Organic aerosols (OA) play a significant role in influencing both climate and human health. However, in source-receptor modelling, a large fraction of OA is typically attributed to highly aged, atmospherically processed species collectively referred to as oxygenated organic aerosol (OOA). Nevertheless, the formation pathways and evolution of OOA as well as their impacts on aerosol optical properties, remain poorly understood. To address this knowledge gap, an experiment was conducted in a suburban site in the Paris region to study the evolution of OOA and their optical properties. Our results show that in regionally transported air masses with mixed biogenic and anthropogenic emissions, the formation of OOA through photochemical processes explains most of the increase in submicron particle mass. Meteorological conditions played a critical role: under dry and strong solar radiation conditions, enhanced formation of more-oxidized OOA (MO-OOA) was observed. BrC absorption increased concurrently, with short-wavelength absorption rising by similar to 35 % over relatively similar to 24 h of photochemical aging. Conversely, under humid, low-radiation conditions, the OA composition shifted toward less-oxidized OOA (LO-OOA). Suppressed photochemistry limited MO-OOA production, resulting in a lower overall OA oxidation state. These findings highlight the role of photochemistry in shaping both the chemical evolution and resultant optical properties of OA, underscoring the need to consider meteorological dynamics when evaluating aerosol-climate interactions in suburban forest environments.

Abstract. Reactive nitrogen species (NOy) are involved in the formation of secondary pollutants, thus following their interconversions is crucial for understanding their impact on air quality. While nitric oxide (NO) and nitrogen dioxide (NO2), are important contributors to total NOy, further constituents such as the nitrate radical (NO3) and dinitrogen pentoxide (N2O5), formed from NO2 oxidation, are key intermediates in nocturnal oxidation processes. Even though NO3 and N2O5 affect the NOy budget in the following day, their role is poorly constrained in isolated measurements. We present the “NOyBOx”, a new instrumental framework combining measurements of NO, NO2 and NOy from commercial analyzers with a custom-built incoherent broadband cavity-enhanced absorption spectrometer (IBBCEAS) that measures NO3 directly and N2O5 via a thermal dissociation inlet (TDI) and channel differencing. We describe the design of the IBB-CEAS subsystem, characterize the TDI, cavity optics, transmission losses, and, using a synthetic spectra simulation approach, the bias of ambient NO2 and water vapor on our concentration retrieval procedure. Under optimal laboratory conditions we find a limit of detection (LOD) of 1.5 and 2.2 pptv (1σ Allan deviation, 15 s) for NO3 and N2O5, respectively. Under field conditions, interference by water vapor and NO2 worsens these LODs to 3.1 pptv with a total measurement uncertainty (TMU) of 12 % for NO3, and to 4.4 pptv with a mean TMU of 21 % for N2O5. Exemplary results from its first deployment during a field campaign in a temperate forest atop a 40 m tall tower are discussed.

Abstract. Aerosols play a critical role in Earth’s climate, but substantial evolution in their physicochemical properties after emission introduces uncertainties in predicting their climate impacts. Observational constraints on how aging modifies aerosol properties remain limited. Here, we investigate the effects of ~2–6 h of aging on aerosol physicochemical properties using coordinated airborne and ground-based measurements in Paris and its downwind regions. Urban plumes contributed modestly to particle number concentrations in the 80–200 nm size range and resulted in a moderate enhancement of submicron particle (PM1) mass relative to out-plume background levels. Organic aerosol (Org) dominated PM1 mass both near the urban source and downwind. Aircraft observations showed enhanced Org and non-refractory PM1 relative to excess CO (CO above surrounding background) in downwind plumes, indicating net secondary organic aerosol production during aging. Aerosol optical properties evolved concurrently. Downwind plume-average single-scattering albedo (SSA) at 450 and 630 nm was higher than near-source values. Consistently, the complex refractive index shifted from lower real (~1.35–1.40) and higher imaginary (~0.03–0.08) components near source to higher real (~1.45–1.50) and lower imaginary (~0.015–0.02) components downwind. The absorption Ångström exponent also increased, indicating a greater fractional contribution of brown carbon to light absorption. These results demonstrate that urban plume aging alters aerosol composition and optical properties and highlight the need to represent evolving aerosol characteristics in atmospheric models and remote-sensing retrievals.

Abstract. Brown carbon (BrC) absorbs solar radiation, thereby partially offsetting the cooling effect of aerosols on the climate. However, its contribution to climate forcing remains poorly constrained and is currently not explicitly included in the overall radiative forcing estimates of the IPCC. In this work, we quantify the contributions of BrC to light absorption in urban outflows from Paris using aircraft observations. We observe a progressive "browning" of the plume over 6 hours of transport time, characterised by a significant increase in the relative contribution of BrC to aerosol light absorption. This optical shift is strongly correlated with photochemical ageing as evidenced by the increase in the organic aerosols-to-BC mass ratio and the simultaneous depletion of aromatic volatile organic precursors. The corresponding rise in the Absorption Angström Exponent from 1.1 to 2.6 provides further evidence that secondary organic aerosol formation is the primary driver of enhanced light absorption at 450 nm in the aged plume. Our observations indicate that BrC is photochemically produced during transport at a rate of ~0.25 h-1 over 6 hours of atmospheric processing under the conditions examined in this study. For these cases, the findings suggest that urban outflows are not merely diluted during transport but undergo significant chemical transformations that alter their radiative impact. Accounting for these time-dependent optical changes provides a way to refine how climate models simulate radiative impacts of large metropolitan areas to climate change.

The complex refractive index (CRI; n−ik) and the single scattering albedo (SSA) are key parameters driving the aerosol direct radiative effect. Their spatial, temporal, and spectral variabilities in anthropogenic–biogenic mixed environments are poorly understood. In this study, we retrieve the spectral CRI and SSA (370–950 nm wavelength range) from in situ surface optical measurements and the number size distribution of submicron aerosols at three sites in the greater Paris area, representative of the urban city, as well as its peri-urban and forested rural environments. Measurements were taken as part of the ACROSS (Atmospheric Chemistry of the Suburban Forest) campaign in June–July 2022 under diversified conditions: (1) two heatwaves leading to high aerosol levels, (2) an intermediate period with low aerosol concentrations, and (3) an episode of long-range-transported fire emissions. The retrieved CRI and SSA exhibit an urban-to-rural gradient, whose intensity is modulated by the weather conditions. A full campaign average CRI of 1.41−0.037i (urban), 1.52−0.038i (peri-urban), and 1.50−0.025i (rural) is retrieved. The imaginary part of the CRI (k) increases and the SSA decreases at the peri-urban and forest sites when exposed to the influence of the Paris urban plume. Values of k > 0.1 and SSA < 0.6 at 520 nm are related to a black carbon mass fraction larger than 10 %. Organic aerosols are found to contribute to more than 50 % of the aerosol mass and up to 10 % (urban), 17 % (peri-urban), and 22 % (forest) of the aerosol absorption coefficient at 370 nm. A k value of 0.022 (370 nm) was measured at the urban site for the long-range-transported fire episode.

Semi-volatile organic compounds (SVOCs) exist in both gaseous and particulate phases in the atmosphere, and are important intermediate species for the formation of secondary organic aerosols. In this study, both phases of SVOCs are studied in the vicinity of Paris, France, and within São Paulo, Brazil aiming to better understand the coupling between anthropogenic and biogenic emissions in distinct urban settings. Both regions are representative of strong anthropogenic and biogenic sources of pollutants. These areas were within the scope of the ACROSS (Atmospheric Chemistry of the Suburban Forest) and BIOMASP+ (Biogenic emissions, chemistry, and impacts in the Metropolitan Area of São Paulo) projects, respectively. The ACROSS campaign took place from June to July 2022 and data analyzed here were acquired at the Rambouillet (RMB) forested site, about 50 km southwest of Paris. BIOMASP+ conducted intensive observations in April and May 2023, and data were collected at the Institute of Astronomy, Geophysics, and Atmospheric Sciences (Matão-IAG) urban site, within the University of São Paulo campus. Continuous measurements of ambient organics through a CHemical Analysis of aeRosols ON-line (CHARON) inlet coupled to a high-resolution proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) were carried out at both sites, as well as complementary variables such as aerosol chemical composition, regulated pollutants, and meteorological parameters, among others.The concentration of submicron bulk organic aerosol was comparable at both sites during ACROSS and BIOMASP+, reaching 5.0 µg/m3 and 7.3 µg/m3 for RMB and Matão-IAG, respectively. These are higher than typical 1-year averages observed at urban sites in Europe (3-4 µg/m3) [1] and previous observations near Matão-IAG in October 2012 (4.8 µg/m3) [2]. Biogenic VOCs showed distinct concentrations and temporal variabilities between sites with isoprene levels of 0.51 ppb vs 0.26 ppb of monoterpene in Brazil and Paris (0.35 ppb of isoprene vs 0.23 ppb of monoterpene) thus potentially leading to important differences in the subsequent secondary organic aerosol formation. Additionally, toluene, an anthropogenic marker, was higher at Matão-IAG (1.52 ppb) compared to RMB (0.25 ppb). This study will focus on SVOCs according to their mass spectra and temporal evolution and will compare the field observations to chamber experiments of biogenic and anthropogenic secondary organic aerosol formation. Those observations shall aid in understanding secondary formation processes and improve air quality modelling, as well as efficient pollution mitigation strategies in two contrasting large urbanized areas.Keywords: SVOC, Sao Paulo, Paris, SOA, CHARON-PTR-ToF-MSAcknowledgments: This work is funded and supported by Labex CaPPA, CPER ECRIN, ANR, and INSU LEFE-CHAT within the framework of BIOMASP and ACROSS projects. O. Murana’s field campaign in Brazil was supported by the Graduate Program “Science for a Changing Planet”, funded by the Program “Investissements d’avenir” (I-SITE ULNE / ANR-16-IDEX-0004 ULNE).References[1] Chen, G. et al., Environment International, vol. 166, p. 107325, 2022[2] Almeida, G. P. et al., Atmos. Chem. Phys., vol. 14, no. 14, pp. 7559-7572, 2014

Understanding the interaction of anthropogenic and biogenic emissions around large urban agglomerations remains an important question for atmospheric research and the key question of the ACROSS (Atmospheric Chemistry of the Suburban Forest) project. ACROSS is based on an intensive field campaign in the Paris area, including ground-based measurements in the urban inner centre and suburban and forested sites and measurements made on board aircraft, during the exceptionally hot and dry summer of 2022. In addition, 3D modelling represents an important tool in ACROSS, and here we use the available measurements from the campaign together with observations from air quality and meteorological networks to evaluate the WRF–CHIMERE model simulation for the ACROSS period. We find that the WRF model is able to reproduce the meteorological variability during the campaign, in particular during two heatwaves. The model reproduces the daily ozone maxima well but overestimates PM2.5 by a factor of 1.5–2, partly due to an overestimation of secondary aerosol, both organic and inorganic. For organic aerosol in the Île-de-France area, the biases are reduced to about ±20 %. These differences are in contrast with those of the existing literature and might have been increased by the hot conditions of summer 2022. For case studies during two heatwave days, the model shows the sources for two organic aerosol peaks above 20–30 µg m−3, on one occasion due to biogenic secondary organic aerosol formation in different forests around Paris and on another occasion due to the advection of wildfire aerosols in combination with secondary formation mainly from forest-emitted biogenic volatile organic compounds (BVOCs).

In order to study aerosols in environments influenced by anthropogenic and biogenic emissions to variable extents, PM1 samples were collected during summer 2022 in the greater Paris area (ACROSS campaign, Atmospheric Chemistry Of the Suburban Forest, 14 June to 25 July) at two locations that represent the urban Paris and the suburban forested areas. They were analyzed using high-resolution mass spectrometry (HRMS) together with total carbon (TC) with a thermo-optical method. Both sites are compared here to explore differences in aerosol composition from urban and forested environments. The TC analysis shows similar organic carbon (OC) concentrations at both sites (3.2 ± 1.8 µg m−3 for Paris and 2.9 ± 1.5 µg m−3 for Rambouillet) and higher elemental carbon (EC) values in the urban area. Both OC and EC concentrations did not show significant variations for daytime and nighttime conditions. This work highlights the influence of anthropogenic inputs on the chemical composition of urban and forested areas, derived from the presence of CHO and CHON compounds but also the detection of two sulfur-containing compounds (C5H12SO7 and C10H17NSO7), which could be tentatively assigned as organosulfates. A smaller number of aromatic compounds were observed for clean periods that better represent the local biogenic and anthropogenic contributions in Rambouillet and Paris, respectively.

Alkyl nitrates (ANs) and peroxycarboxylic nitric anhydrides (PANs) are important reservoirs of reactive nitrogen that contribute significantly to the rate of formation and growth of secondary organic aerosols and support the transport of reactive nitrogen from polluted areas to remote areas. It is therefore critical to understand their sources and sinks in different environments. In this study we use measurements of OH, O3, NO3 reactivity, volatile organic compounds, ∑ANs and ∑PANs during the ACROSS (Atmospheric ChemistRy Of the Suburban foreSt) campaign to investigate different production and loss processes of ANs and PANs in a temperate forest. During the daytime OH-initiated processes were the dominant source of ANs (69 %–72 %), followed by NO3 (18 %–20 %) and O3 (8 %–12 %). During the nighttime the contribution from OH decreased to 43 %–53 %, and NO3 increased to 26 %–40 % with that of O3 largely unchanged. Of the measured ∑PANs, 48 %–78 % were modelled to be peroxyacetic nitric anhydride (PAN, commonly known as peroxyacetyl nitrate). Physical loss (e.g. deposition) was an important sink for both ANs and PANs and contributed significantly to the very short lifetimes of 1–4 h for ANs and 0.08–1.5 h for PANs observed during the campaign.

Nighttime NO3-initiated oxidation of monoterpenes plays a crucial role as source of organic nitrates (ONs) and secondary organic aerosol (SOA), impacting climate, air quality, and human health. Nevertheless, monoterpene reactions with NO3 remain poorly understood. This study provides an in-depth investigation of the NO3-initiated oxidation of alpha- and beta-phellandrenes through simulation chamber experiments and a combination of various analytical techniques (FTIR, PTR-ToF-MS, ACSM, nitrate-CI-APi-ToF-MS, Orbitrap, SMPS). SOA yields were measured, and oxidation products, including highly oxygenated organic molecules (HOMs), were investigated in gas and aerosol phases. Numerical simulations were also performed to investigate the dominant chemical regimes for RO2 radicals. We found that alpha- and beta-phellandrenes are efficient SOA precursors with yields reaching up to 35 % and 60 %, respectively, with beta-phellandrene generating significantly more SOA than alpha-phellandrene. Both monoterpenes produce large quantities of ONs in gas and aerosol phases with total molar yields of 40 %-60 %. Similar gas-phase products were detected for alpha- and beta-phellandrenes. In particular, carbonyl nitrates, dicarbonyl nitrates, and dicarbonyls were detected as first-generation products. Autooxidation processes were also shown to occur with numerous gas-phase HOM monomers and dimers detected. Chemical mechanisms have been proposed to explain the formation of the detected products. Since gas-phase products were similar for both monoterpenes, they do not explain the differences in SOA yields. However, some differences in aerosol-phase composition were observed, which may explain why beta-phellandrene is a more efficient SOA precursor. This study is the first mechanistic investigation of the reactions of alpha- and beta-phellandrenes with the NO3 radical.

​​​​​​​Volatile organic compounds (VOCs) play a key role in tropospheric chemistry, giving rise to secondary products such as highly oxygenated organic molecules (HOMs) and secondary organic aerosols (SOAs). HOMs, a group of low-volatility gas-phase products, are formed through the autoxidation process of peroxy radicals (RO2) originating from the oxidation of VOCs. The measurement of HOMs is made by a NO3- ToFCIMS instrument, which also detects other species like small highly oxygenated VOCs (e.g., dicarboxylic acids) and sulfuric acid (H2SO4). The instrument response to HOMs is typically estimated using H2SO4, as HOMs are neither commercially available nor easily synthesized in the laboratory. The resulting calibration factor is then applied to quantify all species detected using this technique. In this study, we explore the sensitivity of the instrument to commercially available small organic compounds, primarily dicarboxylic acids, given the limitations associated with producing known amounts of HOMs for calibration. We compare these single-compound calibration factors to the one obtained for H2SO4 under identical operational conditions. The study found that the sensitivity of the NO3- ToFCIMS varies depending on the specific type of organic compound, illustrating how a single calibration factor derived from sulfuric acid is clearly inadequate for quantifying all detected species using this technique. The results highlighted substantial variability in the calibration factors for the tested organic compounds, with 4-nitrocatechol exhibiting the highest sensitivity and pyruvic acid the lowest. The obtained sulfuric acid calibration factor agreed well with the previous values from the literature. In summary, this research emphasized the need to develop reliable and precise calibration methods for progressively oxygenated reaction products measured with a NO3- chemical-ionization mass spectrometer (CIMS), for example, HOMs.

Atmospheric aerosols impact the Earth's climate system directly by scattering and absorbing solar radiation, and it is important to characterise the aerosol optical properties in detail. This study reports the development and validation of an airborne dual-wavelength cavity-attenuated phase-shift (CAPS) single monitor, named A2S2 (Aerosol Absorption Spectral Sizer), based on the commercial CAPS single-scattering albedo monitor (CAPS-PMSSA; Aerodyne), to simultaneously measure the aerosol optical scattering and extinction at both 450 and 630 nm wavelengths. Replaced pressure and temperature sensors and an additional flow control system were incorporated into the A2S2 for its utilisation on board research aircraft measuring within the troposphere. The evaluation of A2S2 characteristics was performed in the laboratory and included the investigation of the signal-to-noise ratio, validation of performance at various pressure levels, optical closure studies and intercomparing with the currently validated techniques. The chamber experiments show that the A2S2 can perform measurements at sample pressures as low as 550 hPa and at sample temperatures as high as 315 K. Based on the Allan analysis results, we have evaluated that the minimum detection limit of the measurements shows that the measurements have a limit accuracy of ∼ 2 Mm−1 at 450 nm and ∼ 1 Mm−1 at 630 nm for 1 Hz measurements of both scattering coefficients (σsca) and extinction coefficients (σext). The optical closure study with size-selected polystyrene latex (PSL) particles shows that the truncation error of the A2S2 is negligible for particles with particle volume diameter (Dp) < 200 nm, while, for the larger sub-micrometre particles, the measurement uncertainty of A2S2 increases but remains less than 20 %. The average factors to correct the truncation error are 1.13 and 1.05 for 450 and 630 nm, respectively. A simplified truncation correction, dependent on the scattering Ångström exponent (SAE), was developed to rectify truncation errors of the future A2S2 field measurement data. The σsca and σext measured by the A2S2 show good agreement with the concurrent measured results from the nephelometer and the CAPS particle extinction monitor (CAPS-PMex). The absorption coefficient (σabs) derived through the extinction-minus-scattering (EMS) method by the A2S2 also corresponds with the results obtained from the aethalometer. The A2S2 was successfully deployed during an aircraft measurement campaign (Atmospheric ChemistRy Of the Suburban foreSt – ACROSS) conducted in the vicinity of Paris and the surrounding regions. The average SSA measured during the entire ACROSS flight campaign is 0.86 and 0.88 at 450 and 630 nm, respectively, suggesting that light-absorbing organic aerosols play a significant role. The average SAE and absorption Ångström exponent (AAE) varied due to measurements in various pollution conditions. The results presented in this study indicate that the A2S2 instrument is reliable for measuring aerosol σsca and σext at both blue and red wavelengths, and it stands as a viable substitute for future airborne evaluations of aerosol optical properties.

High-resolution mass spectrometry (HRMS) has become an indispensable tool in the characterization of organic aerosols (OA) providing information on air quality, health assessment, climate trends, reactions, and source apportionment. Spectra-derived lists of formulas and their relative abundances are used to compare ambient OA from different sources or to monitor secondary OA formation under controlled laboratory conditions in smog chamber experiments. Various techniques are implemented to visualize common and unique features, series of precursors, and products. The disadvantage of this conventional approach is in associating elemental compositions to specific compounds, while due to several analytical limitations, the structural information remains hidden. We argue that some of the conclusions derived from this data analysis can be misleading. In this study, we applied in-ESI source H/D exchange (HDX), which facilitated enumeration of labile protons in molecules behind elemental compositions of OA. We applied this technique to compare OA from three different locations representing urban, forest, and marine environments and to examine tentative chemical information derived from Kendrick mass defect (KMD) series analysis. Significant discrepancies were found between numbers of labile protons in protogenic functional groups and the stoichiometry of chemical reactions, which are associated with KMD series. Only a portion of chemical pairs matched target stoichiometries, which highlights the existing limitations in environmental applications of conventional formula-based HRMS data interpretation strategies.

The budget of reactive nitrogen species, which play a central role in atmospheric chemistry (e.g. in photochemical O-3 production), is poorly understood in forested regions. In this study, through observations of NO, NO2, NOy, and O-3 in the Rambouillet Forest near Paris, France, we have examined nighttime processes controlling NOx in an anthropogenically impacted forest environment. The O-3 mixing ratios displayed a strong diel profile at the site that was driven by a variable but generally rapid deposition to soil and foliar surfaces. The O-3 diel profile was strongly influenced by relative humidity and temperature inversion. Only when the O-3 mixing ratio was sufficiently low (and thus the NO lifetime sufficiently long) were sustained NO peaks observed above the instrumental detection limit, enabling the derivation of average NO emission rates of similar to 1.4 ppbvh-1 from the soil. Observations of the lack of increase in NO2 at night, despite a significant production rate from the reaction of NO with O-3, enabled an effective lifetime of NO2 of similar to 0.5-3 h to be derived. As the loss of NO2 was not compensated for by the formation of gas- or particle-phase reactive nitrogen species, it was presumably either driven by deposition to soil and foliar surfaces or any products formed were themselves short-lived with respect to deposition. By comparison, the daytime lifetime of NO2 with respect to loss by reaction with OH is about 1 d. Our results indicate that the nighttime deposition of NO2 is a major sink of boundary layer NOx in this temperate forest environment.

Biogenic volatile organic compounds (BVOCs) are emitted by vegetation and react with other compounds to form ozone and secondary organic matter (OM). In regional air quality models, biogenic emissions are often calculated using a plant functional type approach, which depends on the land use category. However, over cities, the land use is urban, so trees and their emissions are not represented. Here, we develop a bottom-up inventory of urban tree biogenic emissions in which the location of trees and their characteristics are derived from the tree database of the Paris city combined with allometric equations. Biogenic emissions are then computed for each tree based on their leaf dry biomass, tree-species-dependent emission factors, and activity factors representing the effects of light and temperature. Emissions are integrated in WRF-CHIMERE air quality simulations performed over June–July 2022. Over Paris city, the urban tree emissions have a significant impact on OM, inducing an average increase in the OM of about 5 %, reaching 14 % locally during the heatwaves. Ozone concentrations increase by 1.0 % on average and by 2.4 % during heatwaves, with a local increase of up to 6 %. The concentration increase remains spatially localized over Paris, extending to the Paris suburbs in the case of ozone during heatwaves. The inclusion of urban tree emissions improves the estimation of OM concentrations compared to in situ measurements, but they are still underestimated as trees are still missing from the inventory. OM concentrations are sensitive to terpene emissions, highlighting the importance of favoring urban tree species with low-terpene emissions.

We present direct measurements of biogenic volatile organic compound (BVOC)-induced nitrate radical (NO3) reactivity (kVOC) through the diel cycle in the suburban, temperate forest of Rambouillet near Paris (France). The data were obtained in a 6-week summer period in 2022 as part of the Atmospheric ChemistRy Of the Suburban foreSt (ACROSS) campaign. kVOC was measured in a small (700 m2) clearing mainly at a height of 5.5 m above ground level but also at 40 m (for 5 d and nights). At nighttime, mean values (and 25th–75th percentile ranges) of knightVOC(5.5m) = (0.24-0.06+0.32) s−1 and knightVOC(40m) = (0.016-0.007+0.018) s−1 indicate a significant vertical gradient and low NO3 reactivity above the canopy, whereas knightVOC(5.5 m) showed peak values of up to 2 s−1 close to the ground. The strong vertical gradient in NO3 reactivity could be confirmed by measurements between 0 and 24 m on one particular night characterized by a strong temperature inversion and is a result of the decoupling of air masses aloft from the ground- and canopy-level sources of BVOCs (and nitric oxide, NO). No strong vertical gradient was observed in the mean daytime NO3 reactivity, with kdayVOC(5.5m) = (0.12 ± 0.04) s−1 for the entire campaign and kdayVOC(40m) = (0.07 ± 0.02) s−1 during the 5 d period. Within the clearing, the fractional contribution of VOCs to the total NO3 loss rate coefficient (ktot, determined by photolysis, reaction with NO and VOCs) was 80 %–90 % during the night and ∼ 50 % during the day. In terms of chemical losses of α-pinene below canopy height in the clearing, we find that at nighttime hydroxyl radicals (OH) and ozone (O3) dominate, with NO3 contributing “only” 17 %, which decreases further to 8.5 % during the day. Based on measured OH, measured O3, and calculated NO3 concentrations, the chemical lifetime of BVOCs at noon is about 1 h and is likely to be longer than timescales of transport out of the canopy (typically of the order of minutes), thus significantly reducing the importance of daytime in-canopy processing. Clearly, in forested regions where sufficient nitric oxide and nitrogen dioxide (NOx) is available, the role of NO3 and OH as initiators of BVOC oxidation is not strictly limited to nighttime and daytime, respectively, as often implied in e.g. atmospheric chemistry textbooks.

Abstract. Nighttime NO3-initiated oxidation of monoterpenes plays a crucial role as source of organic nitrates (ONs) and secondary organic aerosols (SOA), impacting climate, air quality, and human health. Nevertheless, monoterpene reactions with NO3 remain poorly understood. This study provides an in-depth investigation of the NO3-initiated oxidation of α- and β-phellandrenes, through simulation chamber experiments and a combination of various analytical techniques (FTIR, PTR-ToF-MS, ACSM, nitrate-CI-APi-ToF-MS, Orbitrap, SMPS). SOA yields were measured, and oxidation products, including highly oxygenated organic molecules (HOMs), were investigated in gas and aerosol phases. Numerical simulations were also performed to investigate the dominant chemical regimes for RO2 radicals. We found that α- and β-phellandrenes are efficient SOA precursors with yields reaching up to 35 % and 60 %, respectively, with b-phellandrene generating significantly more SOA than α-phellandrene. Both monoterpenes produce large amounts of ONs in gas and aerosol phases with total molar yields of 40–60 %. Similar gas-phase products were detected for α- and β-phellandrenes. In particular, carbonyl nitrates, dicarbonyl nitrates and dicarbonyls were detected as first-generation products. Autooxidation processes were also shown to occur with numerous gas-phase HOM monomers and dimers detected. Chemical mechanisms have been proposed to explain products formation. Since gas-phase products were similar for both monoterpenes, they do not explain the differences in SOA yields. However, some differences in aerosol-phase composition were observed which may explain why β-phellandrene is a more efficient SOA precursor. This study is the first mechanistic investigation of the reactions of α- and β-phellandrenes with NO3 radical.