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    Complex refractive indices of Saharan dust samples at visible and near UV wavelengths: A laboratory study
    (München : European Geopyhsical Union, 2012) Wagner, R.; Ajtai, T.; Kandler, K.; Lieke, K.; Linke, C.; Müller, T.; Schnaiter, M.; Vragel, M.
    We have retrieved the wavelength-dependent imaginary parts of the complex refractive index for five different Saharan dust aerosol particles of variable mineralogical composition at wavelengths between 305 and 955 nm. The dust particles were generated by dispersing soil samples into a laboratory aerosol chamber, typically yielding particle sizes with mean diameters ranging from 0.3 to 0.4 μm and maximum diameters from 2 to 4 μm. The extinction and absorption coefficients as well as the number size distribution of the dust particles were simultaneously measured by various established techniques. An inversion scheme based on a spheroidal dust model was employed to deduce the refractive indices. The retrieved imaginary parts of the complex refractive index were in the range from 0.003 to 0.005, 0.005 to 0.011, and 0.016 to 0.050 at the wavelengths 955, 505, and 305 nm. The hematite content of the dust particles was determined by electron-microscopical single particle analysis. Hematite volume fractions in the range from 1.1 to 2.7% were found for the different dusts, a range typical for atmospheric mineral dust. We have performed a sensitivity study to assess how accurately the retrieved imaginary refractive indices could be reproduced by calculations with mixing rule approximations using the experimentally determined hematite contents as input.
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    Vertical distribution of aerosol optical properties in the Po Valley during the 2012 summer campaigns
    (Katlenburg-Lindau : EGU, 2018) Bucci, Silvia; Cristofanelli, Paolo; Decesari, Stefano; Marinoni, Angela; Sandrini, Silvia; Größ, Johannes; Wiedensohler, Alfred; Di Marco, Chiara F.; Nemitz, Eiko; Cairo, Francesco; Di Liberto, Luca; Fierli, Federico
    Studying the vertical distribution of aerosol particle physical and chemical properties in the troposphere is essential to understand the relative importance of local emission processes vs. long-range transport for column-integrated aerosol properties (e.g. the aerosol optical depth, AOD, affecting regional climate) as well as for the aerosol burden and its impacts on air quality at the ground. The main objective of this paper is to investigate the transport of desert dust in the middle troposphere and its intrusion into the planetary boundary layer (PBL) over the Po Valley (Italy), a region considered one of the greatest European pollution hotspots for the frequency that particulate matter (PM) limit values are exceeded. Events of mineral aerosol uplift from local (soil) sources and phenomena of hygroscopic growth at the ground are also investigated, possibly affecting the PM concentration in the region as well. During the PEGASOS 2012 field campaign, an integrated observing-modelling system was set up based on near-surface measurements (particle concentration and chemistry), vertical profiling (backscatter coefficient profiles from lidar and radiosoundings) and Lagrangian air mass transport simulations by FLEXPART model. Measurements were taken at the San Pietro Capofiume supersite (44°39′ĝ€N, 11°37′ĝ€E; 11ĝ€mĝ€a.s.l.), located in a rural area relatively close to some major urban and industrial emissive areas in the Po Valley. Mt. Cimone (44°12′ĝ€N, 10°42′ĝ€E; 2165ĝ€mĝ€a.s.l.) WMO/GAW station observations are also included in the study to characterize regional-scale variability. Results show that, in the Po Valley, aerosol is detected mainly below 2000ĝ€mĝ€a.s.l. with a prevalent occurrence of non-depolarizing particles ( > 50ĝ€% throughout the campaign) and a vertical distribution modulated by the PBL daily evolution. Two intense events of mineral dust transport from northern Africa (19-21 and 29 June to 2 July) are observed, with layers advected mainly above 2000ĝ€m, but subsequently sinking and mixing in the PBL. As a consequence, a non-negligible occurrence of mineral dust is observed close to the ground ( ĝ1/4 7ĝ€% of occurrence during a 1-month campaign). The observations unambiguously show Saharan dust layers intruding the Po Valley mixing layer and directly affecting the aerosol concentrations near the surface. Finally, lidar observations also indicate strong variability in aerosol on shorter timescales (hourly). Firstly, these highlight events of hygroscopic growth of anthropogenic aerosol, visible as shallow layers of low depolarization near the ground. Such events are identified during early morning hours at high relative humidity (RH) conditions (RHĝ€ > 80ĝ€%). The process is observed concurrently with high PM1 nitrate concentration (up to 15ĝ€μgĝ€cmĝ'3) and hence mainly explicable by deliquescence of fine anthropogenic particles, and during mineral dust intrusion episodes, when water condensation on dust particles could instead represent the dominant contribution. Secondly, lidar images show frequent events (mean daily occurrence of ĝ1/4 ĝ€22ĝ€% during the whole campaign) of rapid uplift of mineral depolarizing particles in afternoon-evening hours up to 2000ĝ€mĝ€a.s.l. height. The origin of such particles cannot be directly related to long-range transport events, being instead likely linked to processes of soil particle resuspension from agricultural lands.
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    The efficiency of secondary organic aerosol particles acting as ice-nucleating particles under mixed-phase cloud conditions
    (Katlenburg-Lindau : EGU, 2018) Frey, Wiebke; Hu, Dawei; Dorsey, James; Alfarra, M. Rami; Pajunoja, Aki; Virtanen, Annele; Connolly, Paul; McFiggans, Gordon
    Secondary organic aerosol (SOA) particles have been found to be efficient ice-nucleating particles under the cold conditions of (tropical) upper-tropospheric cirrus clouds. Whether they also are efficient at initiating freezing under slightly warmer conditions as found in mixed-phase clouds remains undetermined. Here, we study the ice-nucleating ability of photochemically produced SOA particles with the combination of the Manchester Aerosol Chamber and Manchester Ice Cloud Chamber. Three SOA systems were tested resembling biogenic and anthropogenic particles as well as particles of different phase state. These are namely α-pinene, heptadecane, and 1,3,5-trimethylbenzene. After the aerosol particles were formed, they were transferred into the cloud chamber, where subsequent quasi-adiabatic cloud activation experiments were performed. Additionally, the ice-forming abilities of ammonium sulfate and kaolinite were investigated as a reference to test the experimental setup.

    Clouds were formed in the temperature range of −20 to −28.6 °C. Only the reference experiment using dust particles showed evidence of ice nucleation. No ice particles were observed in any other experiment. Thus, we conclude that SOA particles produced under the conditions of the reported experiments are not efficient ice-nucleating particles starting at liquid saturation under mixed-phase cloud conditions.
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    The sensitivity of the colour of dust in MSG-SEVIRI Desert Dust infrared composite imagery to surface and atmospheric conditions
    (Göttingen : Copernicus GmbH, 2019) Banks, J.R.; Hünerbein, A.; Heinold, B.; Brindley, H.E.; Deneke, H.; Schepanski, K.
    Infrared "Desert Dust" composite imagery taken by the Spinning Enhanced Visible and InfraRed Imager (SEVIRI), onboard the Meteosat Second Generation (MSG) series of satellites above the equatorial East Atlantic, has been widely used for more than a decade to identify and track the presence of dust storms from and over the Sahara Desert, the Middle East, and southern Africa. Dust is characterised by distinctive pink colours in the Desert Dust false-colour imagery; however, the precise colour is influenced by numerous environmental properties, such as the surface thermal emissivity and skin temperature, the atmospheric water vapour content, the quantity and height of dust in the atmosphere, and the infrared optical properties of the dust itself. For this paper, simulations of SEVIRI infrared measurements and imagery have been performed using a modelling system, which combines dust concentrations simulated by the aerosol transport model COSMO-MUSCAT (COSMO: COnsortium for Small-scale MOdelling; MUSCAT: MUltiScale Chemistry Aerosol Transport Model) with radiative transfer simulations from the RTTOV (Radiative Transfer for TOVS) model. Investigating the sensitivity of the synthetic infrared imagery to the environmental properties over a 6-month summertime period from 2011 to 2013, it is confirmed that water vapour is a major control on the apparent colour of dust, obscuring its presence when the moisture content is high. Of the three SEVIRI channels used in the imagery (8.7, 10.8, and 12.0 μm), the channel at 10.8 μm has the highest atmospheric transmittance and is therefore the most sensitive to the surface skin temperature. A direct consequence of this sensitivity is that the background desert surface exhibits a strong diurnal cycle in colour, with light blue colours possible during the day and purple hues prevalent at night. In dusty scenes, the clearest pink colours arise from high-altitude dust in dry atmospheres. Elevated dust influences the dust colour primarily by reducing the contrast in atmospheric transmittance above the dust layer between the SEVIRI channels at 10.8 and 12.0 μm, thereby boosting red and pink colours in the imagery. Hence, the higher the dust altitude, the higher the threshold column moisture needed for dust to be obscured in the imagery: for a sample of dust simulated to have an aerosol optical depth (AOD) at 550 nm of 2-3 at an altitude of 3-4 km, the characteristic colour of the dust may only be impaired when the total column water vapour is particularly moist ('39 mm). Meanwhile, dust close to the surface (altitude < 1 km) is only likely to be apparent when the atmosphere is particularly dry and when the surface is particularly hot, requiring column moisture/13 mm and skin temperatures '314 K, and is highly unlikely to be apparent when the skin temperature is/300 K. Such low-altitude dust will regularly be almost invisible within the imagery, since it will usually be beneath much of the atmospheric water vapour column. It is clear that the interpretation of satellite-derived dust imagery is greatly aided by knowledge of the background environment.
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    Comparison of particle number size distribution trends in ground measurements and climate models
    (Katlenburg-Lindau : EGU, 2022) Leinonen, Ville; Kokkola, Harri; Yli-Juuti, Taina; Mielonen, Tero; Kühn, Thomas; Nieminen, Tuomo; Heikkinen, Simo; Miinalainen, Tuuli; Bergman, Tommi; Carslaw, Ken; Decesari, Stefano; Fiebig, Markus; Hussein, Tareq; Kivekäs, Niku; Krejci, Radovan; Kulmala, Markku; Leskinen, Ari; Massling, Andreas; Mihalopoulos, Nikos; Mulcahy, Jane P.; Noe, Steffen M.; van Noije, Twan; O'Connor, Fiona M.; O'Dowd, Colin; Olivie, Dirk; Pernov, Jakob B.; Petäjä, Tuukka; Seland, Øyvind; Schulz, Michael; Scott, Catherine E.; Skov, Henrik; Swietlicki, Erik; Tuch, Thomas; Wiedensohler, Alfred; Virtanen, Annele; Mikkonen, Santtu
    Despite a large number of studies, out of all drivers of radiative forcing, the effect of aerosols has the largest uncertainty in global climate model radiative forcing estimates. There have been studies of aerosol optical properties in climate models, but the effects of particle number size distribution need a more thorough inspection. We investigated the trends and seasonality of particle number concentrations in nucleation, Aitken, and accumulation modes at 21 measurement sites in Europe and the Arctic. For 13 of those sites, with longer measurement time series, we compared the field observations with the results from five climate models, namely EC-Earth3, ECHAM-M7, ECHAM-SALSA, NorESM1.2, and UKESM1. This is the first extensive comparison of detailed aerosol size distribution trends between in situ observations from Europe and five earth system models (ESMs). We found that the trends of particle number concentrations were mostly consistent and decreasing in both measurements and models. However, for many sites, climate models showed weaker decreasing trends than the measurements. Seasonal variability in measured number concentrations, quantified by the ratio between maximum and minimum monthly number concentration, was typically stronger at northern measurement sites compared to other locations. Models had large differences in their seasonal representation, and they can be roughly divided into two categories: for EC-Earth and NorESM, the seasonal cycle was relatively similar for all sites, and for other models the pattern of seasonality varied between northern and southern sites. In addition, the variability in concentrations across sites varied between models, some having relatively similar concentrations for all sites, whereas others showed clear differences in concentrations between remote and urban sites. To conclude, although all of the model simulations had identical input data to describe anthropogenic mass emissions, trends in differently sized particles vary among the models due to assumptions in emission sizes and differences in how models treat size-dependent aerosol processes. The inter-model variability was largest in the accumulation mode, i.e. sizes which have implications for aerosol-cloud interactions. Our analysis also indicates that between models there is a large variation in efficiency of long-range transportation of aerosols to remote locations. The differences in model results are most likely due to the more complex effect of different processes instead of one specific feature (e.g. the representation of aerosol or emission size distributions). Hence, a more detailed characterization of microphysical processes and deposition processes affecting the long-range transport is needed to understand the model variability.
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    CAMP: An instrumented platform for balloon-borne aerosol particle studies in the lower atmosphere
    (Katlenburg-Lindau : Copernicus, 2022) Pilz, Christian; Düsing, Sebastian; Wehner, Birgit; Müller, Thomas; Siebert, Holger; Voigtländer, Jens; Lonardi, Michael
    Airborne observations of vertical aerosol particle distributions are crucial for detailed process studies and model improvements. Tethered balloon systems represent a less expensive alternative to aircraft to probe shallow atmospheric boundary layers (ABLs). This study presents the newly developed cubic aerosol measurement platform (CAMP) for balloon-borne observations of aerosol particle microphysical properties. With an edge length of 35 cm and a weight of 9 kg, the cube is an environmentally robust instrument platform intended for measurements at low temperatures, with a particular focus on applications in cloudy Arctic ABLs. The aerosol instrumentation on board CAMP comprises two condensation particle counters with different lower detection limits, one optical particle size spectrometer, and a miniaturized absorption photometer. Comprehensive calibrations and characterizations of the instruments were performed in laboratory experiments. The first field study with a tethered balloon system took place at the Leibniz Institute for Tropospheric Research (TROPOS) station in Melpitz, Germany, in the winter of 2019. At ambient temperatures between-8 and 15 C, the platform was operated up to a 1.5 km height on 14 flights under both clear-sky and cloudy conditions. The continuous aerosol observations at the ground station served as a reference for evaluating the CAMP measurements. Exemplary profiles are discussed to elucidate the performance of the system and possible process studies. Based on the laboratory instrument characterizations and the observations during the field campaign, CAMP demonstrated the capability to provide comprehensive aerosol particle measurements in cold and cloudy ABLs.
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    An overview of the Lagrangian experiments undertaken during the North Atlantic regional Aerosol Characterisation Experiment (ACE-2)
    (Milton Park : Taylor & Francis, 2016) Johnson, Doug W.; Osborne, Simon; Wood, Robert; Suhre, Karsten; Johnson, Randy; Businger, Steven; Quinn, Patricia K.; Wiedensohler, Alfred; Durkee, Philip A.; Russell, Lynn M.; Andreae, Meinrat O.; O’Dowd, Colin; Noone, Kevin J.; Bandy, Brian; Rudolph, J.; Rapsomanikis, Spyros
    One of the primary aims of the North Atlantic regional Aerosol Characterisation Experiment (ACE-2) was to quantify the physical and chemical processes affecting the evolution of the major aerosol types over the North Atlantic. The best, practical way of doing this is in a Lagrangian framework where a parcel of air is sampled over several tens of hours and its physical and chemical properties are intensively measured. During the intensive observational phase of ACE-2, between 15 June 1997 and 24 July 1997, 3 cloudy Lagrangian experiments and 3 cloud-free, Lagrangian experiments were undertaken between the south west tip of the Iberian Peninsula and the Canary Islands. This paper gives an overview of the aims and logistics of all of the Lagrangian experiments and compares and contrasts them to provide a framework for the more focused Lagrangian papers in this issue and future process modelling studies and parametrisation development. The characteristics of the cloudy Lagrangian experiments were remarkably different, enabling a wide range of different physical and chemical processes to be studied. In the 1st Lagrangian, a clean maritime air mass was sampled in which salt particle production, due to increased wind speed, dominated the change in the accumulation mode concentrations. In the 2nd Lagrangian, extensive cloud cover resulted in cloud processing of the aerosol in a polluted air mass, and entrainment of air from the free troposphere influenced the overall decrease in aerosol concentrations in the marine boundary layer (MBL). Very little change in aerosol characteristics was measured in the 3rd Lagrangian, where the pollution in the MBL was continually being topped up by entraining air from a residual continental boundary layer (CBL) above. From the analysis of all the Lagrangian experiments, it has been possible to formulate, and present here, a generalised description of a European continental outbreak of pollution over the sub-tropical North Atlantic.
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    3+2 + X : what is the most useful depolarization input for retrieving microphysical properties of non-spherical particles from lidar measurements using the spheroid model of Dubovik et al. (2006)?
    (Katlenburg-Lindau : Copernicus, 2019) Tesche, Matthias; Kolgotin, Alexei; Haarig, Moritz; Burton, Sharon P.; Ferrare, Richard A.; Hostetler, Chris A.; Müller, Detlef
    The typical multiwavelength aerosol lidar data set for inversion of optical to microphysical parameters is composed of three backscatter coefficients (β) at 355, 532, and 1064 nm and two extinction coefficients (α) at 355 and 532 nm. This data combination is referred to as a 3β C 2α or 3 + 2 data set. This set of data is sufficient for retrieving some important microphysical particle parameters if the particles have spherical shape. Here, we investigate the effect of including the particle linear depolarization ratio (δ) as a third input parameter for the inversion of lidar data. The inversion algorithm is generally not used if measurements show values of d that exceed 0.10 at 532 nm, i.e. in the presence of nonspherical particles such as desert dust, volcanic ash, and, under special circumstances, biomass-burning smoke. We use experimental data collected with instruments that are capable of measuring d at all three lidar wavelengths with an inversion routine that applies the spheroidal light-scattering model of Dubovik et al. (2006) with a fixed axis-ratio distribution to replicate scattering properties of non-spherical particles. The inversion gives the fraction of spheroids required to replicate the optical data as an additional output parameter. This is the first systematic test of the effect of using all theoretically possible combinations of d taken at 355, 532, and 1064 nm as input in the lidar data inversion. We find that depolarization information of at least one wavelength already provides useful information for the inversion of optical data that have been collected in the presence of non-spherical mineral dust particles. However, any choice of d will give lower values of the single-scattering albedo than the traditional 3 + 2 data set. We find that input data sets that include d355 give a spheroid fraction that closely resembles the dust ratio we obtain from using β532 and d532 in a methodology applied in aerosol-type separation. The use of d355 in data sets of two or three d? reduces the spheroid fraction that is retrieved when using d532 and d1064. Use of the latter two parameters without accounting for d355 generally leads to high spheroid fractions that we consider not trustworthy. The use of three d instead of two δ, including the constraint that one of these is measured at 355 nm does not provide any advantage over using 3 + 2 + d355 for the observations with varying contributions of mineral dust considered here. However, additional measurements at wavelengths different from 355 nm would be desirable for application to a wider range of aerosol scenarios that may include non-spherical smoke particles, which can have values of d355 that are indistinguishable from those found for mineral dust. We therefore conclude that - depending on measurement capability - the future standard input for inversion of lidar data taken in the presence of mineral dust particles and using the spheroid model of Dubovik et al. (2006) might be 3+2Cδ355 or 3 + 2 + δ355 + δ532. © 2019 The Author(s).
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    Importance of secondary organic aerosol formation of iα/i-pinene, limonene, and im/i-cresol comparing day- And nighttime radical chemistry
    (Katlenburg-Lindau : European Geosciences Union, 2021) Mutzel, Anke; Zhang, Yanli; Böge, Olaf; Rodigast, Maria; Kolodziejczyk, Agata; Wang, Xinming; Herrmann, Hartmut
    The oxidation of biogenic and anthropogenic compounds leads to the formation of secondary organic aerosol mass (SOA). The present study aims to investigate span classCombining double low line"inline-formula"iα/i/span-pinene, limonene, and span classCombining double low line"inline-formula"im/i/span-cresol with regards to their SOA formation potential dependent on relative humidity (RH) under night- (NOspan classCombining double low line"inline-formula"3/span radicals) and daytime conditions (OH radicals) and the resulting chemical composition. It was found that SOA formation potential of limonene with NOspan classCombining double low line"inline-formula"3/span 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 span classCombining double low line"inline-formula"iα/i/span-pinene with NOspan classCombining double low line"inline-formula"3/span slightly exceeds that of the OH-radical reaction, independent from RH. On average, span classCombining double low line"inline-formula"iα/i/span-pinene yielded SOA with about 6-7 % from NOspan classCombining double low line"inline-formula"3/span radicals and 3-4 % from OH-radical reaction. Surprisingly, unexpectedly high SOA yields were found for span classCombining double low line"inline-formula"im/i/span-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 span classCombining double low line"inline-formula"iα/i/span-pinene and span classCombining double low line"inline-formula"im/i/span-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 span classCombining double low line"inline-formula"iα/i/span-pinene varied between 2 and 80 % as a function of RH./p pFurthermore, SOA from span classCombining double low line"inline-formula"iα/i/span-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./p © 2021 Copernicus GmbH. All rights reserved.
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    High-molecular-weight esters in α-pinene ozonolysis secondary organic aerosol: Structural characterization and mechanistic proposal for their formation from highly oxygenated molecules
    (Katlenburg-Lindau : EGU, 2018) Kahnt, Ariane; Vermeylen, Reinhilde; Iinuma, Yoshiteru; Safi Shalamzari, Mohammad; Maenhaut, Willy; Claeys, Magda
    Stable high-molecular-weight esters are present in α-pinene ozonolysis secondary organic aerosol (SOA) with the two most abundant ones corresponding to a hydroxypinonyl ester of cis-pinic acid with a molecular weight (MW) of 368 (C19H28O7) and a diaterpenylic ester of cis-pinic acid with a MW of 358 (C17H26O8). However, their molecular structures are not completely elucidated and their relationship with highly oxygenated molecules (HOMs) in the gas phase is still unclear. In this study, liquid chromatography in combination with positive ion electrospray ionization mass spectrometry has been performed on high-molecular-weight esters present in α-pinene ozonolysis SOA with and without derivatization into methyl esters. Unambiguous evidence could be obtained for the molecular structure of the MWg368 ester in that it corresponds to an ester of cis-pinic acid where the carboxyl substituent of the dimethylcyclobutane ring and not the methylcarboxyl substituent is esterified with 7-hydroxypinonic acid. The same linkage was already proposed in previous work for the MWg358 ester (Yasmeen et al., 2010), but could be supported in the present study. Guided by the molecular structures of these stable esters, we propose a formation mechanism from gas-phase HOMs that takes into account the formation of an unstable C19H28O11 product, which is detected as a major species in α-pinene ozonolysis experiments as well as in the pristine forest atmosphere by chemical ionization-atmospheric pressure ionization-time-of-flight mass spectrometry with nitrate clustering (Ehn et al., 2012, 2014). It is suggested that an acyl peroxy radical related to cis-pinic acid (RO2•) and an alkoxy radical related to 7- or 5-hydroxypinonic acid (R′O•) serve as key gas-phase radicals and combine according to a RO2g+gR′O•→ gRO3R′ radical termination reaction. Subsequently, the unstable C19H28O11 HOM species decompose through the loss of oxygen or ketene from the inner part containing a labile trioxide function and the conversion of the unstable acyl hydroperoxide groups to carboxyl groups, resulting in stable esters with a molecular composition of C19H28O7 (MWg368) and C17H26O8 (MWg358), respectively. The proposed mechanism is supported by several observations reported in the literature. On the basis of the indirect evidence presented in this study, we hypothesize that RO2g+gR′O•→ gRO3R′ chemistry is at the underlying molecular basis of high-molecular-weight ester formation upon α-pinene ozonolysis and may thus be of importance for new particle formation and growth in pristine forested environments.