Abstract. We present a new, small-footprint instrument for point measurements of NO3 and N2O5. Both molecules play an important role in nocturnal atmospheric chemistry, impacting the NOx-budget and the oxidation of biogenic volatile organic compounds. NO3 and N2O5 are often present at concentrations of a few parts per trillion by volume (pptv) and their measurements in remote locations requires instrumentation that is easily transported and lightweight, but maintains high sensitivity and accuracy. We have constructed a relatively compact and light instrument for Cavity Ring-Down Spectroscopy (CRDS) with the dimensions (width × depth × height) of 55 cm × 55 cm × 150 cm and a weight of 50 kg that uses two independent cavities to quantify the mixing ratio of NO3 using an inlet at room temperature and the sum of NO3 + N2O5 via a thermal dissociation inlet. Under laboratory conditions, limits of detection (1σ Allan deviation at 1 s integration) for the NO3 and (NO3 + N2O5) channel are < 1 and < 2 pptv, respectively. This improves to about 0.1 and 0.2 pptv for 3 min integration. The total measurement uncertainty for NO3 is 9.8 % and ≥ 11.5 % for N2O5, depending on the NO3-to-N2O5 ratio. In this publication, we present design details of the instrument, discuss its performance in a controlled environment as well as during a field campaign. Additionally, we present measurements of transmission losses for NO3 across different filter types and methods to reduce filter reactivity and allow reusability after a cleaning procedure.

Ambient inorganic and organic particulate nitrate has been connected to cardiovascular and respiratory illness. Accurate measurement of nitrate in both gas- and aerosol-phases is essential for understanding its partitioning and thus its impact on human health. We report modifications to an existing Denuded-Thermal-Dissociation-Cavity-Ring-Down Spectrometer (D-TD-CRDS) system that reliably measured gas-phase NOX (NO and NO2) and NOY (NOX plus other reactive nitrogen species, see below) but suffered from a positive bias in particle nitrate measurement owing to denuder breakthrough and memory effects associated with changes in relative humidity. We describe an air drying system with low particle transmission losses that reduces the relative humidity at the inlet of the denuder to < 5 % so that no measurable denuder breakthrough of NOY (even of highly volatile species such as NO) was observed after continuous use over the course of month-long campaigns. The D-TD-CRDS measurement of particulate nitrate has a limit of detection (1 min) of similar to 0.035 & micro;gm(-3) under laboratory conditions and similar to 0.085 & micro;gm(-3) during field deployment. The associated uncertainty is estimated to be < 15 %. Laboratory experiments in which either inorganic nitrate aerosol (ammonium nitrate) or organic nitrate aerosol (generated from the NO3-induced oxidation of limonene) were sampled simultaneously from an environmental chamber by the D-TD-CRDS and with an Aerosol Mass Spectrometer (AMS) showed excellent agreement.

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.

Understanding the sources, distribution, and lifetime of inorganic chlorine-containing trace gases is crucial to assessing their tropospheric impacts. We report in situ measurements of Cl2, HOCl, ClNO2, and ClONO2 using iodide chemical ionization mass spectrometry during a 2.5-week campaign in June 2024 at a rural continental site in central Germany. Air masses that had passed over "marine-anthropogenic" regions (approximate to 400 km distant) showed significantly higher mixing ratios of chlorine-containing gases than "continental-unpolluted" air masses. From the marine-anthropogenic period, we provide the first quantitative observations of ClONO2 in the lower troposphere (up to 59.8 pptv during daytime). Persistent nonzero ClONO2 at night implies a non-photochemical source of ClONO2 or its precursor ClO, and/or that heterogeneous loss is slower than laboratory uptake coefficients suggest. ClNO2 levels were consistent with production via N2O5 uptake on chloride-containing particles; both ClNO2 and Cl2 were enhanced when O3-/N2O5-rich air entrained into the nocturnal boundary layer. Photolysis of ClNO2, Cl2, and HOCl yielded mean maximum Cl atom production rates of 1.0 & times; 106 cm-3 s-1 under marine-anthropogenic influenced air and 1.6 & times; 105 cm-3 s-1 under continental-unpolluted conditions. In the early morning, Cl production (due to ClNO2 photolysis) exceeded primary-OH production from O3 photolysis, while after noon HOCl photolysis was the dominant Cl source. At low solar zenith angles, HOCl photolysis contributed up to 40% of primary OH. These measurements indicate that Cl atoms can strongly influence hydrocarbon oxidation in similar rural regions, with potential regional and global implications (of up to 15%) for the methane lifetime.

The reaction of the nitrate radical (NO3) with biogenic volatile organic compounds (BVOC) in the atmosphere is a significant source of secondary organic aerosols and can affect the reactive nitrogen budget. Field studies dedicated to NO3-BVOC interaction on elevated platforms have highlighted vertical variability in both NO3 and BVOC mixing ratios. While vertical profiles of NO3 in the upper parts of the troposphere have been studied extensively, height-resolved measurements within the surface layer of BVOC-dominated areas, such as forests, are scarce. During the "Biosphere-Atmosphere Interactions and the Reactive Nitrogen Budget: Vertical Profiles of Key Species" (BAIRN-VIP) campaign, we measured vertical profiles of (B)VOC-induced NO3 reactivity (kVOC) along with the NO3 precursors nitrogen dioxide (NO2) and ozone (O3) at five heights below the canopy (1-16 m) as well as at one height above it (28 m) in order to assess the vertical gradients of both NO3 and BVOCs in the boreal forest of Hyyti & auml;l & auml;, Finland. We find that the stability of the nocturnal boundary layer and decoupling of the sub-canopy flow are the main drivers of the vertical gradients in kVOC. Steady-state calculations indicate that NO3 concentrations on the order of pptv are found exclusively above the canopy during strongly decoupled nights, with BVOCs as the only NO3 sink at heights above 4 m. During the day, BVOCs contribute, on average, 40-60% to the loss of NO3 along the profile. Our results indicate that single-height field measurements of NO3 are insufficient to explain nighttime oxidation chemistry under decoupled conditions.

The photolysis of nitrous acid (HONO) produces hydroxyl radicals (OH), the most important cleaning agent of the troposphere. For decades, HONO has been measured in concentrations which exceed the photo-stationary concentration arising from its gas phase formation via the reaction NO + OH and destruction by photolysis. Several photochemical and heterogeneous formation mechanisms, including the photolysis of nitrate have been proposed which may explain this excess HONO. This study reports on airborne remote sensing measurements of the mini-DOAS instrument over continental Europe, Southeast Asia, and the tropical Atlantic. The observations form a C-shaped profile in the troposphere with maximum volume mixing ratios of approximately 150 ppt in the planetary boundary layer, about 10 ppt in the free troposphere and up to 100 ppt in the tropical upper troposphere. These measurements of HONO throughout the troposphere exceed model predictions by up to an order of magnitude. Together with a host of other measured species and parameters, various formation mechanisms are explored to investigate in situ HONO sources. Although a precise formation mechanism in the polluted boundary layer remains elusive, the photolysis of particulate nitrate may explain excess HONO in the marine boundary layer. The excess HONO observed in the upper troposphere requires a gas phase source with a formation rate of up to 300 ppt h(-1). The possible role of peroxynitrous acid (HOONO), formed by the reactions NO + HO2 + M and NO2 + OH + M, and further oxidation by reactions with NO or O-3, is explored.

The complex interplay between different processes (physical losses, chemical losses and entrainment) defines the vertical gradient of many trace gases in forested environments. Despite this, height-resolved measurements of trace gases within a forest canopy are scarce. We present measurements of O3, PAN (peroxy acetyl nitric anhydride, CH3C(O)OONO2), and PAA (peroxy acetic acid, CH3C(O)OOH) at 6 heights between 1 and 28 m above and below the canopy at the SMEAR II site in the Finnish boreal forest. Through analysis of O3, PAN, and PAA nocturnal time-series we derived their height-dependent net loss rate coefficients. The net lifetimes of O3, PAN and PAA were highly variable with values of 1.5-42 h, 1.0-29 h, and 0.7-15 h, respectively, with the shortest lifetimes often measured at the lowest heights. The relative loss rates of PAN or PAA compared to O3 (kPAN/kO3 and kPAA/kO3) varied between 1.0-3.0 (85% of the measurements) and 1.0-5.0 (78% of the measurements) with medians of 1.7 and 2.4, respectively. The physical loss of O3, PAN and PAA was the major loss process (>90%), with chemical losses playing only a minor role. Entrainment significantly compensated for the physical and chemical losses and led to reduced net loss values of each trace gas, with exceptions encountered on a few nights when the sub-canopy and above-canopy air masses were decoupled as a result of e.g. low wind speed and friction velocity. Our vertical profiles of O3, PAN, and PAA reveal the complex interplay of boundary layer dynamics and chemistry at this forested location.

Multifunctional organic nitrates (LIM-ONO2) were formed from the NO3-initiated oxidation of limonene in the Simulation Chamber for Atmospheric Reactions and Kinetics (SCHARK), detected quantitatively by thermal dissociation cavity ring-down spectroscopy (TD-CRDS) and identified by high-resolution time-of-flight chemical-ionization (iodide) mass spectrometry (HR-ToF-ICIMS). Based on HR-ToF-ICIMS signal intensities, the most abundant LIM-ONO2 were C10H17NO4 and C10H17NO5, together representing >60% of the total LIM-ONO2 signal. We developed a method for cold-trapping LIM-ONO2 from the chamber, enabling us to examine their photolysis in the absence of precursor chemicals after re-injection into the SCHARK. The photolytic loss frequency of C10H17NO4 in the chamber when irradiated with LEDs emitting at 370 ± 13 nm was (1.69 ± 0.06) × 10-4 s-1. By comparison to the photolysis frequency of a chemical actinometer (Cl2), we were able to gain insight into the quantum yield (0.3-0.8) and absorption cross section of C10H17NO4 at these wavelengths and make an estimate of its atmospheric lifetime with respect to photolysis.

PAN (CH3C(O)O2NO2) and PAA (CH3C(O)OOH) are formed in the atmosphere uniquely and competitively from the same precursor radical (CH3C(O)O-2), whereby the relative rates of formation of PAN and PAA are determined by the abundances of HO2 and NO2. Here, we present airborne measurements of PAN and PAA in an altitude range from 0.3-15 km over the Amazon rainforest during the CAFE Brazil campaign, which took place from December 2022 to January 2023. Median PAA mixing ratios (ca. 340 pptv) across all analysed flights exceed those of PAN (ca. 65 pptv) by more than a factor of 5 in the mid troposphere around 6 km altitude, underlining the dominance of HO2 over NO2 chemistry under low NOX conditions of the Amazon rainforest. Within experimental uncertainty, the median vertical profiles of PAA and PAN throughout the entire measurement campaign are reproduced by the EMAC global chemical transport model. The EMAC analysis reveals that the oxidation of isoprene (emitted from the rainforest) is the primary source of the CH3C(O)O-2 radical. With an average contribution of similar to 31%, the main direct precursor of CH3C(O)O-2 is methylglyoxal (CH3C(O)CHO), followed by acetaldehyde (17%) and methyl vinyl ketone (10%). At altitudes between 12 and 14 km, only a few percent (<5%) of CH3C(O)O-2 radicals formed result in the production of PAN and PAA. The dominant sink of CH3C(O)O-2 in the upper troposphere is NO, while at lower altitudes, its reaction with other organic peroxy radicals produces acetic acid and CH3O2. The CH3O2 production resulting (via reactions of CH3C(O)O-2) from isoprene oxidation over the Amazon rainforest contributes between 9 and 44% to total CH3O2 production and surpasses the contribution from methane oxidation at altitudes below 4 km. Through the formation of CH3C(O)O-2, isoprene oxidation in this region thus significantly influences not only PAN and PAA formation but also the yields of acetic acid as well as methylperoxynitrate, methylhydroperoxide and formaldehyde, which are products of CH3O2 reactions with HO2 and NO, and photochemical precursors to HOx.

Measurement of total peroxy nitrates (ΣPNs) and alkyl nitrates (ΣANs) by instruments that use thermal dissociation (TD) inlets to convert the organic nitrate to detectable NO2 may suffer from systematic bias (both positive and negative) resulting from unwanted secondary chemistry in the heated inlets. Here we review the sources of the bias and the methods used to reduce it and/or correct for it and report new experiments using (for the first time) atmospherically relevant, unsaturated, biogenic alkyl nitrates as well as two different peroxyacetyl nitrate (PAN) sources. We show that the commonly used commercial C3 alkyl nitrate (isopropyl nitrate, IPN) for characterising the chemistry of ANs is not appropriate for real-air samples that contain longer-chain nitrates. Mixing ratios of ANs generated in the NO3-induced oxidation of limonene are strongly positively biased in the presence of NO. By detecting NOx rather than NO2, we provide a simple solution to avoid the bias caused by the conversion of NO to NO2 by primary and secondary peroxy radicals resulting from the complex chemistry in the thermal degradation of long-chain, alkyl nitrates in air at TD temperatures. We also show that using a photochemical source of PAN to characterise the TD inlets can result in a much stronger apparent bias from NO to NO2 conversion than for a diffusion source of synthesised (“pure”) PAN at similar mixing ratios, especially if high acetone concentrations (and thus radical concentrations) are involved. This is explained by the presence of thermally labile trace gases such as peracetic acid (CH3C(O)OOH) and hydrogen peroxide (H2O2).

It is well established that the drastic ozone loss in the Antarctic stratosphere, commonly known as the ozone hole, is primarily driven by gas-phase and heterogeneous chemical processes. While chemistry transport models generally reproduce observed ozone depletion well, they fail to capture the rapid early-winter decline of hydrogen chloride. We here examine the impact of the heterogeneous reaction between chlorine peroxide and hydrogen chloride forming HOOCl, followed by its photolysis. Incorporating this reaction and an additional hypochlorous acid loss pathway into a chemical mechanism significantly improves model agreement with observed levels of several chlorine compounds in the lower polar vortex stratosphere. This revised mechanism increases simulated ozone partial column depletion by over 15% between early July and mid-September 2011. Laboratory confirmation of these proposed reactions is needed to validate the mechanism.

It is well established that the drastic ozone loss in the Antarctic stratosphere, commonly known as the ozone hole, is caused by gas-phase and heterogeneous processes. Chemistry models generally reproduce observed ozone depletion reasonable well. However, models have been unable to reproduce observations of rapid HCl loss at the beginning of the polar winter. Here we examine the impact of the heterogeneous reaction between Cl2O2 and HCl to form HOOCl and its subsequent photolysis on chlorine compounds. A chemical mechanism with these reactions added is able to clearly better reproduce the observed temporal development of the chlorine compounds HCl, ClONO2, ClO, and HOCl in the polar vortex lower stratosphere. The proposed chemical mechanism does moderately increase the chemical ozone column depletion, about 10\% in the lower stratospheric vortex core in September. Laboratory measurements of the proposed reactions are needed to confirm this mechanism.

The two trace gases peroxyacetyl nitrate (PAN, CH3C(O)OONO2) and peracetic acid (PAA, CH3C(O)OOH) are products of reactions of the acetylperoxy radical with NO2 and HO2, respectively. They are formed during the oxidation of anthropogenic and biogenic VOCs and in biomass burning. PAN represents an important source of NOx in remote regions, while PAA is an indicator of the fate of peroxy radicals. To date, there have been very few simultaneous measurements of PAN and PAA. In this study, we present airborne measurements of PAN and PAA using a chemical ionization mass spectrometer (CIMS) in the clean troposphere above the Amazon rainforest in the framework of the CAFE Brazil measurement campaign. The absolute and relative abundances of PAN and PAA are analysed using data obtained during 20 flights performed during December 2022 until the end of January 2023.

PAN is an important reservoir of reactive nitrogen above the over the North and tropical Atlantic Ocean west of Africa. In aged biomass-burning plumes (red data) the PAN/(NO X + PAN) ratio is close to 1 at altitudes where PAN is thermally stable.

Trifluoroacetaldehyde (CF3CHO) is formed in the atmosphere by the oxidation of a number of fluorinated, organic compounds of anthropogenic origin. The reaction of CF3CHO with the OH radical is a potential source of atmospheric trifluoroacetic acid (TFA) which is a highly persistent, water-soluble compound that may accumulate in aquatic ecosystems and for which uncertainty about its sources, fate, and potential ecological impact persists. In light of growing concerns about the impact of TFA, we present the first study of the temperature dependence of the rate coefficient for the title reaction over the atmospherically relevant temperature range of 204 K to 361 K. Rate coefficients were determined using pulsed laser photolysis-pulsed laser induced fluorescence (PLP-PLIF) and direct concentration measurements via Fourier Transform Infrared (FTIR) spectroscopy, as well as relative rate experiments in an atmospheric simulation chamber using ethane (C2H6) as a reference compound. The rate coefficient (k1) obtained with PLP-PLIF at room temperature is (5.8 ± 0.5) × 10-13 cm3 molecule-1 s-1. The temperature dependence is described by the expression k1(T) = (3.8 ± 0.2) × 10-13 × (T/300)2 × exp[(131 ± 16)/T]. The relative-rate experiments showed that the rate coefficient obtained can be significantly biased by reactions of the CF3O radical with CF3CHO and/or C2H6 and also reactions of CF3CHO with HO2. Based on the expression of k1 given above, the lifetime of CF3CHO with respect to reaction with the OH radical varies from 22 days at the surface (T ∼ 300 K) to 30 days in the upper troposphere (T ∼ 220 K).

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.