Particle nucleation from trace atmospheric vapours is important for climate since it gives rise to more than half of global cloud condensation nuclei. Sulfuric acid (H2SO4) has long been recognised to drive particle nucleation in the atmosphere and, more recently, highly oxygenated products of biogenic vapours-in particular monoterpenes such as α-pinene (C10H16)-have also been shown to nucleate under atmospheric conditions, without requiring additional vapours. This raises the question of whether a nucleation synergy exists between α-pinene oxygenated organic molecules (AP-OOM) and H2SO4, as has been suggested by early studies. Here we report new particle formation from AP-OOM and H2SO4 in the absence of base vapours such as ammonia (NH3), measured in experiments performed with the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber at cool boundary layer temperatures of -10 °C and +5 °C. We find that AP-OOM nucleation rates increase strongly when H2SO4 concentrations exceed around 106 cm-3. The enhancement is synergistic and cannot be explained as a simple linear addition of independent chemical systems. Above this threshold, the nucleation rate depends approximately linearly on H2SO4 concentration, in contrast with the strong sensitivity to H2SO4 for H2SO4-NH3 nucleation. Nucleation rates are 10-100-fold higher in the presence of ions from galactic cosmic rays or from the CERN pion beam. Based on these measurements, we have parameterised a temperature-dependent H2SO4-AP-OOM nucleation rate in the absence of base vapours and implemented it in the EMAC (ECHAM/MESSy Atmospheric Chemistry) Earth system model. In comparison with a parameterisation developed in an earlier study [Riccobono et al., Science, 2014, 344, 717-721.], the new parameterisation indicates sharply reduced nucleation rates in the boundary layer over warm regions, and increased rates over northern boreal forests.

Dimethyl sulfide (DMS; CH3SCH3) from marine phytoplankton is a notable source of atmospheric sulfur1. Its oxidation products include sulfuric acid (SA; H2SO4) and methanesulfonic acid (MSA; CH3SO3H), which has a higher yield than SA below 10 °C (ref. 2). Although SA is known to drive the formation of new particles3, which may subsequently grow and act as cloud condensation nuclei (CCN), the role of MSA remains unclear4. Here, in experiments performed under atmospheric conditions at the CERN CLOUD (Cosmics Leaving OUtdoor Droplets) chamber, we show that MSA nucleates together with ammonia (NH3) below -10 °C, at rates comparable with SA-NH3. Moreover, MSA and SA nucleate synergistically below -10 °C, forming multi-acid molecular clusters with NH3. Even at ultralow NH3 levels, MSA drives particle growth at or near the kinetic limit below 9 °C and above 40% relative humidity (RH). Because MSA and SA generally coexist at similar concentrations in cool marine regions, our findings indicate that nucleation rates may be accelerated up to tenfold and growth rates up to twofold compared with SA-NH3 alone. Our global model simulations indicate that MSA can enhance CCN concentrations, especially in polar regions. We propose that MSA might be an important driver of biogenic particles in cool, pristine marine regions of both the present-day and pre-industrial atmospheres and yet is unaccounted for in global climate models5.

Intermediate and semivolatile organic compounds (IVOC and SVOC) play a pivotal role in atmospheric secondary organic aerosol (SOA) formation, contributing substantially to fine particulate matter that impacts air quality, climate, and human health. Anthropogenic IVOC, such as hydrocarbons from diesel vehicle emissions, undergo rapid oxidation to yield low-volatility products that partition into aerosols. Similarly, also biogenic IVOC and SVOC like sesquiterpenes emitted from plants enhance SOA yields in forested regions. Quantifying their contributions to SOA formation remains challenging due to detection limitations, underscoring the need for advanced analytical methods. To elucidate the atmospheric fate of IVOC and SVOC, we herein combine a dynamic volatility separation technique with a novel flow-reactor for rapid photochemical oxidation and two FUSION PTR-TOF instruments (IONICON Analytik, Austria) for characterizing gas-phase and condensed organic compounds.The gas-phase volatility separation technique was recently introduced by Morris et al. (2024). This method utilizes the well studied absorption processes of low volatiles onto polymer tubing to separate volatility classes. Hence, via dynamic addition and removal of absorbing polymer tubing, a defined fraction of SVOC and IVOC can be efficiently removed from complex mixtures as present in ambient air. We further improved this method by using an actively cooled conductive PTFE inlet as a volatility separator. Hence, the volatility cutoff to organic precursors can be precisely adjusted by temperature without the need to switch between different types of polymer.To study the SOA formation potential with and without IVOC and SVOC, this optimized volatility separator is periodically added prior to injection of ambient air into the novel IONICON Laminar-flow Oxidation reactor (ILOx) for rapid photochemical ageing. ILOx’s design allows for transmitting particles, IVOC and even SVOC with lowermost losses. All wetted surfaces are passivated, providing best response times, even for reduced volatility gas-phase organics. The ambient air pre and post ILOx is analyzed by two FUSION PTR-TOF, one equipped with a CHARON particle inlet, and a SMPS system (Grimm Aerosol Technik, Germany). For gas-phase measurements, the instruments cover the volatility range from VOC to SVOC and offer limits of detection in the range of 100 ppqV. With the CHARON particle inlet also condensed organics are detected on a molecular composition level at highest analytical precision and lowermost limits of detection (~20 pg/m³).In this presentation we will highlight the capabilities of this new method with an example of a morning rush-hour event in Innsbruck, Austria. Hydrocarbons and aromatic hydrocarbons emitted by vehicles are significantly elevated. Most of these traffic related volatile organics can be classified as volatile and only approximately 13% can be attributed to IVOC and SVOC. Our method allows us to precisely quantify the contribution of this relatively small fraction to the potential SOA formation, revealing an overproportional impact on the SOA yield.Morris et al.: Absorption of volatile organic compounds (VOCs) by polymer tubing: implications for indoor air and use as a simple gas-phase volatility separation technique, Atmos. Meas. Tech., 17, 1545–1559, https://doi.org/10.5194/amt-17-1545-2024, 2024.

Ocean-atmosphere exchange plays an important but uncertain role for many volatile organic compounds (VOCs). Airborne eddy covariance (EC) enables direct flux quantification over large areas, but VOC applications have largely been performed over land. Here we combine the EC methodology with aircraft-based measurements from the North Atlantic Aerosol and Marine Ecosystem Study (NAAMES) and use the results to characterize air-sea VOC fluxes and to elucidate random and systematic drivers of error. Using perturbation experiments, we show that uncorrelated sensor noise (USN) causes flux biases by obscuring the sensor-wind time lag; such biases are avoided by imposing a time-lag constraint (e.g., from a higher-flux compound or time). We define the flux signal-to-noise ratio SNRf and characterize its dependence on USN and sampling regime. Results show a transition from a USN-dominated regime to one where SNRf is limited by turbulent stochasticity. The NAAMES VOC fluxes are noise-limited, whereas H2O and sensible heat fluxes lie respectively in turbulence-limited and transitional regimes. We provide a methodology for determining sensor noise levels needed for robust flux detection: for the NAAMES subset examined here, a factor of 7–23 USN reduction would enable 75 % (rather than 17 %) of measured VOC fluxes to attain SNRf>3. The airborne NAAMES results reveal VOCs with universally upward (e.g., dimethyl sulfide), downward (e.g., acetone), bidirectional (e.g., acetaldehyde), and undetectable (e.g., monoterpenes) air-sea exchange, with controls including wind speed and planktonic activity. Findings highlight the importance of USN for VOC flux quantification by airborne EC and lay a foundation for expanded use of this technique.

Nanoplastics (NP) represent a global anthropogenic pollutant. Aerosolized NP enter the atmosphere and are eventually deposited in precipitation and surface waters, facilitating their entry into the food chain. Due to their small size, these particles can translocate within organisms and penetrate tissues and cells. While the health effects of environmental exposure levels remain poorly understood, the continuous increase in global concentrations is a growing concern. However, the characterization and analysis of aerosolized or deposited NP are analytically challenging.In this study, we present a newly developed low-volume thermal desorption (TD) solution that is directly interfaced to a proton-transfer-reaction mass-spectrometer (PTR-TOF 6000X2, IONICON Analytik GmbH, Austria) for real-time detection of volatile organic compounds at lowermost concentrations. The TD unit allows for precise temporal temperature control up to 400°C. These temperatures are sufficient to efficiently thermolyze a large fraction of common NP into PTR-MS detectable volatile organic compounds (VOCs). In most cases, the released VOCs serve as specific markers for plastic identification: for instance, styrene for polystyrene (PS), methyl methacrylate for polymethyl methacrylate (PMMA), and terephthalic acid for polyethylene terephthalate (PET). Polyvinyl chloride (PVC) is identified via aromatic compounds such as benzene and naphthalene, while polyethylene (PE) exhibits a characteristic homologous series of alkenes and alkanes.To validate this TD method, commercial monodisperse solutions of PS and PMMA were prepared with concentrations ranging from 0 to 60 ng in HPLC-grade water that was prefiltered through a 0.2 µm PTFE syringe filter. These samples were contained in precleaned headspace vials baked in a vacuum oven at 150°C and 10 mbar for >5 h to eliminate potential contaminants. After an evaporation step in a vacuum desiccator, the dry samples were heated in the TD unit and the thermolysis products were transferred in a controlled carrier gas (Air or N2) to the PTR-MS for quantitative analysis. Several replicates were prepared for each sample, along with laboratory blanks. Respective signals were integrated, and linear regressions were calculated. We achieved an R2 of 0.99 with a 3σ limit of detection (LOD) of 2.8 ng for PS, and an R2 of 0.98 with an LOD of 9.2 ng for PMMA.We further present initial results from samples containing deposited NP and explore data analysis methods based on matrix factorization.

Ammonia is a significant precursor of PM2.5 particles and thus contributes to poor air quality in many regions. Furthermore, ammonia concentrations are rising due to the increase of large-scale, intensive agricultural activities, which are often accompanied by greater use of fertilizers and concentrated animal feedlots. Ammonia is highly reactive and thus highly variable and difficult to measure. Satellite-based instruments, such as the Atmospheric Infrared Sounder (AIRS) and the Cross-Track Infrared Sounder (CrIS), have been shown to provide much greater temporal and spatial coverage of ammonia distribution and variability than is possible with in situ networks or aircraft campaigns, but the validation of these data is limited. Here we evaluate MUSES (multi-spectra, multi-species, multi-sensors) ammonia retrievals from AIRS and CrIS against ammonia measurements from aircraft in the California Central Valley and in the Colorado Front Range. These are small datasets taken over high-source regions under very different conditions: winter in California and summer in Colorado. Direct comparisons of the surface values of the retrieved profiles are biased very low in California (∼ 40 ppbv) and slightly high in Colorado (∼ 4 ppbv). This bias appears to be primarily due to smoothing error, since applying the instrument operator effectively reduces the bias to zero; even after the smoothing error is accounted for, the average uncertainty at the surface is in the 20 %–30 % range. We also compare 3 years of CrIS ammonia against an in situ network in the Magic Valley in Idaho We show that CrIS ammonia captures both the seasonal signal and the spatial variability in the Magic Valley, although it is biased low here also. In summary, this analysis substantially adds to the validation record but also points to the need for more validation under many different conditions and at higher altitudes.

Abstract. Particle condensed polycyclic aromatic hydrocarbons (PAHs) are a group of toxic organic compounds that are produced by incomplete combustion of organic material e.g. via biomass burning or traffic emissions. Even at low long-term exposure levels, such as 1 ng m-3 of benzo(a)pyrene, PAHs are recognized to be detrimental to human health. Therefore, a quantitative characterization of PAHs at sub-ng m-3 levels is important to examine precise long-term exposure. A new ultrasensitive generation of proton-transfer-reaction mass-spectrometry (PTR-MS) instruments coupled to the CHARON particle inlet is highly capable of quantitatively detecting this toxic class of compounds at a molecular composition level, while offering a high temporal resolution of < 1 min and sub-ng m-3 limits of detection. To demonstrate the capabilities of this new CHARON FUSION PTR-TOF 10k instrument, we present a thorough characterization of summertime ambient condensed PAHs in Innsbruck, Austria. With a mass resolution of > 14 000 (m/Δm at full width half maximum) and unprecedented sensitivities of up to 40 cps ng-1 m3, a series of 9 condensed PAHs of four (C16H10) to six aromatic rings (C26H16) are identified among a plethora of organic compounds in ambient organic aerosol. With unprecedented one-minute 3-σ limits of detection between 19 to 46 pg m-3, quantitative time-series of these PAHs of lowermost mass concentrations are determined. To understand the sources and processes associated with these condensed summertime PAHs in greater detail, a matrix factorization including the ~ 4 000 ionic signals detected by the CHARON FUSION PTR-TOF 10k is performed, representing the vast majority of ambient organic aerosol. A total of 10 factors and corresponding time-series can be identified. Known tracer compounds like levoglucosan, pinonic acid or nicotine consequently allow the assignment to individual organic aerosol sources and physico-chemical processes. PAH emissions from traffic are found to be minor contributors during this summertime sampling period. The highest concentrations of PAHs are identified in a mixed aged oxygenated organic aerosol, followed by a biomass-burning and a cigarette smoking organic aerosol.

Proton-transfer-reaction mass spectrometry (PTR-MS) is widely used for measuring organic trace gases in air. In traditional PTR-MS, both nonpolar and polar analytes are ionized with unit efficiency, as predicted from ion-molecule collision theories. This well-defined ion chemistry allows for direct quantification of analytes without prior calibration and therefore is an important characteristic of PTR-MS. In an effort to further increase the sensitivity, recently developed ultrahigh sensitivity chemical ionization mass spectrometry (CIMS) analyzers have, however, been reported to have sacrificed unit ionization efficiency for selected analytes or classes of analytes. We herein report on the development of a novel ultrasensitive PTR-MS instrument, the FUSION PTR-TOF 10k, which exhibits the same universal unit response as conventional PTR-MS analyzers. The core component of this analyzer is the newly designed FUSION ion-molecule reactor, which is a stack of concentric ring electrodes generating a static longitudinal electric field superimposed by a focusing transversal radiofrequency (RF) field. The FUSION PTR-TOF 10k instrument is equipped with an improved ion source, capable of switching between different reagent ions (H3O+, O2+, NO+, NH4+) in less than one second. The improved time-of-flight mass spectrometer analyzes m/z signals with a mass resolution in the 10000-15000 range. FUSION PTR-TOF 10k achieves sensitivities up to 80000 cps ppbV-1 and detection limits down to 0.5 pptV for a 1 s measurement time. We show time-series of naphthalene and 13C-napthalene as measured in ambient air in Innsbruck for demonstrating the sub-pptV detection capability of this novel FUSION PTR-TOF 10k.

Toluene represents a large fraction of anthropogenic emissions and significantly contributes to tropospheric ozone and secondary organic aerosol (SOA) formation. Despite the fact that toluene is one of the most studied aromatic compounds, detailed chemical mechanisms still fail to correctly reproduce the speciation of toluene gaseous and condensed oxidation products. This study aims to elucidate the role of initial experimental conditions in toluene SOA mass loadings and to investigate gas–particle partitioning of its reaction products at different relevant temperatures. Gaseous and particulate reaction products were identified and quantified using a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS) coupled to a CHemical Analysis of aeRosol ONline (CHARON) inlet. The chemical system exhibited a volatility distribution mostly in the semi-volatile regime. Temperature decrease caused a shift of saturation concentration towards lower values. The CHARON–PTR-ToF-MS instrument identified and quantified approximately 60 %–80 % of the total organic mass measured by an aerosol mass spectrometer. A detailed mechanism for toluene gaseous oxidation was developed based on the Master Chemical Mechanism (MCM) and Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) deterministic mechanisms, modified following the literature. The new mechanism showed improvements in modeling oxidation product speciation with more observed species represented and more representative concentrations compared to the MCM–GECKO-A reference. Tests on partitioning processes, nonideality, and wall losses highlighted the high dependency of SOA formation on the considered processes. Our results underline the fact that volatility is not sufficient to explain the gas–particle partitioning: the organic and the aqueous phases need to be considered as well as the interactions between compounds in the particle phase.

Aerosols over Earth's remote and spatially extensive ocean surfaces have important influences on planetary climate. However, these aerosols and their effects remain poorly understood, in part due to the remoteness and limited observations over these regions. In this study, we seek to understand factors that shape marine aerosol size distributions and composition in the northwest Atlantic Ocean region. We use the GEOS-Chem model with the TwO-Moment Aerosol Sectional (TOMAS) microphysics algorithm model to interpret measurements collected from ship and aircraft during the four seasonal campaigns of the North Atlantic Aerosols and Marine Ecosystems Study (NAAMES) conducted between 2015 and 2018. Observations from the NAAMES campaigns show enhancements in the campaign-median number of aerosols with diameters larger than 3 nm in the lower troposphere (below 6 km), most pronounced during the phytoplankton bloom maxima (May/June) below 2 km in the free troposphere. Our simulations, combined with NAAMES ship and aircraft measurements, suggest several key factors that contribute to aerosol number and size in the northwest Atlantic lower troposphere, with significant regional-mean (40–60∘ N and 20–50∘ W) cloud-albedo aerosol indirect effect (AIE) and direct radiative effect (DRE) processes during the phytoplankton bloom. These key factors and their associated simulated radiative effects in the region include the following: (1) particle formation near and above the marine boundary layer (MBL) top (AIE: −3.37 W m−2, DRE: −0.62 W m−2); (2) particle growth due to marine secondary organic aerosol (MSOA) as the nascent particles subside into the MBL, enabling them to become cloud-condensation-nuclei-sized particles (AIE: −2.27 W m−2, DRE: −0.10 W m−2); (3) particle formation and growth due to the products of dimethyl sulfide, above and within the MBL (−1.29 W m−2, DRE: −0.06 W m−2); (4) ship emissions (AIE: −0.62 W m−2, DRE: −0.05 W m−2); and (5) primary sea spray emissions (AIE: +0.04 W m−2, DRE: −0.79 W m−2). Our results suggest that a synergy of particle formation in the lower troposphere (particularly near and above the MBL top) and growth by MSOA contributes strongly to cloud-condensation-nuclei-sized particles with significant regional radiative effects in the northwest Atlantic. To gain confidence in radiative effect magnitudes, future work is needed to understand (1) the sources and temperature dependence of condensable marine vapors forming MSOA, (2) primary sea spray emissions, and (3) the species that can form new particles in the lower troposphere and grow these particles as they descend into the marine boundary layer.

Elevated reactive nitrogen (Nr) deposition is a concern for alpine ecosystems, and dry NH3 deposition is a key contributor. Understanding how emission hotspots impact downwind ecosystems through dry NH3 deposition provides opportunities for effective mitigation. However, direct NH3 flux measurements with sufficient temporal resolution to quantify such events are rare. Here, we measured NH3 fluxes at Rocky Mountain National Park (RMNP) during two summers and analyzed transport events from upwind agricultural and urban sources in northeastern Colorado. We deployed open-path NH3 sensors on a mobile laboratory and an eddy covariance tower to measure NH3 concentrations and fluxes. Our spatial sampling illustrated an upslope event that transported NH3 emissions from the hotspot to RMNP. Observed NH3 deposition was significantly higher when backtrajectories passed through only the agricultural region (7.9 ng m-2 s-1) versus only the urban area (1.0 ng m-2 s-1) and both urban and agricultural areas (2.7 ng m-2 s-1). Cumulative NH3 fluxes were calculated using observed, bidirectional modeled, and gap-filled fluxes. More than 40% of the total dry NH3 deposition occurred when air masses were traced back to agricultural source regions. More generally, we identified that 10 (25) more national parks in the U.S. are within 100 (200) km of an NH3 hotspot, and more observations are needed to quantify the impacts of these hotspots on dry NH3 deposition in these regions.

Satellite ammonia (NH 3 ) observations provide unprecedented insights into NH 3 emissions, spatiotemporal variabilities and trends, but validation with in situ measurements remains lacking. Here, total columns from the Infrared Atmospheric Sounding Interferometer (IASI) were intercompared to boundary layer NH 3 profiles derived from aircraft‐ and surface‐based measurements primarily in Colorado, USA, in the summer of 2014. IASI‐NH 3 version 3 near real‐time data set compared well to in situ derived columns (windows ±15 km around centroid, ±1 h around overpass time) with a correlation of 0.58, a slope of 0.78 ± 0.14 and an intercept of 2.1 × 10 15 ±1.5 × 10 15 molecules cm −2 . Agreement degrades at larger spatiotemporal windows, consistent with the short atmospheric lifetime of NH 3 . We also examined IASI version 3R data, which relies on temperature retrievals from the ERA Reanalysis, and a third product generated using aircraft‐measured temperature profiles. The overall agreement improves slightly for both cases, and neither is biased within their combined measurement errors. Thus, spatiotemporal averaging of IASI over large windows can be used to reduce retrieval noise. Nonetheless, sampling artifacts of airborne NH 3 instruments result in significant uncertainties of the in situ‐derived columns. For example, large validation differences exist between ascent and descent profiles, and the assumptions of the free tropospheric NH 3 profiles used above the aircraft ceiling significantly impact the validation. Because short‐lived species like NH 3 largely reside within the boundary layer with complex vertical structures, more comprehensive validation is needed across a wide range of environments. More accurate and widespread in situ NH 3 data sets are therefore required for improved validations of satellite products.

The popularity of Electronic Nicotine Delivery Systems (ENDS) has led to an increase in interest in characterising the aerosol produced by these devices. So far, most published studies utilise traditional offline methods (e.g. GC-MS) which only give information on the average concentrations / amounts of compounds delivered by ENDS over a series of puffs. In order to be able to carry out detailed studies, there is a high demand for scientific data obtained with online methods that allow true puff-by-puff analysis. Proton Transfer Reaction – Time-Of-Flight – Mass Spectrometry (PTR-TOF-MS) with its real-time quantification capability is well-established in food and flavour science and therefore ideally suited for this task. However, there are several difficulties to overcome, for example the enormous concentration differences between the mainstream aerosol and the residual compounds in exhaled breath and the highly condensable nature of the aerosol, leading to extended retention times in mass spectrometric devices. Here, we present a newly developed PTR-TOF-MS multipurpose sampling setup with a well-defined three-stage dilution system that can shift the instrument's dynamic range by more than four orders of magnitude. All surfaces that come in contact with the sample air are specially coated and heated to considerably reduce retention effects. We show results from a proof-of-concept study on exhaled air before, during and after the use of an ENDS. Our results demonstrate the ability of our system to perform highly time-resolved puff-by-puff characterisation of ENDS mainstream aerosol as well as in exhaled breath after ENDS consumption.

Proton-transfer-reaction mass spectrometry (PTR-MS) is widely used in atmospheric sciences for measuring volatile organic compounds in real time. In the most widely used type of PTR-MS instruments, air is directly introduced into a chemical ionization reactor via an inlet capillary system. The reactor has a volumetric exchange time of ∼0.1 s, enabling PTR-MS analyzers to measure at a frequency of 10 Hz. The time response does, however, deteriorate if low-volatility analytes interact with surfaces in the inlet or in the instrument. Herein, we present the extended volatility range (EVR) PTR-MS instrument which mitigates this issue. In the EVR configuration, inlet capillaries are made of passivated stainless steel, and all wetted metal parts in the chemical ionization reactor are surface-passivated with a functionalized hydrogenated amorphous silicon coating. Heating the entire setup (up to 120 ∘C) further improves the time-response performance. We carried out time-response performance tests on a set of 29 analytes having saturation mass concentrations C0 in the range between 10−3 and 105 µg m−3. The 1/e-signal decay times after instant removal of the analyte from the sampling flow were between 0.2 and 90 s for gaseous analytes. We also tested the EVR PTR-MS instrument in combination with the chemical analysis of aerosols online (CHARON) particle inlet, and 1/e-signal decay times were in the range between 5 and 35 s for particulate analytes. We show on a set of example compounds that the time-response performance of the EVR PTR-MS instrument is comparable to that of the fastest flow tube chemical ionization mass spectrometers that are currently in use. The fast time response can be used for rapid (∼1 min equilibration time) switching between gas and particle measurements. The CHARON EVR PTR-MS instrument can thus be used for real-time monitoring of both gaseous and particulate organics in the atmosphere. Finally, we show that the CHARON EVR PTR-MS instrument also rapidly detects highly oxygenated species (with up to eight oxygen atoms) in particles formed by limonene ozonolysis.

The OH-initiated degradation of 2-amino-2-methyl-1-propanol [CH3C(NH2)(CH3)CH2OH, AMP] was investigated in a large atmospheric simulation chamber, employing time-resolved online high-resolution proton-transfer reaction-time-of-flight mass spectrometry (PTR-ToF-MS) and chemical analysis of aerosol online PTR-ToF-MS (CHARON-PTR-ToF-MS) instrumentation, and by theoretical calculations based on M06-2X/aug-cc-pVTZ quantum chemistry results and master equation modeling of the pivotal reaction steps. The quantum chemistry calculations reproduce the experimental rate coefficient of the AMP + OH reaction, aligning k(T) = 5.2 × 10–12 × exp (505/T) cm3 molecule–1 s–1 to the experimental value kexp,300K = 2.8 × 10–11 cm3 molecule–1 s–1. The theoretical calculations predict that the AMP + OH reaction proceeds via hydrogen abstraction from the −CH3 groups (5–10%), −CH2– group, (>70%) and −NH2 group (5–20%), whereas hydrogen abstraction from the −OH group can be disregarded under atmospheric conditions. A detailed mechanism for atmospheric AMP degradation was obtained as part of the theoretical study. The photo-oxidation experiments show 2-amino-2-methylpropanal [CH3C(NH2)(CH3)CHO] as the major gas-phase product and propan-2-imine [(CH3)2C=NH], 2-iminopropanol [(CH3)(CH2OH)C=NH], acetamide [CH3C(O)NH2], formaldehyde (CH2O), and nitramine 2-methyl-2-(nitroamino)-1-propanol [AMPNO2, CH3C(CH3)(NHNO2)CH2OH] as minor primary products; there is no experimental evidence of nitrosamine formation. The branching in the initial H abstraction by OH radicals was derived in analyses of the temporal gas-phase product profiles to be BCH3/BCH2/BNH2 = 6:70:24. Secondary photo-oxidation products and products resulting from particle and surface processing of the primary gas-phase products were also observed and quantified. All the photo-oxidation experiments were accompanied by extensive particle formation that was initiated by the reaction of AMP with nitric acid and that mainly consisted of this salt. Minor amounts of the gas-phase photo-oxidation products, including AMPNO2, were detected in the particles by CHARON-PTR-ToF-MS and GC×GC-NCD. Volatility measurements of laboratory-generated AMP nitrate nanoparticles gave ΔvapH = 80 ± 16 kJ mol–1 and an estimated vapor pressure of (1.3 ± 0.3) × 10–5 Pa at 298 K. The atmospheric chemistry of AMP is evaluated and a validated chemistry model for implementation in dispersion models is presented.