scholarly journals Measurement Report: important contributions of oxygenated compounds to emissions and chemistry of VOCs in urban air

2020 ◽  
Author(s):  
Caihong Wu ◽  
Chaomin Wang ◽  
Sihang Wang ◽  
Wenjie Wang ◽  
Bin Yuan ◽  
...  
Keyword(s):  
Mass Spectrometer ◽  
Mass Spectra ◽  
Total Concentration ◽  
Proton Transfer Reaction ◽  
Urban Atmosphere ◽  
Urban Site ◽  
Oh Radicals ◽  
Secondary Formation ◽  
Primary Emissions ◽  
Tof Ms

Abstract. Volatile organic compounds (VOCs) play important roles in the tropospheric atmosphere. In this study, VOCs were measured at an urban site in Guangzhou, one of the mega-cities in Pearl River Delta (PRD) using a gas chromatograph mass spectrometer/flame ionization detection (GC-MS/FID) and a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS). Diurnal profile analyses show that stronger chemical removal by OH radicals for more reactive hydrocarbons during the daytime. Diurnal profiles of OVOCs indicate evidence of contributions from secondary formation. Detailed source analyses of OVOCs using a photochemical age-based parameterization method suggest important contributions from both primary emissions and secondary formation for measured OVOCs. During the campaign, around 1700 ions were detected in PTR-ToF-MS mass spectra, among of which 462 ions with noticeable concentrations. VOCs signals from these ions without calibration in PTR-TOF-MS are quantified based on sensitivities of available VOCs species. OVOC-related ions dominated PTR-ToF-MS mass spectra with an average contribution of 77.2 %. Combining measurements from PTR-ToF-MS and GC-MS/FID, OVOCs contribute 57.4 % to the total concentration of VOCs. Using concurrent measurement of OH reactivity, OVOCs measured by PTR-ToF-MS contribute greatly to the OH reactivity (19.3 %). In comparison, hydrocarbons account for 20.0 % of OH reactivity. Adding up the contributions from inorganic gases (47.9 %), ∼ 12 % of the OH reactivity remains as missing. Our results demonstrate the important roles of OVOCs in the emission and evolution budget of VOCs in urban atmosphere.

scholarly journals Measurement report: Important contributions of oxygenated compounds to emissions and chemistry of volatile organic compounds in urban air

2020 ◽  
Vol 20 (23) ◽  
pp. 14769-14785
Author(s):  
Caihong Wu ◽  
Chaomin Wang ◽  
Sihang Wang ◽  
Wenjie Wang ◽  
Bin Yuan ◽  
...  
Keyword(s):  
Volatile Organic Compounds ◽  
Organic Compounds ◽  
Mass Spectrometer ◽  
Mass Spectra ◽  
Urban Atmosphere ◽  
Urban Site ◽  
Oh Radicals ◽  
Secondary Formation ◽  
Volatile Organic ◽  
Tof Ms

Abstract. Volatile organic compounds (VOCs) play important roles in the tropospheric atmosphere. In this study, VOCs were measured at an urban site in Guangzhou, one of the megacities in the Pearl River Delta (PRD), using a gas chromatograph–mass spectrometer/flame ionization detection (GC–MS/FID) and a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS). Diurnal profile analyses show that stronger chemical removal by OH radicals for more reactive hydrocarbons occurs during the daytime, which is used to estimate the daytime average OH radical concentration. In comparison, diurnal profiles of oxygenated volatile organic compounds (OVOCs) indicate evidence of contributions from secondary formation. Detailed source analyses of OVOCs, using a photochemical age-based parameterization method, suggest important contributions from both primary emissions and secondary formation for measured OVOCs. During the campaign, around 1700 ions were detected in PTR-ToF-MS mass spectra, among which there were 462 ions with noticeable concentrations. VOC signals from these ions are quantified based on the sensitivities of available VOC species. OVOC-related ions dominated PTR-ToF-MS mass spectra, with an average contribution of 73 % ± 9 %. Combining measurements from PTR-ToF-MS and GC–MS/FID, OVOCs contribute 57 % ± 10 % to the total concentration of VOCs. Using concurrent measurements of OH reactivity, OVOCs measured by PTR-ToF-MS contribute greatly to the OH reactivity (19 % ± 10 %). In comparison, hydrocarbons account for 21 % ± 11 % of OH reactivity. Adding up the contributions from inorganic gases (48 % ± 15 %), ∼ 11 % (range of 0 %–19 %) of the OH reactivity remains `missing”, which is well within the combined uncertainties between the measured and calculated OH reactivity. Our results demonstrate the important roles of OVOCs in the emission and evolution budget of VOCs in the urban atmosphere.

scholarly journals Simultaneous factor analysis of organic particle and gas mass spectra: AMS and PTR-MS measurements at an urban site

2010 ◽  
Vol 10 (4) ◽  
pp. 1969-1988 ◽  
Author(s):  
J. G. Slowik ◽  
A. Vlasenko ◽  
M. McGuire ◽  
G. J. Evans ◽  
J. P. D. Abbatt
Keyword(s):  
Mass Spectrometer ◽  
Mass Spectra ◽  
Relative Weight ◽  
Proton Transfer Reaction ◽  
Urban Site ◽  
Emission Sources ◽  
Reaction Products ◽  
Aerosol Mass ◽  
Direct Emission ◽  
The Individual

Abstract. During the winter component of the SPORT (Seasonal Particle Observations in the Region of Toronto) field campaign, particulate non-refractory chemical composition and concentration of selected volatile organic compounds (VOCs) were measured by an Aerodyne time-of-flight aerosol mass spectrometer (AMS) and a proton transfer reaction-mass spectrometer (PTR-MS), respectively. Sampling was performed in downtown Toronto ~15 m from a major road. The mass spectra from the AMS and PTR-MS were combined into a unified dataset, which was analysed using positive matrix factorization (PMF). The two instruments were given balanced weight in the PMF analysis by the application of a scaling factor to the uncertainties of each instrument. A residual based metric, Δesc, was used to evaluate the instrument relative weight within each solution. The PMF analysis yielded a 6-factor solution that included factors characteristic of regional transport, local traffic emissions, charbroiling and oxidative processing. The unified dataset provides information on emission sources (particle and VOC) and atmospheric processing that cannot be obtained from the datasets of the individual instruments: (1) apportionment of oxygenated VOCs to either direct emission sources or secondary reaction products; (2) improved correlation of oxygenated aerosol factors with photochemical age; and (3) increased detail regarding the composition of oxygenated organic aerosol factors. This analysis represents the first application of PMF to a unified AMS/PTR-MS dataset.

scholarly journals Chemical evolution of organic aerosol in Los Angeles during the CalNex 2010 study

2013 ◽  
Vol 13 (5) ◽  
pp. 12867-12911 ◽  
Author(s):  
R. Holzinger ◽  
A. H. Goldstein ◽  
P. L. Hayes ◽  
J. L. Jimenez ◽  
J. Timkovsky
Keyword(s):  
Los Angeles ◽  
Mass Spectrometer ◽  
Mass Spectra ◽  
Total Concentration ◽  
Carbon Number ◽  
Organic Aerosol ◽  
Proton Transfer Reaction ◽  
Organic N ◽  
Aerosol Mass ◽  
Photochemical Age

Abstract. During the CalNex study (15 May to 16 June 2010) a large suite of instruments was operated at the Los Angeles area ground supersite to characterize the sources and atmospheric processing of atmospheric pollution. The thermal-desorption proton-transfer-reaction mass-spectrometer (TD-PTR-MS) was deployed to an urban area for the first time and detected 691 organic ions in aerosol samples, the mean total concentration of which was estimated as 3.3 μg m−3. Based on comparison to total organic aerosol (OA) measurements, we estimate that approximately 50% of the OA mass at this site was directly measured by the TD-PTR-MS. Based on correlations with aerosol mass spectrometer (AMS) OA components, the ions were grouped to represent hydrocarbon-like OA (HOA), local OA (LOA), semi-volatile oxygenated OA (SV-OOA), and low volatility oxygenated OA (LV-OOA). Mass spectra and thermograms of the ion groups are mostly consistent with the assumed sources and/or photochemical origin of the OA components. The mass spectra of ions representing the primary components HOA and LOA included the highest m/z, consistent with their higher resistance to thermal decomposition, and they were volatilized at lower temperatures. Photochemical ageing weakens C-C bond strengths (also resulting in chemical fragmentation), and produces species of lower volatility (through the addition of functional groups). Accordingly the mass spectra of ions representing the oxidized OA components (SV-OOA, and LV-OOA) lack the highest masses and they are volatilized at higher temperatures. Chemical parameters like mean carbon number (nC), mean carbon oxidation state (OSC), and the atomic ratios O/C and H/C of the ion groups are consistent with the expected sources and photochemical processing of the aerosol components. Our data suggest that chemical fragmentation gains importance over functionalization as photochemical age of OA increases. Surprisingly, the photochemical age of OA decreases during the daytime hours, demonstrating the importance of rapid production of new (photochemically young) SV-OOA during daytime. The PTR detects higher organic N concentrations than the AMS, the reasons for which are not well understood and cannot be explained by known artifacts related to PTR or the AMS. The median atomic N/C ratio (6.4%) of the ion group representing LV-OOA is a factor 2 higher than N/C of any other ion group. This suggests a multiphase chemical source involving ammonium ions is contributing to LV-OOA.

scholarly journals Simultaneous factor analysis of organic particle and gas mass spectra: AMS and PTR-MS measurements at an urban site

2009 ◽  
Vol 9 (2) ◽  
pp. 6739-6785 ◽  
Author(s):  
J. G. Slowik ◽  
A. Vlasenko ◽  
M. McGuire ◽  
G. J. Evans ◽  
J. P. D. Abbatt
Keyword(s):  
Mass Spectrometer ◽  
Mass Spectra ◽  
Relative Weight ◽  
Proton Transfer Reaction ◽  
Urban Site ◽  
Equal Weight ◽  
Reaction Products ◽  
Aerosol Mass ◽  
Direct Emission ◽  
The Individual

Abstract. During the winter component of the SPORT (Seasonal Particle Observations in the Region of Toronto) field campaign, particulate non-refractory chemical composition and concentration of selected volatile organic compounds (VOCs) were measured by an Aerodyne time-of-flight aerosol mass spectrometer (AMS) and a proton transfer reaction-mass spectrometer (PTR-MS), respectively. Sampling was performed in downtown Toronto ~15 m from a major road. The mass spectra from the AMS and PTR-MS were combined into a unified dataset, which was analyzed using positive matrix factorization (PMF). The two instruments were given equal weight in the PMF analysis by application of a scaling factor to the uncertainties of each instrument. A residual based metric, Δesc, was used to evaluate the relative weight. The PMF analysis yielded a 5-factor solution that included factors characteristic of regional transport, local traffic emissions, charbroiling, and oxidative processing. The unified dataset provides information on particle and VOC sources and atmospheric processing that cannot be obtained from the datasets of the individual instruments, such as apportionment of oxygenated VOCs to direct emission sources vs. secondary reaction products, improved correlation of oxygenated aerosol factors with photochemical age, and increased detail regarding the composition of oxygenated organic aerosol factors. This analysis represents the first application of PMF to a unified AMS/PTR-MS dataset.

Emissions and chemistry of volatile organic compounds (VOCs) in urban air: important contributions from oxygenated compounds

2020 ◽  
Author(s):  
Bin Yuan ◽  
Caihong Wu ◽  
Chaomin Wang ◽  
Sihang Wang ◽  
Wenjie Wang ◽  
...  
Keyword(s):  
Volatile Organic Compounds ◽  
Organic Compounds ◽  
Mass Spectrometer ◽  
Proton Transfer Reaction ◽  
Urban Site ◽  
Oxygenated Compounds ◽  
Secondary Formation ◽  
Volatile Organic ◽  
Particle Pollution ◽  
Comparative Reactivity

<p>Volatile organic compounds (VOCs) play central roles in formation of ozone and secondary particles. However, emissions and evolution of VOCs remain uncertain in different environments, including urban regions. A field campaign was conducted at an urban site of Guangzhou in September-November of 2018 to study ozone and particle pollution in this region. VOCs species were measured by both a gas chromatography mass spectrometer (GC-MS/FID) and a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS). Another PTR-MS associated with comparative reactivity method (CRM) was used for quantifing OH reactivity in the atmosphere. In total, around 1700 ions were detected in mass spectra of PTR-TOF during this campaign, among of which 438 ions are with noticeable concentrations in the atmosphere. For all of the measured VOCs species, the total average concentrations of oxygenated VOCs was 28.2 ppb, which are significantly higher than other VOCs classes, namely alkanes (19.6 ppb), aromatics (4.4 ppb) and alkenes (2.9 ppb). These oxygenated VOCs contribute large fractions (campaign-average: 28%) of the total measured OH reactivity, which leaves only a small fraction of measured reactivity as “missing”. We will show that primary emission and secondary formation both contribute to the measured OVOCs. These results indicate important roles of OVOCs in emissions and evolution budget of VOCs in the atmosphere.</p>

scholarly journals Chemical evolution of organic aerosol in Los Angeles during the CalNex 2010 study

2013 ◽  
Vol 13 (19) ◽  
pp. 10125-10141 ◽  
Author(s):  
R. Holzinger ◽  
A. H. Goldstein ◽  
P. L. Hayes ◽  
J. L. Jimenez ◽  
J. Timkovsky
Keyword(s):  
Los Angeles ◽  
Mass Spectrometer ◽  
Mass Spectra ◽  
Total Concentration ◽  
Carbon Number ◽  
Organic Aerosol ◽  
Proton Transfer Reaction ◽  
Organic N ◽  
Aerosol Mass ◽  
Photochemical Age

Abstract. During the CalNex study (15 May to 16 June 2010) a large suite of instruments was operated at the Los Angeles area ground supersite to characterize the sources and atmospheric processing of atmospheric pollution. The thermal-desorption proton-transfer-reaction mass-spectrometer (TD-PTR-MS) was deployed to an urban area for the first time and detected 691 organic ions in aerosol samples, the mean total concentration of which was estimated as 3.3 μg m−3. Based on comparison to total organic aerosol (OA) measurements, we estimate that approximately 50% of the OA mass at this site was directly measured by the TD-PTR-MS. Based on correlations with aerosol mass spectrometer (AMS) OA components, the ions were grouped to represent hydrocarbon-like OA (HOA), local OA (LOA), semi-volatile oxygenated OA (SV-OOA), and low volatility oxygenated OA (LV-OOA). Mass spectra and thermograms of the ion groups are mostly consistent with the assumed sources and/or photochemical origin of the OA components. The mass spectra of ions representing the primary components HOA and LOA included the highest m/z, consistent with their higher resistance to thermal decomposition, and they were volatilized at lower temperatures (~ 150 °C). Photochemical ageing weakens C-C bond strengths (also resulting in chemical fragmentation), and produces species of lower volatility (through the addition of functional groups). Accordingly the mass spectra of ions representing the oxidized OA components (SV-OOA, and LV-OOA) lack the highest masses and they are volatilized at higher temperatures (250–300 °C). Chemical parameters like mean carbon number (nC), mean carbon oxidation state (OSC), and the atomic ratios O / C and H / C of the ion groups are consistent with the expected sources and photochemical processing of the aerosol components. Our data suggest that chemical fragmentation gains importance over functionalization as photochemical age of OA increases. Surprisingly, the photochemical age of OA decreases during the daytime hours, demonstrating the importance of rapid production of new (photochemically young) SV-OOA during daytime. The PTR detects higher organic N concentrations than the AMS, the reasons for which are not well understood and cannot be explained by known artifacts related to PTR or the AMS. The median atomic N / C ratio (6.4%) of the ion group representing LV-OOA is a factor 2 higher than N / C of any other ion group. This suggests a multiphase chemical source involving ammonium ions is contributing to LV-OOA.

Composition of ultrafine particles in urban Beijing: Measurement using a thermal desorption chemical ionization mass spectrometer

2021 ◽  
Author(s):  
Xiaoxiao Li ◽  
Yuyang Li ◽  
Michael Lawler ◽  
Jiming Hao ◽  
James Smith ◽  
...  
Keyword(s):  
Mass Spectrometer ◽  
Thermal Desorption ◽  
Ultrafine Particles ◽  
Chemical Ionization ◽  
Urban Atmosphere ◽  
Ionization Mass ◽  
Radioactive Materials ◽  
Desorption Chemical Ionization ◽  
Wide Range ◽  
Primary Emissions

<p>Ultrafine particles (UFPs) dominate the particle number population in the urban atmosphere and revealing their chemical composition is important. The thermal desorption chemical ionization mass spectrometer (TDCIMS) can semi-continuously measure UFP composition at the molecular level. We modified a TDCIMS and deployed it in urban Beijing. Radioactive materials in the TDCIMS for aerosol charging and chemical ionization were replaced by soft X-ray ionizers so that it can be operated in countries with tight regulations on radioactive materials. Protonated N-methyl-2-pyrrolidone ions were used as the positive reagent ion, which selectively detects ammonia and low-molecular weight-aliphatic amines and amides vaporized from the particle phase. With superoxide as the negative reagent ion, a wide range of inorganic and organic compounds were observed, including nitrate, sulfate, aliphatic acids with carbon numbers up to 18, and highly oxygenated CHO, CHON, and CHOS compounds. The latter two can be attributed to parent ions or the decomposition products of organonitrates and organosulfates/organosulfonates, respectively. Components from both primary emissions and secondary formation of UFPs were identified. Compared to the UFPs measured at forest and marine sites, those in urban Beijing contain more nitrogen-containing and sulfur-containing compounds. These observations illustrate unique features of the UFPs in this polluted urban environment and provide insights into their origins.</p>

scholarly journals Measurements of higher alkanes using NO<sup>+</sup> chemical ionization in PTR-ToF-MS: important contributions of higher alkanes to secondary organic aerosols in China

2020 ◽  
Vol 20 (22) ◽  
pp. 14123-14138
Author(s):  
Chaomin Wang ◽  
Bin Yuan ◽  
Caihong Wu ◽  
Sihang Wang ◽  
Jipeng Qi ◽  
...  
Keyword(s):  
Chemical Ionization ◽  
Secondary Organic Aerosols ◽  
Organic Aerosols ◽  
Proton Transfer Reaction ◽  
Rural Site ◽  
Urban Site ◽  
Accurate Estimation ◽  
High Time Resolution ◽  
The North ◽  
Tof Ms

Abstract. Higher alkanes are a major class of intermediate-volatility organic compounds (IVOCs), which have been proposed to be important precursors of secondary organic aerosols (SOA) in the atmosphere. Accurate estimation of SOA from higher alkanes and their oxidation processes in the atmosphere is limited, partially due to the difficulty of their measurement. High-time-resolution (10 s) measurements of higher alkanes were performed using NO+ chemical ionization in proton transfer reaction time-of-flight mass spectrometry (NO+ PTR-ToF-MS) at an urban site in Guangzhou in the Pearl River Delta (PRD) and at a rural site in the North China Plain (NCP). High concentrations were observed in both environments, with significant diurnal variations. At both sites, SOA production from higher alkanes is estimated from their photochemical losses and SOA yields. Higher alkanes account for significant fractions of SOA formation at the two sites, with average contributions of 7.0 % ± 8.0 % in Guangzhou and 9.4 % ± 9.1 % in NCP, which are comparable to or even higher than both single-ring aromatics and naphthalenes. The significant contributions of higher alkanes to SOA formation suggests that they should be explicitly included in current models for SOA formation. Our work also highlights the importance of NO+ PTR-ToF-MS in measuring higher alkanes and quantifying their contributions to SOA formation.

scholarly journals Gas-to-particle partitioning of major biogenic oxidation products: a study on freshly formed and aged biogenic SOA

2018 ◽  
Vol 18 (17) ◽  
pp. 12969-12989 ◽  
Author(s):  
Georgios I. Gkatzelis ◽  
Thorsten Hohaus ◽  
Ralf Tillmann ◽  
Iulia Gensch ◽  
Markus Müller ◽  
...  
Keyword(s):  
Proton Transfer ◽  
Organic Compounds ◽  
Mass Spectrometer ◽  
Transfer Reaction ◽  
Theoretical Calculations ◽  
Proton Transfer Reaction ◽  
Oxidation Products ◽  
Particle Phase ◽  
Tof Ms ◽  
Good Agreement

Abstract. Secondary organic aerosols (SOAs) play a key role in climate change and air quality. Determining the fundamental parameters that distribute organic compounds between the phases is essential, as atmospheric lifetime and impacts change drastically between the gas and particle phase. In this work, gas-to-particle partitioning of major biogenic oxidation products was investigated using three different aerosol chemical characterization techniques. The aerosol collection module, the collection thermal desorption unit, and the chemical analysis of aerosols online are different aerosol sampling inlets connected to a proton-transfer reaction time-of-flight mass spectrometer (ACM-PTR-ToF-MS, TD-PTR-ToF-MS, and CHARON-PTR-ToF-MS, respectively, referred to hereafter as ACM, TD, and CHARON). These techniques were deployed at the atmosphere simulation chamber SAPHIR to perform experiments on the SOA formation and aging from different monoterpenes (β-pinene, limonene) and real plant emissions (Pinus sylvestris L.). The saturation mass concentration C* and thus the volatility of the individual ions was determined based on the simultaneous measurement of their signal in the gas and particle phase. A method to identify and exclude ions affected by thermal dissociation during desorption and ionic dissociation in the ionization chamber of the proton-transfer reaction mass spectrometer (PTR-MS) was developed and tested for each technique. Narrow volatility distributions with organic compounds in the semi-volatile (SVOCs – semi-volatile organic compounds) to intermediate-volatility (IVOCs – intermediate-volatility organic compounds) regime were found for all systems studied. Despite significant differences in the aerosol collection and desorption methods of the proton-transfer-reaction (PTR)-based techniques, a comparison of the C* values obtained with different techniques was found to be in good agreement (within 1 order of magnitude) with deviations explained by the different operating conditions of the PTR-MS. The C* of the identified organic compounds were mapped onto the two-dimensional volatility basis set (2D-VBS), and results showed a decrease in C* with increasing oxidation state. For all experiments conducted in this study, identified partitioning organic compounds accounted for 20–30 % of the total organic mass measured from an aerosol mass spectrometer (AMS). Further comparison between observations and theoretical calculations was performed for species found in our experiments that were also identified in previous publications. Theoretical calculations based on the molecular structure of the compounds showed, within the uncertainties ranges, good agreement with the experimental C* for most SVOCs, while IVOCs deviated by up to a factor of 300. These latter differences are discussed in relation to two main processes affecting these systems: (i) possible interferences by thermal and ionic fragmentation of higher molecular-weight compounds, produced by accretion and oligomerization reactions, that fragment in the m∕z range detected by the PTR-MS and (ii) kinetic influences in the distribution between the gas and particle phase with gas-phase condensation, diffusion in the particle phase, and irreversible uptake.

scholarly journals Evolving mass spectra of the oxidized component of organic aerosol: results from aerosol mass spectrometer analyses of aged diesel emissions

2008 ◽  
Vol 8 (5) ◽  
pp. 1139-1152 ◽  
Author(s):  
A. M. Sage ◽  
E. A. Weitkamp ◽  
A. L. Robinson ◽  
N. M. Donahue
Keyword(s):  
Mass Spectrometer ◽  
Mass Spectra ◽  
Explicit Knowledge ◽  
Organic Aerosol ◽  
Diesel Emissions ◽  
Vapor Pressures ◽  
Aerosol Mass Spectrometer ◽  
Aerosol Mass ◽  
Primary Emissions ◽  
Organic Particles

Abstract. The species and chemistry responsible for secondary organic aerosol (SOA) formation remain highly uncertain. Laboratory studies of the oxidation of individual, high-flux SOA precursors do not lead to particles with mass spectra (MS) matching those of ambient aged organic material. Additionally, the complexity of real organic particles challenges efforts to identify their chemical origins. We have previously hypothesized that SOA can form from the atmospheric oxidation of a large suite of precursors with varying vapor pressures. Here, we support this hypothesis by using an aerosol mass spectrometer to track the chemical evolution of diesel exhaust as it is photochemically oxidized in an environmental chamber. With explicit knowledge of the condensed-phase MS of the primary emissions from our engine, we are able to decompose each recorded MS into contributing primary and secondary spectra throughout the experiment. We find that the SOA becomes increasingly oxidized as a function of time, quickly approaching a final MS that closely resembles that of ambient aged organic particulate matter. This observation is consistent with our hypothesis of an evolving suite of SOA precursors. Low vapor pressure, semi-volatile organic emissions can form condensable products with even a single generation of oxidation, resulting in an early-arising, relatively less-oxidized SOA. Continued gas-phase oxidation can form highly oxidized SOA in surprisingly young air masses via reaction mechanisms that can add multiple oxygen atoms per generation and result in products with sustained or increased reactivity toward OH.

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