Sponsored by TOFWERKReviewed by Aimee MolineuxJun 18 2025
Organic aerosols constitute up to 90 % of the mass of atmospheric aerosols globally and originate from the oxidation of volatile organic compounds (VOCs).
Image Credit: TOFWERK
Oxygenated VOCs with low to extremely low vapor pressures are particularly significant for climate, as they contribute to the formation of atmospheric aerosols by either condensing onto pre-existing aerosol particles or forming new particles.
A critical initial step in the formation and growth of particles is the generation of highly oxygenated organic molecules (HOMs) through autoxidation.
In addition to organic compounds, inorganic species are also essential in the formation and growth of atmospheric particles. Among these, sulfur oxides (SOx) are major air pollutants directly affecting Earth’s atmosphere.
Sulfur dioxide (SO2) is the most prevalent of these oxides and is primarily emitted through human activities, such as fuel combustion, as well as natural events like volcanic eruptions.
Once in the atmosphere, SO2 is rapidly oxidized to form sulfur trioxide (SO3). This compound serves as a key intermediate in the formation of sulfuric acid, which plays a critical role in atmospheric new particle formation.
SO3 can also react on organic surfaces, contributing to the formation of sulfur-containing compounds that significantly influence aerosol physicochemical properties.
Although the atmospheric lifetime of gaseous SO3 is considered extremely short due to its rapid reaction with water dimer molecules (resulting in sulfuric acid), its reaction with other atmospherically relevant compounds has been anticipated to be negligible.
However, laboratory and field observations indicate the formation and presence of gaseous sulfur-containing compounds in the atmosphere.
Following these observations, several studies have explored the gas-phase formation mechanisms of sulfur-containing compounds resulting from the reactions of SO3 with atmospheric organics, particularly carboxylic acids.
Analytical Challenges
Understanding the evolution of sulfur-containing compounds in the atmosphere is essential for elucidating the physicochemical processes that lead to particle formation.
Due to their very low vapor pressure and acidity, sulfur-containing compounds are expected to play a vital role in the processes that generate new particles.
However, accurately characterizing these species presents analytical challenges for researchers, primarily due to the similar masses of compounds containing oxygen and sulfur, which render them virtually indistinguishable in typical mass spectrometry analyses.
In many instances, specific spectral fitting techniques are required to resolve overlapping peaks at the same integer mass; however, such mathematical approaches may fail to accurately retrieve the true composition of isobaric species.
Addressing Analytical Challenges with the Vocus HR
The Vocus High Resolution (HR) online chemical ionization mass spectrometer utilizes multi-reflection technology to provide a 5.5-meter flight path within a compact frame, enabling the real-time separation of such compounds for laboratory or field measurements.
The study discussed in this article simulated atmospheric conditions to assess the capability of the newly developed Vocus HR to effectively resolve sulfur-containing compounds from other closed-shell products, determining whether this instrument can offer new insights into these critical species.
Experimental Setup
To assess the performance of the Vocus HR system, gaseous species were generated in an 18-liter Pyrex glass aerosol flow tube reactor (12 cm i.d. × 158 cm length) through the O3/OH-initiated oxidation of α-pinene in the presence of SO2 at room temperature and atmospheric pressure.
Ozone was produced at a stable concentration of 80 ppb by passing a flow of 0.6 standard liters per minute (SLPM) of synthetic air through a UV lamp and was monitored with a Thermo 49C analyzer.
A continuous injection of 5 ppb of α-pinene was introduced into the flow reactor, while SO2 was continuously added in stepped concentrations ranging from 0.5 to 10 ppb.
A mixture of nitrogen and oxygen (80:20) served as a carrier gas, providing a total flow of 18 SLPM and a reaction time of approximately 60 seconds.
The Vocus HR was integrated with an Aerodyne atmospheric pressure chemical ionization (CI) inlet utilizing nitrate (NO3–) ion chemistry. NO3– ions were generated from a nitric acid solution (65%) that was continuously flushed with 2 standard cubic centimeters per minute (sccm) of pure N2, which was then mixed with 25 LPM of N2 serving as sheath flow (resulting in a total flow of 34 LPM) and subsequently ionized with a soft X-ray photoionizer.
Enhanced Detection Using the Vocus HR
To evaluate the detection capabilities of the Vocus HR, oxidation products from the initiated oxidation of α-pinene, both with and without SO2, were analyzed using a steady-state flow tube setup.
This reaction mechanism was selected due to its extensive study and the resultant suite of oxidation products that span a wide range of functionalities. It notably produces molecules with varying molecular masses, from lightly oxidized monomers to heavily oxidized dimers.
As illustrated in Figure 1, the enhanced mass resolving power (20,000 – 25,000 Th Th−1) of the Vocus HR, in comparison to conventional online mass spectrometers (which typically have a mass resolving power of 10,000 Th Th−1), allows for the clear identification of isobaric compounds.
The increase in mass resolving power enhances the robustness of peak fitting, yielding more accurate and reliable identification and quantification of the compounds of interest.
Figure 1. An example of traditional HR peak fitting. A potential peak fitting at m∕Q 512 and 514 Th of products generated during the OH/O3 initiated oxidation of α-pinene, utilizing the nitrate-based CI-Vocus HR at a resolving power of (a, c) 10,000 Th Th−1 and (b, d) 22,000 Th Th−1. Image Credit: TOFWERK
Detection of Sulfur-Containing Compounds Using the Vocus HR
Identifying C10H16O5S species clustered with NO3– is exclusively achievable with the Vocus HR, as depicted in Figure 2.
In contrast, employing a conventional time-of-flight spectrometer would necessitate additional information to accurately ascertain the presence of such compounds and facilitate their identification, particularly at low concentrations.
While identification may be feasible, the spectral overlap between C10H16O5S and C10H16O7 is significant enough that obtaining quantitative information becomes challenging due to ambiguous peak fitting, resulting in unreliable signal separation.
Figure 2. Potential peak fitting at m∕Q 310 and 326 Th of oxygenated products generated during the OH/O3 initiated oxidation of α-pinene in the presence of SO2, utilizing the nitrate-based CI-Vocus HR at a resolving power of 22,000 Th Th−1. Image Credit: TOFWERK
Leveraging the superior resolving power of the Vocus HR, a total of 75 sulfur-containing species among approximately 250 total species were detected during the OH/O3-initiated oxidation of α-pinene in the presence of 5 ppb of SO2.
Figure 3 illustrates the subsequent formation of gaseous sulfur-containing species and suggests that sulfur trioxide may influence the oxidation of biogenic compounds, leading to the formation of very low vapor-pressure compounds that could contribute to the formation of new particles in the atmosphere.
Figure 3. Mass defect plot of non-sulfur-containing (orange) and sulfur-containing (purple) species measured by the nitrate-based CI-Vocus HR generated via the OH/O3-initiated oxidation of α-pinene in the presence of 5 ppb of SO2. The x-axis represents the mass-to-charge ratio of the neutral analyte; the y-axis represents the corresponding mass defect, which is the difference between their exact mass and nominal mass; and the size of the circle represents the square root of the signal intensity measured for each ion. Image Credit: TOFWERK
Conclusions
The integration of sensitivity and selectivity provided by CI reactors, along with the enhanced mass resolving power of the Vocus HR, enables the real-time separation of sulfur and oxygen-containing compounds.
This capability offers researchers a valuable opportunity to gain deeper insights into these critical precursors of atmospheric particle formation.
Analyzing these species could significantly contribute to the development of improved regulatory frameworks, ultimately reducing pollution-related mortality in humans and mitigating detrimental environmental impacts.
Acknowledgments
Produced from material originally authored by Matthieu Riva, Katie Schmidt, Vasyl Yatsyna, and Felipe Lopez-Hilfiker from TOFWERK.
This information has been sourced, reviewed and adapted from materials provided by TOFWERK.
For more information on this source, please visit TOFWERK.