acid with a molecular weight (MW) of 368 C19H28O7) and a hydroxypinonyl ester of cis-pinic acid with a MW of 358 (C17H26O8). However, their molecular structures are not completely elucidated and their relationship with highly oxygenated molecules (HOMs) in the gas phase is still unclear. In this study, liquid chromatography in combination with positive ion electrospray ionization mass spectrometry has been performed on high-molecular-weight esters present in α-pinene/O3 SOA with and without derivatization into methyl esters. Unambiguous evidence could be obtained for the molecular structure of the MW 368 ester in that it corresponds to an ester of cis-pinic acid where the carboxyl substituent of the dimethylcyclobutane ring and not the methylcarboxyl substituent is esterified with 7-hydroxypinonic acid. The same linkage was already proposed in previous work for the MW 358 ester (Yasmeen et al., 2010), but could be supported in the present study. Guided by the molecular structures of these stable esters, we propose a formation mechanism from gas-phase HOMs that takes into account the formation of an unstable C19H28O11 product, which is detected as a major species in α-pinene ozonolysis experiments as well as in the pristine forest atmosphere by chemical ionization – atmospheric pressure ionization – time-of-flight mass spectrometry with nitrate clustering (Ehn et al., 2012, 2014). It is suggested that an acyl peroxy radical related to cis-pinic acid (RO2·) and an alkoxy radical related to 7- or 5-hydroxypinonic acid (R'O·) serve as key gas-phase radicals and combine according to a RO2· + R'O· → RO3R' radical termination reaction. Subsequently, the unstable C19H28O11

Historic records of organic compounds from a high Alpine glacier: influences of biomass burning, anthropogenic emissions, and dust transport

Historic records of α-dicarbonyls (glyoxal, methylglyoxal), carboxylic acids (C6–C12 dicarboxylic acids, pinic acid, p-hydroxybenzoic acid, phthalic acid, 4-methylphthalic acid), and ions (oxalate, formate, calcium) were determined with annual resolution in an ice core from Grenzgletscher in the southern Swiss Alps, covering the time period from 1942 to 1993. Chemical analysis of the organic compounds was conducted using ultra-high-performance liquid chromatography (UHPLC) coupled to electrospray ionization high-resolution mass spectrometry (ESI-HRMS) for dicarbonyls and long-chain carboxylic acids and ion chromatography for short-chain carboxylates. Long-term records of the carboxylic acids and dicarbonyls, as well as their source apportionment, are reported for western Europe. This is the first study comprising long-term trends of dicarbonyls and long-chain dicarboxylic acids (C6–C12) in Alpine precipitation. Source assignment of the organic species present in the ice core was performed using principal component analysis. Our results suggest biomass burning, anthropogenic emissions, and transport of mineral dust to be the main parameters influencing the concentration of organic compounds. Ice core records of several highly correlated compounds (e.g., p-hydroxybenzoic acid, pinic acid, pimelic, and suberic acids) can be related to the forest fire history in southern Switzerland. P-hydroxybenzoic acid was found to be the best organic fire tracer in the study area, revealing the highest correlation with the burned area from fires. Historical records of methylglyoxal, phthalic acid, and dicarboxylic acids adipic acid, sebacic acid, and dodecanedioic acid are comparable with that of anthropogenic emissions of volatile organic compounds (VOCs). The small organic acids, oxalic acid and formic acid, are both highly correlated with calcium, suggesting their records to be affected by changing mineral dust transport to the drilling site.