Fragmentation patterns of particulate organic nitrates in an Aerosol Mass Spectrometer

Atmospheric aerosols affect the Earth's radiative balance, visibility and human health. Therefore the formation processes and growth of these particles are important and should be studied to understand how human and natural processes affects these processes. One poorly understood and relatively little studied part of aerosols is particulate organic nitrates (pONs). These pONs are mostly formed during nighttime when NOx, mainly emitted from fossil fuel combustion and industrial processes, and volatile organic compounds (VOCs), from both natural and anthropogenic sources, reacts in the atmosphere. The quantification of these pONs is still hard due to instrumental restrictions, although much improvement has happened during recent years. One main reason for these challenges is the difficulty to separate inorganic nitrates from organic nitrates with real-time instruments. During this work, we generated pure pON in well controlled laboratory conditions and sampled it with an Aerosol Mass Spectrometer (AMS), an instrument widely used for measuring the chemical composition of atmospheric aerosols. We used four different pON precursors to generate pON. I investigated the fragmentation patterns of pON detected by the AMS, utilizing the high resolution of the newest model of the AMS. As older versions of the AMS has difficulties to separate nitrate-containing organic fragments due to lower resolution than the AMS I used, I was able to study pON mass spectrum with better resolution than anyone before me. I found mass spectral differences for the different pON precursors, and was able to find unique fragments for some of the pON precursors that possibly can be used as marker fragments

Unwanted Indoor Air Quality Effects from Using Ultraviolet C Lamps for Disinfection

Ultraviolet germicidal irradiation (UVGI) is known to inactivate various viruses and bacteria, including SARS-CoV-2, and is widely applied especially in medical facilities. This inactivation results from the high photon energies causing molecular bonds to break, but when nonpathogen molecules are affected, unwanted effects may occur. Here, we explored the effect of a commercial high intensity (similar to 2 kW) UVC disinfection device on the composition and concentration of gases and particles in indoor air. We find that the UVC (254 nm) caused dramatic increases in particle number concentrations, and nearly all (similar to 1000) monitored gas phase species also increased. These responses were unsurprising when considering the typical impacts of UVC on atmospheric chemistry. High particle concentrations are associated with adverse health effects, suggesting that the impact of UVGI devices on indoor air quality (IAQ) should be studied in much more detail. The high-intensity device in this study was intended for short durations in unoccupied rooms, but lower-intensity devices for continuous use in occupied rooms are also widely applied. This makes further studies even more urgent, as the potential IAQ effects of these approaches remain largely unexplored.Peer reviewe

Oxidation product characterization from ozonolysis of the diterpene ent-kaurene

Diterpenes (C20H32) are biogenically emitted volatile compounds that only recently have been observed in ambient air. They are expected to be highly reactive, and their oxidation is likely to form condensable vapors. However, until now, no studies have investigated gas-phase diterpene oxidation. In this paper, we explored the ozonolysis of a diterpene, ent-kaurene, in a simulation chamber. Using state-of-the-art mass spectrometry, we characterized diterpene oxidation products for the first time, and we identified several products with varying oxidation levels, including highly oxygenated organic molecules (HOM), monomers, and dimers. The most abundant monomers measured using a nitrate chemical ionization mass spectrometer were C19H28O8 and C20H30O5, and the most abundant dimers were C38H60O6 and C39H62O6. The exact molar yield of HOM from kaurene ozonolysis was hard to quantify due to uncertainties in both the kaurene and HOM concentrations, but our best estimate was a few percent, which is similar to values reported earlier for many monoterpenes. We also monitored the decrease in the gas-phase oxidation products in response to an increased condensation sink in the chamber to deduce their affinity to condense. The oxygen content was a critical parameter affecting the volatility of products, with four to five O atoms needed for the main monomeric species to condense onto 80 nm particles. Finally, we report on the observed fragmentation and clustering patterns of kaurene in a Vocus proton-transfer-reaction time-of-flight mass spectrometer. Our findings highlight similarities and differences between diterpenes and smaller terpenes during their atmospheric oxidation, but more studies on different diterpenes are needed for a broader view of their role in atmospheric chemistry.Peer reviewe

Detecting and Characterizing Particulate Organic Nitrates with an Aerodyne Long-ToF Aerosol Mass Spectrometer

Particulate organic nitrate (pON) can be a major part of secondary organic aerosol (SOA) and is commonly quantified by indirect means from aerosol mass spectrometer (AMS) data. However, pON quantification remains challenging. Here, we set out to quantify and characterize pON in the boreal forest, through direct field observations at Station for Measuring Ecosystem Atmosphere Relationships (SMEAR) II in Hyytia''la'', Finland, and targeted single precursor laboratory studies. We utilized a long time-of-flight AMS (LToF-AMS) for aerosol chemical characterization, with a particular focus to identify CxHyOzN+ ("CHON+") fragments. We estimate that during springtime at SMEAR II, pON (including both the organic and nitrate part) accounts for similar to 10% of the particle mass concentration (calculated by the NO+/NO2+ method) and originates mainly from the NO3 radical oxidation of biogenic volatile organic compounds. The majority of the background nitrate aerosol measured is organic. The CHON+ fragment analysis was largely unsuccessful at SMEAR II, mainly due to low concentrations of the few detected fragments. However, our findings may be useful at other sites as we identified 80 unique CHON+ fragments from the laboratory measurements of SOA formed from NO3 radical oxidation of three pON precursors (beta-pinene, limonene, and guaiacol). Finally, we noted a significant effect on ion identification during the LToF-AMS high-resolution data processing, resulting in too many ions being fit, depending on whether tungsten ions (W+) were used in the peak width determination. Although this phenomenon may be instrument-specific, we encourage all (LTOF-) AMS users to investigate this effect on their instrument to reduce the possibility of incorrect identifications.Peer reviewe

A combined gas- and particle-phase analysis of highly oxygenated organic molecules (HOMs) from alpha-pinene ozonolysis

Highly oxygenated organic molecules (HOMs) are important for the formation of secondary organic aerosol (SOA), which poses serious health risks and exerts great influence on Earth's climate. However, the speciation of particle-phase HOMs and its relationship with gas-phase HOM formation has been limited by the lack of suitable analytical techniques. Here, combining a novel particle evaporation inlet, the VIA (Vaporization Inlet for Aerosols), with a nitrate chemical ionization mass spectrometer (NO3-CIMS), gas- and particle-phase HOM products of a-pinene ozonolysis were studied under different conditions. Within the 50 min residence time of our Teflon chamber, we observed enhancement of C-16-C-19 HOM dimers in particles compared to the HOMs that were condensing. In particular, gas-phase dimer formation was considerably suppressed in experiments with the addition of CO or NO, but dimers still made up a considerable fraction of the observed SOA. In addition to the generally shorter carbon skeletons of the particle-phase dimers (i.e., C-16-C-19) compared to the gas phase (C-19-C-20), average O/C ratios of the HOMs (especially in the dimer range) also decreased slightly in the particle phase. C17H26Oz compounds, which have often been reported by previous offline measurements, dominate the particle-phase HOM mass spectra in a-pinene ozonolysis experiments. Our results indicate that these C-17 compounds might be related to particle-phase processes within 1 h after HOM condensation. However, the new VIA-NO3-CIMS system used in this work will require more detailed characterization to better understand how the thermal desorption and wall effects may modify the measured particle-phase HOM distributions. Nevertheless, organic nitrate, for example, measured by this novel VIA-NO3-CIMS system was consistent with the measurements of an Aerodyne aerosol mass spectrometer (AMS), showing the capability of this system as a promising technique for particle-phase HOM measurements. Taken together, we believe that this system is a promising technique for combined online gas- and particle-phase HOM measurements.Peer reviewe

Online measurement of highly oxygenated compounds from organic aerosol

Highly oxygenated compounds are important contributors to the formation and growth of atmospheric organic aerosol and thus have an impact on Earth's radiation balance and global climate. However, knowledge of the contribution of highly oxygenated compounds to organic aerosol and their fate after condensing into the particle phase has been limited by the lack of suitable detection techniques. Here, we present a new online method for measuring highly oxygenated compounds from organic aerosol. The method includes thermal evaporation of particles in a new inlet, the vaporization inlet for aerosols (VIA), followed by identification of the evaporated highly oxygenated compounds by a nitrate chemical ionization mass spectrometer (NO3-CIMS). The method does not require sample collection, enabling highly time-resolved measurements of particulate compounds. We evaluate the performance of the method by measuring the detection limit and performing background measurements. We estimate a detection limit of below 1 ng m(-3) for a single compound and below 1 mu g m(-3) for SOA with the sampling setup used here. These detection limits can be improved upon by optimizing the flow setup. Furthermore, we detect hundreds of particulate highly oxygenated compounds from organic aerosol generated from different precursors. Our results are consistent with previous studies showing that the volatility of organic compounds decreases with increasing m/z ratio and level of oxygenation and that organic aerosol consists of monomers and oligomeric compounds. By comparing the gas- and particle-phase compounds, we found indications of potential particle-phase reactions occurring in organic aerosol. Future work will focus both on further improving the sampling design and on better understanding the evaporation dynamics of the system, but already these initial tests show that the VIA coupled to the NO3-CIMS is a promising method for investigating the transformations and fate of the compounds after condensing into the particle phase.Peer reviewe

Towards more accurate particulate organic nitrate quantification through aerosol mass spectrometry

A large fraction of particulate organic nitrates (pON) arise from reactions involving volatile organic compounds (VOC) and nitrate radicals (NO3) contributing significantly to tropospheric particulate matter. However, their quantification remains challenging. Previous studies with Aerosol Mass Spectrometers (AMS) show that the fragmentation patterns of inorganic ammonium nitrate and pON significantly differ from each other resulting in a clear contrast in the ratio between the major nitrate fragments, i.e., the NO+/NO2+ ratio. Detection of pONs leads to larger NO+/NO2+ values, but with high variability depending on the pON precursors. Additional uncertainty is also introduced while performing unit mass resolution (UMR) analysis as an organic ion fragment (CH2O+) is typically present at the same unit mass as NO+. Underestimation of this organic fragment can lead to significant overestimation of pON. As part of the Aerosol Chemical Monitor Calibration Centre (ACMCC) pON experiment, we generated pON via NO3 oxidation of four different VOCs (limonene, β-pinene, guaiacol, and acenaphthylene) in a Potential Aerosol Mass (PAM) oxidation flow reactor. The pON were detected with a High Resolution (HR) AMS equipped with a Long Time-of-Flight chamber facilitating a mass resolution approaching 8000 M/ΔM, further enabling separation of nitrogen containing peaks in the mass spectrum. We observed precursor-dependent differences in the nitrate loading when comparing UMR and HR results mainly linked to CH2O+ underestimation with the default AMS fragmentation table. Here we present a modified fragmentation table with the goal of providing more accurate pON quantification in ambient air via UMR aerosol mass spectrometry such as Aerosol Chemical Speciation Monitor (ACSM). We also compare the abundance of other nitrogen-containing HR-AMS ion signals (e.g. combinations of carbon/hydrogen/nitrogen and carbon/hydrogen/nitrogen/oxygen atoms) obtained for the different pON types. We acknowledge the European COST Action CA16109 COLOSSAL, the H2020 ACTRIS-2 project (grant agreements no. 654109 and the ERC Starting Grant 638703-COALA

Characterization of the Vaporization Inlet for Aerosols (VIA) for online measurements of particulate highly oxygenated organic molecules (HOMs)

Particulate matter has major climate and health impacts, and it is therefore of utmost importance to be able to measure the composition of these particles to gain insights into their sources and characteristics. Many methods, both offline and online, have been employed over the years to achieve this goal. One of the most recent developments is the Vaporization Inlet for Aerosols (VIA) coupled to a nitrate Chemical Ionization Mass Spectrometer (NO3-CIMS), but a thorough understanding of the VIA–NO3-CIMS system remains incomplete. In this work, we ran a series of tests to assess the impacts from different systems and sampling parameters on the detection efficiency of highly oxygenated organic molecules (HOMs) in the VIA–NO3-CIMS system. Firstly, we found that the current VIA system (which includes an activated carbon denuder and a vaporization tube) efficiently transmits particles (> 90 % for particles larger than 50 nm) while also removing gaseous compounds (> 97% for tested volatile organic compounds – VOCs). One of the main differences between the VIA and traditional thermal desorption (TD) techniques is the very short residence time in the heating region, on the order of 0.1 s. We found that this short residence time, and the corresponding short contact with heated surfaces, is likely one of the main reasons why relatively reactive or weakly bound peroxides, for example, were observable using the VIA. However, the VIA also requires much higher temperatures in order to fully evaporate the aerosol components. For example, the evaporation temperature of ammonium sulfate particles using the VIA was found to be about 100–150 °C higher than in typical TD systems. We also observed that the evaporation of particles with larger sizes occurred at slightly higher temperatures compared to smaller particles. Another major aspect that we investigated was the gas-phase wall losses of evaporated molecules. With a more optimized interface between the VIA and the NO3-CIMS, we were able to greatly decrease wall losses and thus improve the sensitivity compared to our earlier VIA work. This interface included a dedicated sheath flow unit to cool the heated sample and provide the NO3-CIMS with the needed high flow (10 L min−1). Our results indicate that most organic molecules observable by the NO3-CIMS can evaporate and be transported efficiently in the VIA system, but upon contact with the hot walls of the VIA, the molecules are instan taneously lost. This loss potentially leads to fragmentation products that are not observable by the NO3-CIMS. Thermograms, obtained by scanning the VIA temperature, were found to be very valuable for both quantification purposes and for estimating the volatility of the evaporating compounds. We developed a simple one-dimensional model to account for the evaporation of particles and the temperaturedependent wall losses of the evaporated molecules, and we thereby estimate the concentration of HOMs in secondary organic aerosol (SOA) particles. Overall, our results provide much-needed insights into the key processes underlying the VIA–NO3-CIMS method. Although there are still some limitations that could be addressed through hardware improvements, the VIA–NO3-CIMS system is a very promising and useful system for fast online measurements of HOMs in the particle phase

Photochemical Production of Light-absorbing Syringol Secondary Organic Aerosol in Droplets using an Atmospheric Simulation Chamber

International audienceThe photooxidation of syringol, a substituted phenol emitted primarily from lignin pyrolysis during wildfires, was used to explore the formation of secondary brown carbon (BrC) under dry, moist, and cloud-like conditions in aerosol and droplet-phase reactions using the CESAM multiphase atmospheric simulation chamber at the University of Paris-Est LISA in Créteil, France. Optical properties were monitored using a particle-into-liquid (PILS) waveguide with total organic carbon (TOC) analysis system while chemical properties were interrogated using two high-resolution aerosol time-of-flight mass spectrometers (HR-ToF-AMS, Aerodyne). Syringol oxidation produced brown, water-soluble products in deliquesced ammonium sulfate (AS) aerosol and in cloud droplets; the limited secondary organic aerosol (SOA) produced on dry AS seed aerosol did not absorb visible light. Browning occurred in simulated sunlight both with and without OH radicals generated by hydrogen peroxide photolysis, as well as in dark conditions with hydrogen peroxide. Brown products formed under dark conditions were different from those formed under light, while products formed in simulated sunlight were chemically and optically very similar whether or not HOOH was present. The aqueous BrC formed without light featured an absorbance peak at 470 nm consistent with a dimer observed previously in this chemical system and disappeared immediately upon illumination. HR-ESI-MS of chamber filter extracts indicates that most products detected by this technique contain N although HR-AMS spectra indicate only a very small contribution from N-containing fragments. UV/visible absorbance spectra and aerosol mass spectra suggest that the products formed in sunlit, OH-mediated reactions are highly similar to those formed under light without an OH radical source, suggesting that direct syringol photolysis may be capable of initiating similar radical-driven chemistry in the absence of additional oxidants like OH or 3C*. However, the actinic flux of the wavelengths of light necessary for this direct mechanism of syringol SOA formation (λ< 280 nm) is likely too small in the lower atmosphere to be relevant for most biomass burning plumes. Our findings from droplet phase reactions support earlier bulk phase studies suggesting that syringol is capable of forming light absorbing products rapidly in sunlit reactions, especially when liquid water is available