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The composition, sources and chemical transformation of volatile organic compounds (VOCs) and organic aerosol (OA) particles were investigated during July–August 2021 at a rural forested site in southwest Germany, 10 km north of the city of Karlsruhe. VOCs and semi-volatile OA particles were measured with a proton-transfer-reaction mass spectrometer coupled to the CHemical Analysis of aeRosols ONline particle inlet (CHARON–PTR-MS). The CHARON-measured OA mass accounted for 62 ± 18 % on average of the total OA mass (4.2 ± 2.8 µ g m -3) measured concurrently with an aerosol mass spectrometer (AMS). The total concentrations of measured VOCs ranged from 7.6 to 88.9 ppb with an average of 31.2 ± 13.4 ppb. Positive matrix factorization (PMF) was used to identify major source factors of VOCs and OA particles. Three types of oxygenated VOC (OVOC), namely aromatic OVOCs, biogenic OVOCs and aged OVOCs, contributed on average 11 % ± 9 %, 37 % ± 29 % and 29 % ± 21 % of total VOC concentrations, respectively. The results of AMS–PMF indicated substantial contributions of oxygenated organic compounds to OA particle mass. Three secondary OA (SOA) factors determined by CHARON–PMF analysis, namely aromatic SOA (5 % ± 7 %), daytime biogenic SOA (17 % ± 17 %) and nighttime biogenic SOA (28 % ± 21 %), consistently showed high contributions to the total CHARON-measured OA mass. Nighttime particle growth was observed regularly in this area, which was mainly attributed to the semi-volatile organic compounds and organic nitrates formed from the oxidation of monoterpenes and sesquiterpenes. This study presents the major sources, real-time transformations of VOCs and OA particles, and nighttime particle formation characteristics for central European forested areas. [ABSTRACT FROM AUTHOR]

Knowledge of chemical composition and mixing state of aerosols at a single particle level is critical for gaining insights into atmospheric processes. One common tool to make these measurements is single particle mass spectrometry. There remains a need to compare the performance of different single particle mass spectrometers (SPMSs). An intercomparison of SPMSs was conducted at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) chamber at the Karlsruhe Institute of Technology (KIT) in November 2014, as the first phase of the Fifth International Workshop on Ice Nucleation (FIN-01). In this paper we compare size distributions and mass spectra of atmospherically relevant particle types measured by five SPMSs. These include different minerals, desert and soil dusts, soot, bioaerosol (Snomax; protein granule), secondary organic aerosol (SOA) and SOA coated mineral particles. All SPMSs reported similar vacuum aerodynamic diameter (dva) within typical instrumental ranges from ~100‒200 nm (lower limit) to ~2‒3 μm (upper limit). In general, all SPMSs exhibited a wide dynamic range (up to ~103) and high signal to noise ratio (up to ~104) in mass spectra. Common spectral features with small diversities in mass spectra were found with high average Pearson's correlation coefficients, i.e., for average positive spectra ravg-pos = 0.74 ± 0.12 and average negative spectra ravg-neg = 0.67 ± 0.22. The highest correlation of average mass spectra across all instruments was observed for Snomax (ravg-pos = 0.92 ± 0.04 and ravg-neg = 0.90 ± 0.05), attributed to the prevalence of common markers and similar spectral patterns. The poorest correlation was found for propane soot (ravg-pos = 0.51 ± 0.23 and ravg-neg = 0.35 ± 0.26), primarily because of low detection efficiency (DE) due to small particle size. We found that instrument-specific DE was more dependent on particle size than particle type. We also found that particle identification favored the use of bipolar, rather than monopolar, instruments. Particle classification from "blind experiments" showed that all instruments differentiated SOA, soot, and soil dust, and detected subtle changes in the particle internal mixing, but had difficulties differentiating among specific mineral types and dusts. This study helps to further understand the capabilities and limitations of the single particle mass spectrometry technique in general, as well as the specific instrument performance in characterizing atmospheric aerosol particles. We propose that intercomparison workshops should continue as new SPMSs are developed and, ideally, should include both laboratory and field activities. [ABSTRACT FROM AUTHOR]

Indole (ind) is a nitrogen-containing heterocyclic volatile organic compound commonly emitted from animal husbandry and from different plants like maize with global emissions of 0.1 Tg yr -1. The chemical composition and optical properties of indole secondary organic aerosol (SOA) and brown carbon (BrC) are still not well understood. To address this, environmental chamber experiments were conducted to investigate the oxidation of indole at atmospherically relevant concentrations of selected oxidants (OH radicals and O 3) with or without NO 2. In the presence of NO 2 , the SOA yields decreased by more than a factor of 2, but the mass absorption coefficient at 365 nm (MAC 365) of ind-SOA was 4.3 ± 0.4 m 2 g -1 , which was 5 times higher than that in experiments without NO 2. In the presence of NO 2 , C 8 H 6 N 2 O 2 (identified as 3-nitroindole) contributed 76 % to all organic compounds detected by a chemical ionization mass spectrometer, contributing ∼ 50 % of the light absorption at 365 nm (Abs 365). In the absence of NO 2 , the dominating chromophore was C 8 H 7 O 3 N, contributing to 20 %–30 % of Abs 365. Indole contributes substantially to the formation of secondary BrC and its potential impact on the atmospheric radiative transfer is further enhanced in the presence of NO 2 , as it significantly increases the specific light absorption of ind-SOA by facilitating the formation of 3-nitroindole. This work provides new insights into an important process of brown carbon formation by interaction of two pollutants, NO 2 and indole, mainly emitted by anthropogenic activities. [ABSTRACT FROM AUTHOR]

Aerosol distribution is of great relevance for air quality especially for the cities like Stuttgart which is located in a basin. To understand the impact of boundary layer mixing processes on local air quality and to validate the large eddy simulation (LES) model PAML-4U, we collected a comprehensive set of data from remote sensing, in-situ methods including radiosondes for the urban background of downtown Stuttgart. Stagnant meteorological conditions caused accumulation of aerosols during winter. Case studies show that clouds during previous nights can weaken temperature inversion and accelerate boundary layer mixing after sunrise. This is important for ground-level aerosol dilution during morning rush hours. Furthermore, our observations validate results of the LES model PALM-4U in terms of boundary layer heights and aerosol mixing for 48 hours. The simulated aerosol concentrations follow the trend of our observations but are still underestimated by a factor of 4.5 ± 2.1. This underestimation is mainly due to uncertainties of emissions and boundary conditions of the model. This paper firstly evaluates the PALM-4U model performance in simulating aerosol temporal-spatial distribution, which can help to improve the LES model and to better understand sources and sinks for air pollution as well as the role of horizontal and vertical transport. [ABSTRACT FROM AUTHOR]

The spatiotemporal distribution of aerosol particles in the atmosphere has a great impact on radiative transfer, clouds, and air quality. Modern remote sensing methods, as well as airborne in situ measurements by unpiloted aerial vehicles (UAV) or balloons, are suitable tools to improve our understanding of the role of aerosol particles in the atmosphere. To validate the measurement capabilities of three relatively new measurement systems and to bridge the gaps that are often encountered between remote sensing and in situ observation, as well as to investigate aerosol particles in and above the boundary layer, we conducted two measurement campaigns and collected a comprehensive dataset employing a scanning aerosol lidar, a balloon-borne radiosonde with the Compact Optical Backscatter Aerosol Detector (COBALD), an optical particle counter (OPC) on a UAV, and a comprehensive set of ground-based instruments. The extinction coefficients calculated from near-ground-level aerosol size distributions measured in situ are well correlated with those retrieved from lidar measurements, with a slope of 1.037±0.015 and a Pearson correlation coefficient of 0.878, respectively. Vertical profiles measured by an OPC-N3 on a UAV show similar vertical particle distributions and boundary layer heights to lidar measurements. However, the sensor, OPC-N3, shows a larger variability in the aerosol backscatter coefficient measurements, with a Pearson correlation coefficient of only 0.241. In contrast, the COBALD data from a balloon flight are well correlated with lidar-derived backscatter data from the near-ground level up to the stratosphere, with a slope of 1.063±0.016 and a Pearson correlation coefficient of 0.925, respectively. This consistency between lidar and COBALD data reflects the good data quality of both methods and proves that lidar can provide reliable and spatial distributions of aerosol particles with high spatial and temporal resolutions. This study shows that the scanning lidar has the capability to retrieve backscatter coefficients near the ground level (from 25 to 50m above ground level) when it conducts horizontal measurement, which is not possible for vertically pointing lidar. These near-ground-level retrievals compare well with ground-level in situ measurements. In addition, in situ measurements on the balloon and UAV validated the scanning lidar retrievals within and above the boundary layer. The scanning aerosol lidar allows us to measure aerosol particle distributions and profiles from the ground level to the stratosphere with an accuracy equal to or better than in situ measurements and with a similar spatial resolution. [ABSTRACT FROM AUTHOR]

Stabilized Criegee intermediates (SCIs) and organic peroxy radicals (RO 2) are critical in atmospheric oxidation processes and secondary organic aerosol (SOA) formation. However, the influence of temperature on their corresponding reaction mechanisms in SOA formation is unclear. Through utilizing formic acid as a SCI scavenger and regulating the ratio of hydroperoxyl radials (HO 2) to RO 2 ([HO 2]/[RO2 ]) from ∼ 0.3 to ∼ 1.9 using different concentrations of CO, the roles of RO 2 and SCIs in SOA formation were investigated from 248 to 298 K, particularly for dimer formation in β -pinene ozonolysis. The SOA yield increased by 21 % from 298 to 273 K, while it decreased by 40 % from 273 to 248 K. Both changing [HO 2]/[RO2 ] and scavenging SCIs significantly affect SOA yield and composition. SCI reactions accounted for more than 40 % of dimer and SOA mass formation for all temperatures. Increasing [HO 2]/[RO2 ] inhibited dimer and SOA formation, and this inhibition became larger with decreasing temperature. Compared to low [HO 2]/[RO2 ] (0.30–0.34), the dimer abundance at high [HO 2]/[RO2 ] (1.53–1.88) decreased by about 31 % at 298 K and 70 % at 248 K. [HO 2]/[RO2 ] has a specific impact on SCI-controlled dimers at lower temperatures by especially influencing the C 9 –SCI reactions with RO 2. The dimers formed from C 9 –SCI reactions with RO 2 were estimated to decrease by 61 % at high [HO 2]/[RO2 ] compared to low [HO 2]/[RO2 ] at 248 K. The high reactivity and substantial contribution to SOA of β -pinene-derived SCIs at lower temperatures observed in this study suggest that monoterpene-derived SCI reactions should be accounted for in describing colder regions of the atmosphere. [ABSTRACT FROM AUTHOR]

The composition, sources and chemical transformation of volatile organic compounds (VOCs) and organic aerosol (OA) particles were investigated during July–August 2021 at a rural forested site in southwest Germany. VOCs and semi-volatile OA particles were measured with a proton-transfer-reaction mass spectrometer coupled with a particle inlet (CHARON-PTR-MS). The CHARON-measured OA mass accounted on average for 63 ± 18 % of the total OA mass (4.2 ± 2.8 μg m-3) concurrently measured by an aerosol mass spectrometer (AMS). The total concentrations of measured VOCs ranged from 7.6 to 88.9 ppb with an average of 31.2 ± 13.4 ppb. Positive matrix factorization (PMF) was used to identify major source factors of VOCs and OA. Three factors of oxygenated VOC (OVOC), namely aromatic-OVOCs, biogenic-OVOCs and aged-OVOCs contributed on average 11 % ± 9 %, 37 % ± 29 %, 29 % ± 21 % of total VOC concentrations, respectively. The results of AMS-PMF indicated substantial contributions of oxygenated organic compounds to OA particle mass. Consistently, three secondary OA (SOA) factors determined by CHARON-PMF analysis, namely aromatic-SOA (5 % ± 7 %), daytime-biogenic SOA (17 % ± 17 %), nighttime-biogenic SOA (28 % ± 21 %), showed high contributions to total CHARON-measured OA mass. Nighttime particle growth was observed regularly at this area, which was mainly attributed by the semi-volatile organic compounds and organic nitrates formed from the oxidation of monoterpenes and sesquiterpenes. This study presents major sources, real-time transformations of VOCs and OA, and nighttime particle formation characteristic for central European forested areas. [ABSTRACT FROM AUTHOR]

The composition, sources and chemical transformation of volatile organic compounds VOCs) and organic aerosol (OA) particles were investigated during July-August 2021 at a rural forested site in southwest Germany. VOCs and semi-volatile OA particles were measured with proton-transfer-reaction mass spectrometer coupled with a particle inlet (CHARON-PTRMS). The CHARON-measured OA mass accounted on average for 63 ± 18% of the total OA mass (4.2 ± 2.8 µg m-3) concurrently measured by an aerosol mass spectrometer (AMS). The total concentrations of measured VOCs ranged from 7.6 to 88.9 ppb with an average of 31.2 ± 13.4 ppb. Positive matrix factorization (PMF) was used to identify major source factors of VOCs and OA. Three factors of oxygenated VOC (OVOC), namely aromatic-OVOCs, biogenic-OVOCs and aged-OVOCs contributed on average 11% ± 9%, 37% ± 29%, 29% ± 21% of total VOC concentrations, respectively. The results of AMS-PMF indicated substantial contributions of oxygenated organic compounds to OA particle mass. Consistently, three secondary OA (SOA) factors determined by CHARON-PMF analysis, namely aromatic-SOA 5% ± 7%), daytime-biogenic SOA (17% ± 17%), nighttime-biogenic SOA (28% ± 21%), showed high contributions to total CHARON-measured OA mass. Nighttime particle growth was observed regularly at this area, which was mainly attributed by the semi-volatile organic compounds and organic nitrates formed from the oxidation of monoterpenes and sesquiterpenes. This study presents major sources, real-time transformations of VOCs and OA, and nighttime particle formation characteristic for central European forested areas. [ABSTRACT FROM AUTHOR]

The apparent volatility of atmospheric organic aerosol (OA) particles is determined by their chemical composition and environmental conditions (e.g., ambient temperature). A quantitative, experimental assessment of volatility and the respective importance of these two factors remains challenging, especially in ambient measurements. We present molecular composition and volatility of oxygenated OA (OOA) particles in different rural, urban, and remote environments across the globe (including Chacaltaya, Bolivia; Alabama, U.S.; Hyytiälä, Finland; Stuttgart and Karlsruhe, Germany; and Delhi, India) based on deployments of a filter inlet for gases and aerosols coupled to a high-resolution time-of-flight chemical ionization mass spectrometer (FIGAERO-CIMS). We find on average larger carbon numbers (nC) and lower oxygen-to-carbon (O:C) ratios at the urban sites (nC: 9.8±0.7; O:C: 0.76±0.03; average ± 1 standard deviation), compared to the rural (nC: 8.8±0.6; O:C: 0.80±0.05) and remote mountain stations (nC: 8.1±0.8; O:C: 0.91±0.07), indicative of different emission sources and chemistry. Compounds containing only carbon, hydrogen, and oxygen atoms (CHO) contribute highest to the total OOA mass at the rural sites (79.9±5.2 %), in accordance with their proximity to forested areas (66.2±5.5 % at the mountain sites and 72.6±4.3 % at the urban sites). The largest contribution of nitrogen-containing compounds (CHON) are found at the urban stations (27.1±4.3 %), consistent with their higher NOx levels. Besides, we parametrize OOA volatility (saturation mass concentrations, C sat) using molecular composition information and compare it with the bulk apparent volatility derived from thermal desorption of the OOA particles within the FIGAERO. We find differences in C sat values of up to ~3 orders of magnitude, and variation in thermal desorption profiles (thermograms) across different locations and systems. From our study, we draw the general conclusion that environmental conditions (e.g., ambient temperature) do not directly affect OOA apparent volatility, but rather indirectly by influencing the sources and chemistry of the environment and thus the chemical composition. The comprehensive global dataset provides results that show the complex thermodynamics and chemistry of OOA and their changes during its lifetime in the atmosphere, and that generally the chemical description of OOA suffices to predict its apparent volatility, at least qualitatively. Our study thus provides new insights that will help guide choices of e.g. descriptions of OOA volatility in different model frameworks which has been previously simplified due to challenges to measure and represent it in models. [ABSTRACT FROM AUTHOR]

The apparent volatility of atmospheric organic aerosol (OA) particles is determined by their chemical composition and environmental conditions (e.g., ambient temperature). A quantitative, experimental assessment of volatility and the respective importance of these two factors remains challenging, especially in ambient measurements. We present molecular composition and volatility of oxygenated OA (OOA) particles in different rural, urban, and remote environments across the globe (including Chacaltaya, Bolivia; Alabama, U.S.; Hyytiälä, Finland; Stuttgart and Karlsruhe, Germany; and Delhi, India) based on deployments of a filter inlet for gases and aerosols coupled to a high-resolution time-of-flight chemical ionization mass spectrometer (FIGAERO CIMS). We find on average larger carbon numbers (nC) and lower oxygen-to-carbon (O:C) ratios at the urban sites (nC: 9.8±0.7; O:C: 0.76±0.03; average ± 1 standard deviation), compared to the rural (nC: 8.8±0.6; O:C: 0.80±0.05) and remote mountain stations (nC: 8.1±0.8; O:C: 0.91±0.07), indicative of different emission sources and chemistry. Compounds containing only carbon, hydrogen, and oxygen atoms (CHO) contribute highest to the total OOA mass at the rural sites (79.9±5.2%), in accordance with their proximity to forested areas (66.2±5.5% at the mountain sites and 72.6±4.3% at the urban sites). The largest contribution of nitrogen-containing compounds (CHON) are found at the urban stations (27.1±4.3%), consistent with their higher NOx levels. Besides, we parametrize OOA volatility (saturation mass concentrations, Csat) using molecular composition information and compare it with the bulk apparent volatility derived from thermal desorption of the OOA particles within the FIGAERO. We find differences in Csat values of up to ~3 orders of magnitude, and variation in thermal desorption profiles (thermograms) across different locations and systems. From our study, we draw the general conclusion that environmental conditions (e.g., ambient temperature) do not directly affect OOA apparent volatility, but rather indirectly by influencing the sources and chemistry of the environment and thus the chemical composition. The comprehensive global dataset provides results that show the complex thermodynamics and chemistry of OOA and their changes during its lifetime in the atmosphere, and that generally the chemical description of OOA suffices to predict its apparent volatility, at least qualitatively. Our study thus provides new insights that will help guide choices of e.g. descriptions of OOA volatility in different model frameworks which has been previously simplified due to challenges to measure and represent it in models. [ABSTRACT FROM AUTHOR]

Indole (ind) is a nitrogen-containing heterocyclic volatile organic compound commonly emitted from animal husbandry and from different plants like maize with global emissions of 0.1 Tg y-1. The chemical composition and optical properties of indole secondary organic aerosol (SOA) and brown carbon (BrC) are still not well understood. To address this, environmental chamber experiments were conducted to investigate the oxidation of indole at atmospherically relevant concentrations of selected oxidants (OH radicals and O3) with/without NO2. In the presence of NO2, the SOA yields decreased by more than a factor of two but the mass absorption coefficient at 365 nm (MAC 365) of ind -SOA was 4.3 ± 0.4 m2 g-1, which was 5 times higher than that in experiments without NO2. In the presence of NO2, C8H6N2O2 (identified as 3-nitroindole) contributed 76 % to the all organic compounds detected by a chemical ionization mass spectrometer, contributing ~50 % of the light absorption at 365 nm (Abs 365). In the absence of NO2, the dominating chromophore was C8H7O3N contributing to 20–30 % of Abs 365. Indole contributes substantially to the formation of secondary BrC and its potential impact on the atmospheric radiative transfer is further enhanced in the presence of NO2, as it significantly increases the specific light absorption of ind -SOA by facilitating the formation of 3-nitroindole. This work provides new insights into an important process of brown carbon formation by interaction of two pollutants, NO2 and indole, mainly emitted by anthropogenic activities. [ABSTRACT FROM AUTHOR]

Indole (ind) is a nitrogen-containing heterocyclic volatile organic compound commonly emitted from animal husbandry and from different plants like maize with global emissions of 0.1 Tg y-1. The chemical composition and optical properties of indole secondary organic aerosol (SOA) and brown carbon (BrC) are still not well understood. To address this, environmental chamber experiments were conducted to investigate the oxidation of indole at atmospherically relevant concentrations of selected oxidants (OH radicals and O3) with/without NO2. In the presence NO2, the SOA yields decreased by more than a factor of two but the mass absorption coefficient at 365 nm (MAC365) ind-SOA was 4.3 ± 0.4 m² g-1, which was 5 times higher than that in experiments without NO2. In the presence of NO2, C8H6N2O2 (identified as 3-nitroindole) contributed 76% to the all organic compounds detected by a chemical ionization mass spectrometer, contributing ~50% of the light absorption at 365 nm (Abs365). In the absence of NO2, the dominating chromophore was C8H7O3N contributing to 20-30% of Abs365. Indole contributes substantially to the formation of secondary BrC and its potential impact on the atmospheric radiative transfer is further enhanced in the presence of NO2, as it significantly increases the specific light absorption of ind-SOA by facilitating the formation of nitroindole. This work provides new insights into an important process of brown carbon formation by interaction of two pollutants, NO2 and indole, mainly emitted by anthropogenic activities. [ABSTRACT FROM AUTHOR]

Stabilized Criegee intermediates (SCIs) and organic peroxy radicals (RO2), as important reactive species in the atmosphere, are critical in oxidation processes and secondary organic aerosol (SOA) formation. However, the influence of temperature on these reactive intermediates and the corresponding reaction mechanisms in SOA formation are still not well defined. In this study, through utilizing SCIs scavengers and regulating [HO2]/[RO2] from ~0.3 to ~1.9, the roles of RO2 and SCIs in SOA formation were investigated at 298 K, 273 K, and 248 K, respectively, particularly for dimers formation in β-pinene ozonolysis. The SOA yield increased by 21 % from 298 K to 273 K, while further reducing the temperature to 248 K led to a decrease of 40 % in SOA yield. This cannot be explained by partitioning or wall losses and is attributed to the temperature impact on rate coefficients and product branching ratios of some specific reactions. Both changing [HO2]/[RO2] and scavenging SCIs significantly affect SOA yield and composition. SCIs reactions accounted for more than 40 % of dimers and SOA mass formation for all temperatures, and the dimers formed from the SCIs channel did not show obvious suppression at subzero temperature. Increasing [HO2]/[RO2] inhibited dimers and SOA formation with a higher sensitivity at lower temperatures. Compared to low [HO2]/[RO2] condition, the dimers abundance at high [HO2]/[RO2] decreased by about 31 % at 298 K and 70 % at 248 K. The correlation between dimers and [RO2]2 demonstrates that RO2 cross reactions cannot explain the impact of RO2 concentration on dimers formation at low temperatures. The specific impact of [HO2]/[RO2] on SCIs-controlled dimers at lower temperatures indicates the influence of changing [HO2]/[RO2] on dimers formed from the reaction of C9-SCIs and RO2 with a negative temperature dependence. The higher contribution of this SCIs reaction channel to dimers at lower temperatures is confirmed by chemical kinetic modeling. The dimers formed from C9-SCIs reaction with RO2 were estimated to decrease by 61 % at high [HO2]/[RO2] compared to low [HO2]/[RO2] at 248 K, providing explanations for the observed [HO2]/[RO2] impact. The high reactivity and substantial contribution to SOA of β-pinene-derived SCIs at lower temperatures observed in this study suggest that monoterpene-derived SCIs reactions should be accounted for in describing colder regions of the atmosphere. [ABSTRACT FROM AUTHOR]

Stabilized Criegee intermediates (SCIs) and organic peroxy radicals (RO2), as important reactive species in the atmosphere, are critical in oxidation processes and secondary organic aerosol (SOA) formation. However, the influence of temperature on these reactive intermediates and the corresponding reaction mechanisms in SOA formation are still not well defined. In this study, through utilizing SCIs scavengers and regulating [HO2]/[RO2] from ~0.3 to ~1.9, the roles of RO2 and SCIs in SOA formation were investigated at 298 K, 273 K, and 248 K, respectively, particularly for dimers formation in ß-pinene ozonolysis. The SOA yield increased by 21% from 298 K to 273 K, while further reducing the temperature to 248 K led to a decrease of 40% in SOA yield. This cannot be explained by partitioning or wall losses and is attributed to the temperature impact on rate coefficients and product branching ratios of some specific reactions. Both changing [HO2]/[RO2] and scavenging SCIs significantly affect SOA yield and composition. SCIs reactions accounted for more than 40% of dimers and SOA mass formation for all temperatures, and the dimers formed from the SCIs channel did not show obvious suppression at subzero temperature. Increasing [HO2]/[RO2] inhibited dimers and SOA formation with a higher sensitivity at lower temperatures. Compared to low [HO2]/[RO2] condition, the dimers abundance at high [HO2]/[RO2] decreased by about 31% at 298 K and 70% at 248 K. The correlation between dimers and [RO2]2 demonstrates that RO2 cross reactions cannot explain the impact of RO2 concentration on dimers formation at low temperatures. The specific impact of [HO2]/[RO2] on SCIscontrolled dimers at lower temperatures indicates the influence of changing [HO2]/[RO2] on dimers formed from the reaction of C9-SCIs and RO2 with a negative temperature dependence. The higher contribution of this SCIs reaction channel to dimers at lower temperatures is confirmed by chemical kinetic modeling. The dimers formed from C9-SCIs reaction with RO2 were estimated to decrease by 61% at high [HO2]/[RO2] compared to low [HO2]/[RO2] at 248 K, providing explanations for the observed [HO2]/[RO2] impact. The high reactivity and substantial contribution to SOA of ß-pinene-derived SCIs at lower temperatures observed in this study suggest that monoterpene-derived SCIs reactions should be accounted for in describing colder regions of the atmosphere. [ABSTRACT FROM AUTHOR]

Satellite- and aircraft-based mid-infrared measurements of polar stratospheric clouds (PSCs) have provided spectroscopic evidence for the presence of β -NAT (nitric acid trihydrate) particles. Metastable nitric acid hydrate phases such as α -NAT and α -NAD (nitric acid dihydrate) have been frequently observed in laboratory experiments but not yet detected as a constituent of PSCs in atmospheric measurements. As for the β -NAD modification, its formation was first observed in X-ray diffraction measurements when the low-temperature α -NAD phase was warmed to a temperature above 210 K. Its infrared spectrum has been reported, but so far no optical constants have been derived that could be used as input for infrared retrievals of PSC composition. In this work, we show that β -NAD particles were efficiently formed in isothermal heterogeneous crystallisation experiments at 190 K from supercooled HNO 3 /H 2 O solution droplets containing an embedded mineral dust or meteoric smoke particle analogue. An inversion algorithm based on a T-matrix optical model was used to derive for the first time the mid-infrared complex refractive indices of the β -NAD modification from the measured extinction spectrum of the particles. In contrast to the heterogeneous crystallisation experiments, the α -NAD phase was formed when the HNO 3 /H 2 O solution droplets did not contain a solid nucleus and crystallised homogeneously. Using a light-scattering detector that recorded two-dimensional scattering patterns of the crystallised NAD particles, we were able to determine predominant shapes of the α - and β -NAD crystals. We found that α -NAD grew into elongated, needle-shaped crystals, while β -NAD particles were compact in shape. This agrees with previously reported images of α - and β -NAD particles grown on the cryo-stage of an environmental scanning electron microscope. While direct evidence for the existence of metastable NAD in the polar stratosphere is still lacking, our experiments add to the wealth of previous laboratory studies that have identified various conditions for the rapid growth of metastable compositions. In the atmosphere, these could be intermediate states that transform into thermodynamically stable NAT on longer timescales in aged PSCs. [ABSTRACT FROM AUTHOR]