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.

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 ∼ 35 % over relatively ∼ 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.

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 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.

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.

Measurements in the remote unpolluted atmosphere have tremendous power to reveal processes that are happening on a global scale. In the marine atmosphere where nitrogen oxide (NOx) levels are very low, the photochemical loss rate of tropospheric ozone dominates over production, allowing loss processes to be sensitively explored. We showed that bromine and iodine emitted from open-ocean marine sources initiate important global-scale catalytic ozone-destroying cycles and found that the deposition of ozone and subsequent reactions at the sea surface are a substantial pathway for production of volatile iodine. Production of ozone in the remote atmosphere is predominantly regulated by the abundance of NOx, which also exerts substantial control over the hydroxyl radical (OH), the most important oxidant in the atmosphere. It is now emerging that NOx regeneration pathways, namely the photolysis of particulate nitrate, could provide the dominant source of NOx to the marine atmosphere. This has significant implications for our understanding of the chemistry of the remote troposphere. This presentation discusses advances made in understanding these important, predominantly natural, cycles and their impacts on the atmosphere.

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.

Recent observations of nitrous acid (HONO) in the remote troposphere show much higher concentrations than can be explained through known sources, with important implications for air quality and climate. Laboratory evidence and modelling of field observations suggests that nitrate aerosol photolysis is the likely mechanism providing the additional HONO, offering a rapid route for recycling of NOx from nitric acid (HNO3). Previous studies of the global impact of this chemistry have used either very restricted HONO data or a “top-down” approach to parameterize the HONO source by reconciling simulated and observed NOx concentrations. Here, we use multiple, independent tropospheric HONO observations from different locations to parameterize nitrate photolysis, and evaluate its impacts on global atmospheric chemistry using GEOS-Chem. The simulations improve agreement between modelled and observed HONO concentrations relative to previous studies, decreasing the model bias by 5 %–20 %. The remaining (and large) underestimate of HONO in the model is due predominantly to an underestimate of total nitrate aerosol (−95 %) and is reduced to 20 % when accounting for low model nitrate. Despite the low bias in the model HONO, we find that nitrate aerosol photolysis leads to substantial global increases in NOx, O3 and OH concentrations, likely beyond the observational constraints. The additional source of NOx (∼ 48 Tg N yr−1 globally) is comparable to total NOx emissions from all sources (∼ 55 Tg yr−1). These HONO observations in the remote troposphere, thus imply a large uncertainty in the NOx budget and an incomplete understanding of atmospheric chemistry. Improved techniques to measure HONO at the low concentrations typical of remote areas, coupled with more measurements in these areas and improved process level understanding of nitrate photolysis are needed to provide quantitative assessment of its potentially global-scale atmospheric impacts.

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.

The budget of reactive nitrogen species, which play a central role in atmospheric chemistry (e.g. in photochemical O3 production), is poorly understood in forested regions. In this study, through observations of NO, NO2, NOy, and O3 in the Rambouillet Forest near Paris, France, we have examined nighttime processes controlling NOx in an anthropogenically impacted forest environment. The O3 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 O3 diel profile was strongly influenced by relative humidity and temperature inversion. Only when the O3 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 ∼1.4 ppbv h−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 O3, enabled an effective lifetime of NO2 of ∼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.

New particle formation (NPF) in the tropical upper troposphere is a globally important source of atmospheric aerosols1, 2, 3-4. It is known to occur over the Amazon basin, but the nucleation mechanism and chemical precursors have yet to be identified2. Here we present comprehensive in situ aircraft measurements showing that extremely low-volatile oxidation products of isoprene, particularly certain organonitrates, drive NPF in the Amazonian upper troposphere. The organonitrates originate from OH-initiated oxidation of isoprene from forest emissions in the presence of nitrogen oxides from lightning. Nucleation bursts start about 2 h after sunrise in the outflow of nocturnal deep convection, producing high aerosol concentrations of more than 50,000 particles cm-3. We report measurements of characteristic diurnal cycles of precursor gases and particles. Our observations show that the interplay between biogenic isoprene, deep tropical convection with associated lightning, oxidation photochemistry and the low ambient temperature uniquely promotes NPF. The particles grow over time, undergo long-range transport and descend through subsidence to the lower troposphere, in which they can serve as cloud condensation nuclei (CCN) that influence the Earth's hydrological cycle, radiation budget and climate1,4, 5, 6, 7-8.

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.

Particulate nitrate ( pNO 3 − ) has long been considered a permanent sink for NO x (NO and NO 2 ), removing a gaseous pollutant that is central to air quality and that influences the global self-cleansing capacity of the atmosphere. Evidence is emerging that photolysis of pNO 3 − can recycle HONO and NO x back to the gas phase with potentially important implications for tropospheric ozone and OH budgets; however, there are substantial discrepancies in “renoxification” photolysis rate constants. Using aircraft and ground-based HONO observations in the remote Atlantic troposphere, we show evidence for renoxification occurring on mixed marine aerosols with an efficiency that increases with relative humidity and decreases with the concentration of pNO 3 − , thus largely reconciling the very large discrepancies in renoxification photolysis rate constants found across multiple laboratory and field studies. Active release of HONO from aerosol has important implications for atmospheric oxidants such as OH and O 3 in both polluted and clean environments.

Satellite-based retrievals of tropospheric NO2 columns are widely used to infer NOx (≡ NO + NO2) emissions. These retrievals rely on model information for the vertical distribution of NO2. The free tropospheric background above 2 km is particularly important because the sensitivity of the retrievals increases with altitude. Free tropospheric NOx also has a strong effect on tropospheric OH and ozone concentrations. Here we use observations from three aircraft campaigns (SEAC4RS, DC3, and ATom) and four atmospheric chemistry models (GEOS-Chem, GMI, TM5, and CAMS) to evaluate the model capabilities for simulating NOx in the free troposphere and attribute it to sources. NO2 measurements during the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS) and Deep Convective Clouds and Chemistry (DC3) campaigns over the southeastern U.S. in summer show increasing concentrations in the upper troposphere above 10 km, which are not replicated by the GEOS-Chem, although the model is consistent with the NO measurements. Using concurrent NO, NO2, and ozone observations from a DC3 flight in a thunderstorm outflow, we show that the NO2 measurements in the upper troposphere are biased high, plausibly due to interference from thermally labile NO2 reservoirs such as peroxynitric acid (HNO4) and methyl peroxy nitrate (MPN). We find that NO2 concentrations calculated from the NO measurements and NO–NO2 photochemical steady state (PSS) are more reliable to evaluate the vertical profiles of NO2 in models. GEOS-Chem reproduces the shape of the PSS-inferred NO2 profiles throughout the troposphere for SEAC4RS and DC3 but overestimates NO2 concentrations by about a factor of 2. The model underestimates MPN and alkyl nitrate concentrations, suggesting missing organic NOx chemistry. On the other hand, the standard GEOS-Chem model underestimates NO observations from the Atmospheric Tomography Mission (ATom) campaigns over the Pacific and Atlantic oceans, indicating a missing NOx source over the oceans. We find that we can account for this missing source by including in the model the photolysis of particulate nitrate on sea salt aerosols at rates inferred from laboratory studies and field observations of nitrous acid (HONO) over the Atlantic. The median PSS-inferred tropospheric NO2 column density for the ATom campaign is 1.7 ± 0.44 × 1014 molec. cm−2, and the NO2 column density simulated by the four models is in the range of 1.4–2.4 × 1014 molec. cm−2, implying that the uncertainty from using modeled NO2 tropospheric columns over clean areas in the retrievals for stratosphere–troposphere separation is about 1 × 1014 molec. cm−2. We find from GEOS-Chem that lightning is the main primary NOx source in the free troposphere over the tropics and southern midlatitudes, but aircraft emissions dominate at northern midlatitudes in winter and in summer over the oceans. Particulate nitrate photolysis increases ozone concentrations by up to 5 ppbv (parts per billion by volume) in the free troposphere in the northern extratropics in the model, which would largely correct the low model bias relative to ozonesonde observations. Global tropospheric OH concentrations increase by 19 %. The contribution of the free tropospheric background to the tropospheric NO2 columns observed by satellites over the contiguous U.S. increases from 25 ± 11 % in winter to 65 ± 9 % in summer, according to the GEOS-Chem vertical profiles. This needs to be accounted for when deriving NOx emissions from satellite NO2 column measurements.

&lt;p&gt;Terpenoids are readily released into the environment via biogenic emissions. One of their major nighttime oxidizing agent is the nitrate radical (NO&lt;sub&gt;3&lt;/sub&gt;), which is formed from the reaction between ozone (O&lt;sub&gt;3&lt;/sub&gt;) and nitrogen dioxide (NO&lt;sub&gt;2&lt;/sub&gt;). The NO&lt;sub&gt;3&lt;/sub&gt;-initiated oxidation of such biogenic volatile organic compounds (BVOCs) leads to the formation of organic nitrates that can deposit on particles to form secondary organic aerosols. This reaction path thus can lead to irreversible removal of NO&lt;sub&gt;2&lt;/sub&gt;, usually of anthropogenic origin, from the gas phase. Alternatively, NO&lt;sub&gt;3&lt;/sub&gt; rapidly reacts with nitric oxide (NO) with reformation of NO&lt;sub&gt;2&lt;/sub&gt;. Assigning the contribution of BVOCs and NO to the NO&lt;sub&gt;3&lt;/sub&gt; reactivity therefore provides insight into the fate of NO&lt;sub&gt;x&lt;/sub&gt; (= NO + NO&lt;sub&gt;2&lt;/sub&gt;).&lt;/p&gt; &lt;p&gt;We used a flow-tube coupled to a cavity-ring-down spectrometer (FT-CRDS) to make direct measurements of the NO&lt;sub&gt;3 &lt;/sub&gt;reactivity in air (at a height of ~ 6 m above ground level) during the ACROSS field campaign in the forest of Rambouillet in the vicinity of Paris, France. Measured reactivities ranged from &lt; 0.006 s&lt;sup&gt;-1&lt;/sup&gt; to 2.3 s&lt;sup&gt;-1&lt;/sup&gt;, with high average daytime and nighttime reactivities of 0.13 s&lt;sup&gt;-1&lt;/sup&gt; and 0.25 s&lt;sup&gt;-1&lt;/sup&gt;, respectively. In addition, vertical profiles of NO&lt;sub&gt;3&lt;/sub&gt; reactivity (up to 40 m) were made and are compared to data previously observed in a boreal forest. Reactivities in Rambouillet forest were highest close the ground (0.36 s&lt;sup&gt;-1 &lt;/sup&gt;at 0 m) and drastically decreased with height (0.08 s&lt;sup&gt;-1&lt;/sup&gt; at 24 m).&lt;/p&gt; &lt;p&gt;By comparing direct NO&lt;sub&gt;3&lt;/sub&gt; reactivity measurements with those calculated from trace gas mixing ratios and their corresponding rate coefficients, we identify the contributions of BVOCs and NO to NO&lt;sub&gt;3&lt;/sub&gt; losses.&lt;/p&gt;

Chemical processing of reactive nitrogen species, especially of NOx (= NO + NO2) and nitrous acid (HONO), determines the photochemical ozone production and oxidation capacity in the troposphere. However, sources of HONO and NOx in the remote marine atmosphere are still poorly understood. In this work, the multiphase chemistry mechanism CAPRAM in the model framework SPACCIM was used to study HONO formation at Cape Verde (CVAO) in October 2017, adopted with the input of current parameterizations for various HONO sources. Three simulations were performed that adequately reproduced ambient HONO levels and its diurnal pattern. The model performance for NOx and O-3 improves significantly when considering dust-surface-photocatalytic conversions of reactive nitrogen compounds with high correlation coefficients up to 0.93, 0.56, and 0.89 for NO, NO2, and O-3, respectively. Photocatalytic conversion of the adsorbed HNO3 on dust is modeled to be the predominant contributor for daytime HONO at CVAO, that is, accounting for about 62% of the chemical formation rate at noontime. In contrast, the ocean-surface-mediated conversion of NO2 to HONO and other discussed pathways are less important. The average OH levels at midday (9:00-16:00) modeled for cluster trajectory 1, 2, and 3 are 5.2, 5.1, and 5.2 x 10(6) molecules cm(-3), respectively. Main OH formation is driven by O-3 photolysis with a contribution of 74.6% to the total source rate, while HONO photolysis is negligible (similar to 1.8%). In summary, this study highlights the key role of dust aerosols for HONO formation and NOx cycling at CVAO and possibly in other dust-affected regions, urgently calling for further investigations using field and model studies.

&lt;p&gt;At nighttime, when concentrations of the OH-radical are low, the nitrate radical, NO&lt;sub&gt;3&lt;/sub&gt;, over takes the role of major initiator of the oxidation of many organic trace gases, especially those containing one or more double bonds. In contrast to daytime, where the lifetime of NO&lt;sub&gt;3&lt;/sub&gt; is very short due to its photolysis and reaction with NO, NO&lt;sub&gt;3&lt;/sub&gt; can reach mixing ratios of several tens of ppt at night. NO&lt;sub&gt;3&lt;/sub&gt; can also react with NO&lt;sub&gt;2&lt;/sub&gt; to form N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt;. As N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt; is thermally stable, the three trace-gases usually exist in equilibrium:&lt;/p&gt; &lt;p&gt;NO&lt;sub&gt;3&lt;/sub&gt; + NO&lt;sub&gt;2&lt;/sub&gt; + M &amp;#160; &amp;#160; &amp;#160; &amp;#160; &amp;#160; &amp;#160; &amp;#160; &amp;#160;&amp;#8652; N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt; + M&lt;/p&gt; &lt;p&gt;Measurements of NO&lt;sub&gt;3&lt;/sub&gt; and N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt; are central to our understanding of the fate of NO&lt;sub&gt;x&lt;/sub&gt; at night. Loss of NO&lt;sub&gt;3&lt;/sub&gt; to gas-phase reactions (forming e.g. organic nitrates) has a different impact on NO&lt;sub&gt;x&lt;/sub&gt; than formation of N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt; which may hydrolyse on aerosol to form particulate nitrate.&lt;/p&gt; &lt;p&gt;During the ACROSS campaign in Rambouillet Forest (France), a recently built two-channel cavity-ring-down spectrometer was deployed for the first time to record mixing ratios of NO&lt;sub&gt;3&lt;/sub&gt; and N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt; at night over a period of several weeks. NO&lt;sub&gt;3&lt;/sub&gt; was detected directly at 662nm in one channel while N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt; was first converted to NO&lt;sub&gt;3&lt;/sub&gt; in a thermal dissociation inlet before being detected in the same way.&lt;/p&gt; &lt;p&gt;In this work, we describe the new instrument in detail and compare obtained data with those measured by an established cavity-ring-down instrument. We show that, at a sampling height of about 6m, NO&lt;sub&gt;3&lt;/sub&gt; and N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt; mixing ratios were low and frequently below the detection limit of both instruments; the likely reasons for this are discussed.&lt;/p&gt;

Enhanced photolysis of particulate nitrate (pNO(3)) to form photolabile species, such as gas-phase nitrous acid (HONO), has been proposed as a potential mechanism to recycle nitrogen oxides (NOx) in the remote boundary layer ("renoxification"). This article presents a series of laboratory experiments aimed at investigating the parameters that control the photolysis of pNO(3) and the efficiency of HONO production. Filters on which artificial or ambient particles had been sampled were exposed to the light of a solar simulator, and the formation of HONO was monitored under controlled laboratory conditions. The results indicate that the photolysis of pNO(3) is enhanced, compared to the photolysis of gas-phase HNO3, at low pNO(3) levels, with the enhancement factor reducing at higher pNO(3) levels. The presence of cations (Na+) and halides (Cl-) and photosensitive organic compounds (imidazole) also enhance pNO(3) photolysis, but other organic compounds such as oxalate and succinic acid have the opposite effect. The precise role of humidity in pNO(3) photolysis remains unclear. While the efficiency of photolysis is enhanced in deliquescent particles compared to dry particles, some of the experimental results suggest that this may not be the case for supersaturated particles. These experiments suggest that both the composition and the humidity of particles control the enhancement of particulate nitrate photolysis, potentially explaining the variability in results among previous laboratory and field studies. HONO observations in the remote marine boundary layer can be explained by a simple box-model that includes the photolysis of pNO(3), in line with the results presented here, although more experimental work is needed in order to derive a comprehensive parametrization of this process.