The chemical reactions driving atmospheric oxidation during the day strongly influence the subsequent nighttime chemistry and vice versa. Here, the interconnection of the alpha-pinene daytime and nighttime chemistry was studied to answer the following question: How does the generated secondary organic aerosol evolve during the time of the day transition? The study conducted two types of simulation chamber experiments reflecting the daily changes in atmospheric oxidants: night-to-day and day-to-night processing, where day processing was reflected as OH radical oxidation, and night processing as NO3 radical oxidation. In the day-to-night transition, alpha-pinene oxidation produced more SOA, with yields increasing under higher relative humidity (RH), indicating that humidity enhances the SOA formation efficiency in this process, in contrast to the night-to-day process, which was less sensitive to humidity changes. The main products of the reaction of alpha-pinene and NO3 radicals were identified as organic nitrates (ON) in all conducted experiments, contributing to the total aerosol mass of around 40%. Under daytime conditions, the fraction of ON decreased, while that of SOA-bound peroxides became more dominant, suggesting a shift in chemical composition with the changing oxidation regime. The calculated fractional contribution of carboxylic acids was two times higher for experiments where SOA formation originated from the OH radical reaction (up to an 89% mass fraction).

Atmospheric simulation chambers (ASCs) are one of the most advanced tools for the experimental investigation of the oxidation of volatile organic compounds (VOCs) and the subsequent secondary organic aerosol (SOA) formation. Toluene is one of the most prevalent anthropogenic VOCs. Its photo-oxidation yields a wide range of products in the gas phase and a significant amount of SOA. Some of the remaining uncertainties about toluene atmospheric chemistry are possibly linked with chamber artifacts. In this study, several atmospheric simulation chambers, characterized by a great diversity (size, shape, material of walls, light source, instrumentation, measurement techniques, etc.), performed several toluene photo-oxidation experiments under different pre-set conditions (levels of toluene, NOx, and relative humidity, presence, or lack of seeds). A model based on the Master Chemical Mechanism (MCM) and a SOA production module were used to facilitate the synthesis of the results. The results of the multiple-chamber toluene experiments suggest that a combination of facilities can provide a better picture of the overall behavior and that significant gaps remain in our understanding of the system, especially in the later oxidation stages. For cresol, a first-generation product, the observed gas-phase yields, ranging from 3

The atmospheric oxidation of isoprene by OH radicals under low NOx conditions primarily leads to hydroxy hydroperoxides (ISOPOOH), and further, to isoprene epoxy diols (IEPOX), which have been identified as important SOA precursors. Recent studies indicate that an additional class of highly oxidized ISOPOOH oxidation products might contribute equally to SOA. Nonetheless, kinetic investigations of the phase transfer of ISOPOOH and of its oxidation products are largely missing, resulting in large uncertainties in understanding the respective atmospheric chemistry and its implications. In the present work, the partitioning behavior of synthetic 1,2-ISOPOOH and its OH oxidation products was investigated in chamber experiments with a (NO3-)-CI-APi-ToFMS under low NOx conditions for sulfate seed particles under variation of relative humidity and particle acidity conditions. For acidic sulfate particles, a reactive uptake coefficient of gamma(( 1,2-ISOPOOH)(pH=0)) = (9 +/- 4) x 10(-3) as determined. For monomeric oxidation products, a parametrization of measured uptake coefficients based on estimated vapor pressures was obtained with gamma( C-d product) = -1.302 x 10(-2) x ln(vap.press.center dot atm(-1)) - 0.1662, n = 15, R-2 = 0.727. Dimeric RO2 accretion products were observed in the gas phase for the first time. In a model study, an unexpectedly large proportion of these products was used to constrain the rate constant of intramolecular hydrogen shift reactions of C5O6H11 radicals to an upper limit of k = 0.002 s(-1). To evaluate the atmospheric relevance, the results of this study were implemented in a remote-case model study performed with F0AM, which resulted in an increased SOA mass formation of 31% by way of the investigated ISOPOOH oxidation and phase transfer pathways.

Abstract The detailed chemical characterization of gas and particle phase species is essential for interpreting the results of atmospheric simulation chamber experiments. Although the application of online techniques has advanced significantly over the last two decades, offline analytical methods such as GC–MS and LC–MS are still frequently used. In this chapter, the approaches commonly employed for gas and particle sampling prior to subsequent offline analysis are described in detail. Methods involving the use of cartridges, canisters, bags and sorbent tubes for gas sampling are described with the support of examples reported in the literature. Technical descriptions related to the application of different types of filters, inertial classifiers and particle-into-liquid samplers for the collection of particles are also provided.

AbstractThe detailed chemical characterization of gas and particle phase species is essential for interpreting the results of atmospheric simulation chamber experiments. Although the application of online techniques has advanced significantly over the last two decades, offline analytical methods such as GC–MS and LC–MS are still frequently used. In this chapter, the approaches commonly employed for gas and particle sampling prior to subsequent offline analysis are described in detail. Methods involving the use of cartridges, canisters, bags and sorbent tubes for gas sampling are described with the support of examples reported in the literature. Technical descriptions related to the application of different types of filters, inertial classifiers and particle-into-liquid samplers for the collection of particles are also provided.

Hydroxy hydroperoxides are formed upon OH oxidation of volatile organic compounds in the atmosphere and may contribute to secondary organic aerosol growth and aqueous phase chemistry after phase transfer to particles. Although the detection methods for oxidized volatile organic compounds improved much over the past decades, the limited availability of synthetic standards for atmospherically relevant hydroxy hydroperoxides prevented comprehensive investigations for the most part. Here, we present a straightforward improved synthetic access to isoprene-derived hydroxy hydroperoxides, i.e., 1,2-ISOPOOH and 4,3-ISOPOOH. Furthermore, we present the first successful synthesis of an α-pinene derived hydroxy hydroperoxide. All products were identified by 1H, 13C NMR spectroscopy for structure elucidation, additional 2D NMR experiments were performed. Furthermore, gas-phase FTIR- and UV/VIS spectra are presented for the first time. Using the measured absorption cross section, the atmospheric photolysis rate of up to 2.1 × 10−3 s−1 was calculated for 1,2-ISOPOOH. Moreover, we present the investigation of synthesized hydroxy hydroperoxides in an aerosol chamber study by online MS techniques, namely PTR-ToFMS and (NO3−)-CI-APi-ToFMS. Fragmentation patterns recorded during these investigations are presented as well. For the (NO3−)-CI-APi-ToFMS, a calibration factor for 1,2-ISOPOOH was calculated as 4.44 × 10−5 ncps·ppbv−1 and a LOD (3σ, 1 min average) = 0.70 ppbv.

The oxidation of biogenic and anthropogenic compounds leads to the formation of secondary organic aerosol mass (SOA). The present study aims to investigate α-pinene, limonene, and m-cresol with regards to their SOA formation potential dependent on relative humidity (RH) under night- (NO3 radicals) and daytime conditions (OH radicals) and the resulting chemical composition. It was found that SOA formation potential of limonene with NO3 under dry conditions significantly exceeds that of the OH-radical reaction, with SOA yields of 15–30 % and 10–21 %, respectively. Additionally, the nocturnal SOA yield was found to be very sensitive towards RH, yielding more SOA under dry conditions. In contrast, the SOA formation potential of α-pinene with NO3 slightly exceeds that of the OH-radical reaction, independent from RH. On average, α-pinene yielded SOA with about 6–7 % from NO3 radicals and 3–4 % from OH-radical reaction. Surprisingly, unexpectedly high SOA yields were found for m-cresol oxidation with OH radicals (3–9 %), with the highest yield under elevated RH (9 %), which is most likely attributable to a higher fraction of 3-methyl-6-nitro-catechol (MNC). While α-pinene and m-cresol SOA was found to be mainly composed of water-soluble compounds, 50–68 % of nocturnal SOA and 22–39 % of daytime limonene SOA are water-insoluble. The fraction of SOA-bound peroxides which originated from α-pinene varied between 2 and 80 % as a function of RH. Furthermore, SOA from α-pinene revealed pinonic acid as the most important particle-phase constituent under day- and nighttime conditions with a fraction of 1–4 %. Other compounds detected are norpinonic acid (0.05–1.1 % mass fraction), terpenylic acid (0.1–1.1 % mass fraction), pinic acid (0.1–1.8 % mass fraction), and 3-methyl-1,2,3-tricarboxylic acid (0.05–0.5 % mass fraction). All marker compounds showed higher fractions under dry conditions when formed during daytime and showed almost no RH effect when formed during night.

During haze periods in the North China Plain, extremely high NO concentrations have been observed, commonly exceeding 1 ppbv, preventing the classical gas-phase H2O2 formation through HO2 recombination. Surprisingly, H2O2 mixing ratios of about 1 ppbv were observed repeatedly in winter 2017. Combined field observations and chamber experiments reveal a photochemical in-particle formation of H2O2, driven by transition metal ions (TMIs) and humic-like substances (HULIS). In chamber experiments, steady-state H2O2 mixing ratios of 116 ± 83 pptv were observed upon the irradiation of TMI- and HULIS-containing particles. Correspondingly, H2O2 formation rates of about 0.2 ppbv h-1 during the initial irradiation periods are consistent with the H2O2 rates observed in the field. A novel chemical mechanism was developed explaining the in-particle H2O2 formation through a sequence of elementary photochemical reactions involving HULIS and TMIs. Dedicated box model studies of measurement periods with relative humidity >50% and PM2.5 ≥ 75 μg m-3 agree with the observed H2O2 concentrations and time courses. The modeling results suggest about 90% of the particulate sulfate to be produced from the SO2 reaction with OH and HSO3- oxidation by H2O2. Overall, under high pollution, the H2O2-caused sulfate formation rate is above 250 ng m-3 h-1, contributing to the sulfate formation by more than 70%.

Atmospheric nitrophenols are pollutants of concern due to their toxicity and light-absorption characteristics and their low reactivity resulting in relatively long residence times in the environment. We investigate multiphase nitrophenol formation from guaiacol in a simulated atmospheric aerosol and support observations with the corresponding chemical mechanisms. The maximal secondary organic aerosol (SOA) yield (42%) is obtained under illumination at 80% relative humidity. Among the identified nitrophenols, 4-nitrocatechol (3.6% yield) is the prevailing species in the particulate phase. The results point to the role of water in catechol and further 4-nitrocatechol formation from guaiacol. In addition, a new pathway of dark nitrophenol formation is suggested, which prevailed in dry air and roughly yielded 1% nitroguaiacols. Furthermore, the proposed mechanism possibly leads to oligomer formation via a phenoxy radical formation by oxidation with HONO.

Abstract. The oxidation of biogenic and anthropogenic compounds leads to the formation of secondary organic aerosol mass (SOA). The present study aims to investigate α-pinene, limonene and m-cresol with regards to their SOA formation potential dependent on relative humidity (RH) under night- (NO3 radicals) and day-time conditions (OH radicals) and the resulting chemical composition. It was found that SOA formation potential of limonene with NO3 significantly exceeds the one of the OH radical reaction, with SOA yields of 15–30 % and 10–21 %, respectively. Additionally, the nocturnal SOA yield was found to be very sensitive towards RH, yielding more SOA under dry conditions. On the contrary, the SOA formation potential of α-pinene with NO3 slightly exceeds that of the OH radical reaction, independent from RH. In average, α-pinene yielded SOA with about 6–7 % from NO3 radicals and 3–4 % from OH radical reaction. Surprisingly, unexpected high SOA yields were found for m-cresol oxidation with OH radicals (3–9 %) with the highest yield under elevated RH (9 %) which is most likely attributed to a higher fraction of 3-methyl-6-nitro-catechol (MNC). While α-pinene and m-cresol SOA was found to be mainly composed of water-soluble compounds, 50–68 % of nocturnal SOA and 22–39 % of daytime limonene SOA is water-insoluble. The fraction of SOA-bound peroxides which originated from α-pinene varied between 2–80 % as a function of RH. Furthermore, SOA from α-pinene revealed pinonic acid as the most important particle-phase constituent under day- and night-time conditions with fraction of 1–4 %. Further compounds detected are norpinonic acid (0.05–1.1 % mass fraction), terpenylic acid (0.1–1.1 % mass fraction), pinic acid (0.1–1.8 % mass fraction) and 3-methyl-1,2,3-tricarboxylic acid (0.05–0.5 % mass fraction). All marker compounds showed higher fractions under dry conditions when formed during daytime and almost no RH effect when formed during night.

The photooxidation and ozonolysis of a wide range of anthropogenic and biogenic compounds results in the formation of polyfunctional oxygenated organic volatile compounds (OVOCs), which are ubiquitous intermediates and important source of radicals, as well as a significant contributor to the organic fraction of aerosol, playing an important role in the air quality and in the health. With the aim of ensuring a good quality of the measurements, an intercomparison campaign on instruments and techniques to measure OVOCs was held at the EUPHORE atmosphere simulation chamber (Valencia, Spain), allowing the simultaneous sampling from the same air mixture under controlled conditions and facilitating the characterization of the instrumental performance. This intercomparison is part of the activities within the EUROCHAMP-2020 project (www.eurochamp.org) and is aligned with the ACTRIS objectives. On-line and off-line techniques were employed. These comprised different approaches of mass spectrometers and optical devices as on-line systems and a suite of commercial and non-commercial cartridges as off-line. Some of the experiments were also modeled. The intercomparison included three types of scenarios, the two later simulating typical urban and semi-urban conditions: 1. Synthetic mixture of carbonyls, 2. Products from photooxidations of biogenic and anthropogenic VOCs and 3. Products from ozonolysis of biogenic VOC and monoterpenes, under different relative humidity conditions. This work represents a step forward in the evaluation and conclusions of the data obtained. An overall good agreement was found among the techniques with good correlations, especially in the synthetic mixtures, as expected, while these parameters worsen under more realistic conditions with complex gas samples. Regarding off-line techniques, good practices on sample storage and use are discussed. Advanced analysis using clusters was also applied to group instruments with similar behaviour, indicating similar performance among the on-line instruments although partitions revealed certain discrepancies. The role of the gas mixture, the calibration set-up and the humidity conditions are discussed in this work. Acknowledgements This project/work has received funding from the European Union’s Horizon 2020 research and innovation programme through the EUROCHAMP-2020 Infrastructure Activity under grant agreement No 730997. CAPOX from the Spanish R+D program, Ref RTU2018-097768-B-C21 and PROMETEO from the GVA excellence R&D program, Ref PROMETEO (2019/110) are acknowledged. F. CEAM is partly supported by Generalitat Valenciana.

Sulfur and nitrogen containing organic compounds, such as organosulfates (OSs) and nitrooxy organosulfates (NOSs), are recognized to be ubiquitously present in secondary organic aerosol (SOA). However, little is known about the chemical mechanisms or the required conditions for the formation of these compounds in the ambient atmosphere. Earlier studies have commonly suggested that OSs are predominantly formed through the reaction of organic gaseous epoxides with acidic sulfate particles. However, this epoxide pathway often fails to explain the formation of (N)OSs from monoterpenes. Moreover, recent studies highlight the potential role of gas-phase SO2 and organic peroxides for the formation of OSs, which might serve as predominant precursors for OSs and NOSs from atmospheric monoterpene oxidation. Here, we conducted a series of chamber experiments to elucidate the formation mechanisms of (N)OSs from α-pinene oxidation during daytime and nighttime conditions. In particular, we focused on the role of organic peroxides and S(IV) (i.e., gas-phase SO2 and particulate SO32–) in contrast to organic epoxides and isotope-labelled particulate sulfate (i.e., S(VI)). SOA particles were analyzed online by extractive electrospray ionization coupled with high-resolution Orbitrap mass spectrometry (EESI-Orbitrap MS) allowing an unambiguous identification of OS and NOS species with a high time resolution. Additionally, filter samples were collected and analyzed by liquid chromatography (LC) coupled with Orbitrap MS to determine the presence of isomeric compounds. Consistently, online and offline Orbitrap MS analysis showed that particulate sulfate played a minor role in the formation of OSs and NOSs. In contrast, (N)OSs were rapidly formed upon addition of either gaseous SO2 or particulate SO32–, suggesting S(IV) to react with organic peroxides that were formed through monoterpene oxidation. Based on these experiments, we identified specific NOS species that are formed only through either daytime or nighttime chemistry, and thus, might serve as marker molecules. Moreover, we present complete formation pathways for these species. Our study indicates that in contrast to previous work, the formation of OSs and NOSs does not require acidic sulfate particles, but rather involves the reaction of organic peroxides with S(IV) in the gas phase or the particle phase.

Organosulfates (OSs), also referred to as organic sulfate esters, are well-known and ubiquitous constituents of atmospheric aerosol particles. Commonly, they are assumed to form upon mixing of air masses of biogenic and anthropogenic origin, that is, through multiphase reactions between organic compounds and acidic sulfate particles. However, in contrast to this simplified picture, recent studies suggest that OSs may also originate from purely anthropogenic precursors or even directly from biomass and fossil fuel burning. Moreover, besides classical OS formation pathways, several alternative routes have been discovered, suggesting that OS formation possibly occurs through a wider variety of formation mechanisms in the atmosphere than initially expected. During the past decade, OSs have reached a constantly growing attention within the atmospheric science community with evermore studies reporting on large numbers of OS species in ambient aerosol. Nonetheless, estimates on OS concentrations and implications on atmospheric physicochemical processes are still connected to large uncertainties, calling for combined field, laboratory, and modeling studies. In this Critical Review, we summarize the current state of knowledge in atmospheric OS research, discuss unresolved questions, and outline future research needs, also in view of reductions of anthropogenic sulfur dioxide (SO2) emissions. Particularly, we focus on (1) field measurements of OSs and measurement techniques, (2) formation pathways of OSs and their atmospheric relevance, (3) transformation, reactivity, and fate of OSs in atmospheric particles, and (4) modeling efforts of OS formation and their global abundance.

This paper presents a new CAPRAM–GECKO-A protocol for mechanism auto-generation of aqueous-phase organic processes. For the development, kinetic data in the literature were reviewed and a database with 464 aqueous-phase reactions of the hydroxyl radical with organic compounds and 130 nitrate radical reactions with organic compounds has been compiled and evaluated. Five different methods to predict aqueous-phase rate constants have been evaluated with the help of the kinetics database: gas–aqueous phase correlations, homologous series of various compound classes, radical reactivity comparisons, Evans–Polanyi-type correlations, and structure–activity relationships (SARs). The quality of these prediction methods was tested as well as their suitability for automated mechanism construction. Based on this evaluation, SARs form the basis of the new CAPRAM–GECKO-A protocol. Evans–Polanyi-type correlations have been advanced to consider all available H atoms in a molecule besides the H atoms with only the weakest bond dissociation enthalpies (BDEs). The improved Evans–Polanyi-type correlations are used to predict rate constants for aqueous-phase NO3 and organic compounds reactions. Extensive tests have been performed on essential parameters and on highly uncertain parameters with limited experimental data. These sensitivity studies led to further improvements in the new CAPRAM–GECKO-A protocol but also showed current limitations. Biggest uncertainties were observed in uptake processes and the estimation of Henry's law coefficients as well as radical chemistry, in particular the degradation of alkoxy radicals. Previous estimation methods showed several deficits, which impacted particle growth. For further evaluation, a 1,3,5-trimethylbenzene oxidation experiment has been performed in the aerosol chamber “Leipziger Aerosolkammer” (LEAK) at high relative humidity conditions and compared to a multiphase mechanism using the Master Chemical Mechanism (MCMv3.2) in the gas phase and using a methylglyoxal oxidation scheme of about 600 reactions generated with the new CAPRAM–GECKO-A protocol in the aqueous phase. While it was difficult to evaluate single particle constituents due to concentrations close to the detection limits of the instruments applied, the model studies showed the importance of aqueous-phase chemistry in respect to secondary organic aerosol (SOA) formation and particle growth. The new protocol forms the basis for further CAPRAM mechanism development towards a new version 4.0. Moreover, it can be used as a supplementary tool for aerosol chambers to design and analyse experiments of chemical complexity and help to understand them on a molecular level.

Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO3) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. This interaction has been recognized for more than 3 decades, during which time a large body of research has emerged from laboratory, field, and modeling studies. NO3-BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone, and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms, and organic aerosol yields for NO3-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO3 radical, the difficulty of characterizing the spatial distributions of BVOC and NO3 within the poorly mixed nocturnal atmosphere, and the challenge of constructing appropriate boundary layer schemes and non-photochemical mechanisms for use in state-of-the-art chemical transport and chemistry-climate models.This review is the result of a workshop of the same title held at the Georgia Institute of Technology in June 2015. The first half of the review summarizes the current literature on NO3-BVOC chemistry, with a particular focus on recent advances in instrumentation and models, and in organic nitrate and secondary organic aerosol (SOA) formation chemistry. Building on this current understanding, the second half of the review outlines impacts of NO3-BVOC chemistry on air quality and climate, and suggests critical research needs to better constrain this interaction to improve the predictive capabilities of atmospheric models.