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The article discusses the measurement of nitrogen oxide concentrations in the atmosphere simulation chamber by using various instruments. It highlights the analytical methods used including pulsed cavity ring-down spectroscopy (CRDS), laser-induced fluorescence (LIF) and broadband cavity ring-down spectroscopy (BBCRDS). It also notes that all of the absorption spectrometers were optimized to discover the main target species during the NO3Comp Campaign.

In the present study, the heterogeneous conversion of nitrogen oxide (NO) and nitrogen dioxide (NO2) was studied at atmospheric humidity levels on flame soot surfaces treated with gaseous nitric acid (HNO3). In addition, the heterogeneous reaction of HNO3 on soot was investigated at atmospheric humidity. For the treatment of soot by pure HNO3 only reversible uptake with a surface coverage of ∼1-2×1014 HNO3 cm-2 was observed for HNO3 mixing ratios in the range 250-800 ppbv. Only for higher HNO3 mixing ratios of >800 ppbv the formation of NO and NO2 was observed. The results were not affected by the addition of NO. In none of the experiments with HNO3 the formation of nitrous acid (HONO) was observed. For HNO3 mixing ratios <600 ppbv the upper limit yields for HONO, NO2 and NO were found to be <0.2%, <0.5% and <1%, respectively. Compared to untreated soot, the product formation of the reaction of NO2 with soot was not significantly affected when the soot surface was treated with gaseous HNO3 prior to the experiment. Only for high surface coverage of HNO3 the formation of HONO was suppressed in the initial phase of the reaction, probably caused by the blocking of active sites by adsorbed HNO3. Under the assumption that the experimental findings for the used model flame soot can be extrapolated to atmospheric soot particles, the results show that the reactions of HNO3 and HNO3+NO on soot surfaces are unimportant for a "renoxification" of the atmosphere and do not represent an atmospheric HONO source. In addition, the integrated HONO yield of ca. 1014 cm-2 in the reaction of NO2 with soot is not significantly influenced by simulated atmospheric processing of the soot surface by HNO3, and is still too small to explain HONO formation in the atmosphere. [ABSTRACT FROM AUTHOR]

Abstract: A simple O3-LOPAP (Long Path Absorption Photometer) instrument for the selective detection of O3 in the atmosphere is presented, which is mainly intended to be used as an extension of a recently developed NO2-LOPAP. O3 is sampled in a stripping coil by a selective chemical reaction with the highly absorbing Indigo dye. The reduction of the optical absorption of the dye is detected in a liquid core waveguide. The instrument has a detection limit of 0.4 ppbv, an accuracy of 10%, a precision of 2% for 6 min time resolution and allows the absolute quantification of O3 according to Lambert–Beer''s law. Interferences towards nitrogen oxides (NO and NO2), N2O5, H2O2 and several VOCs were quantified in the laboratory and found to be negligible for atmospheric conditions. The new instrument was successfully validated against a commercial UV-absorption instrument during an urban field campaign and against the FTIR technique in a smog chamber under complex photosmog conditions. For the UV-absorption instrument significant positive interferences towards aromatic species were observed in the smog chamber. [Copyright &y& Elsevier]

Nitrogen oxides (NO x  = NO + NO 2 ) are important atmospheric pollutants that are directly harmful to human health. Recently in urban and industrial areas, synthetic materials have been developed and deployed to photocatalytically oxidize NO x to nitrate (NO 3 - ) in order to improve air quality. We show that the natural presence of small amounts (≤5%) of titanium oxides, such as anatase and rutile, can also drive NO x oxidation to nitrate in soils under UV-visible irradiation. The NO uptake coefficients ranged between 0.1 × 10 -6 for sandy soils to 6.4 × 10 -5 in the case of tropical clay soils; the latter comparable in efficiency to current industrial man-made catalysts. This photocatalytic N-fixation mechanism offers a new strategy for NO x mitigation from the atmosphere by transforming it into nitrate, and simultaneously provides an energy efficient source of essential fertilizer to agriculture., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Atmospheric nitrophenols are pollutants of concern due to their toxicity and light-absorption characteristics and their low reactivity resulting in relatively long residence times in the environment. We investigate multiphase nitrophenol formation from guaiacol in a simulated atmospheric aerosol and support observations with the corresponding chemical mechanisms. The maximal secondary organic aerosol (SOA) yield (42%) is obtained under illumination at 80% relative humidity. Among the identified nitrophenols, 4-nitrocatechol (3.6% yield) is the prevailing species in the particulate phase. The results point to the role of water in catechol and further 4-nitrocatechol formation from guaiacol. In addition, a new pathway of dark nitrophenol formation is suggested, which prevailed in dry air and roughly yielded 1% nitroguaiacols. Furthermore, the proposed mechanism possibly leads to oligomer formation via a phenoxy radical formation by oxidation with HONO., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

The photocatalytic decomposition of nitrogen oxides (NOx) has attracted significant interest as a potential measure of reducing NOx levels in the urban atmosphere. Since photocatalytic activity is highly variable depending on atmospheric conditions, the uptake of NO, NO2 and HONO was studied on a commercial photocatalytic dispersion paint in a flow photoreactor as a function of the relative humidity and temperature. Since the relative humidity is a function of the surface's temperature, here both dependencies were carefully decoupled for the first time. In addition, for the first time the temperature dependence of the whole NOx reaction system including the important intermediate HONO was investigated. While for NO and NO2 strong negative humidity dependencies were observed, the photocatalytic uptake of HONO increased with humidity. For constant relative humidity no temperature dependence of the photocatalytic oxidation of NO was observed, whereas the photocatalytic NO2 uptake decreased with increasing temperature, which is explained by a temperature dependent adsorption equilibrium of the surface active NO2. HONO uptake showed a positive temperature dependence confirming the proposed photocatalysis of nitrite in a layer of adsorbed water on the surface of the photocatalyst. The missing/negative temperature dependencies of the photocatalysis of NO/NO2 are overcompensated by their strong negative relative humidity dependencies, leading to increasing uptake for both pollutants when photocatalytic surfaces are heated by solar irradiation in the atmosphere.

In the present study, emission indices for NO, NO 2 , HONO, HCHO, CO, particle mass, and particle numbers including particle size distributions for three different offering candles were determined. The candles investigated showed similar emission characteristics with emission indices (g/kg) in good agreement with former candle emission studies. An average HONO/NO x emission ratio of 6.6 ± 1.1% was obtained, which is much higher compared to most other combustion sources, indicating that candles may be a significant indoor source of this important trace gas. The particle size distributions indicate that the majority of the emitted particles are in the size range 7 - 15 nm. Three modes were observed during burning the candles with very different emission profiles: a "normal burning" mode characterized by low particle number emission rates and small particles; an initial "sooting" behavior after ignition, and a final "smoldering" phase upon candle extinction with higher particle number emission rates and larger particles. The particle emission upon extinction is dependent on the extinction method. The NO x emission indices were applied in a simple box model to calculate typical indoor NO x concentration levels from candle emissions, which were in excellent agreement with direct measurements in a typical indoor environment., (© 2020 The Authors. Indoor Air published by John Wiley & Sons Ltd.)

Nitrous acid (HONO) is a major precursor of tropospheric hydroxyl radical (OH) that accelerates the formation of secondary pollutants. The HONO sources, however, are not well understood, especially in polluted areas. Based on a comprehensive winter field campaign conducted at a rural site of the North China Plain, a box model (MCM v3.3.1) was used to simulate the daytime HONO budget and nitrate formation. We found that HONO photolysis acted as the dominant source for primary OH with a contribution of more than 92%. The observed daytime HONO could be well explained by the known sources in the model. The heterogeneous conversion of NO 2 on ground surfaces and the homogeneous reaction of NO with OH were the dominant HONO sources with contributions of more than 36 and 34% to daytime HONO, respectively. The contribution from the photolysis of particle nitrate and the reactions of NO 2 on aerosol surfaces was found to be negligible in clean periods (2%) and slightly higher during polluted periods (8%). The relatively high OH levels due to fast HONO photolysis at the rural site remarkably accelerated gas-phase reactions, resulting in the fast formation of nitrate as well as other secondary pollutants in the daytime.

Objectives: Percutaneous tracheostomy (PT) is common on ICUs. An increase of intracranial pressure (ICP) can be observed in patients with acute cerebral diseases. Factors determining ICP increase remain unclear., Patients and Methods: Data for all PTs were collected prospectively. ICP, cerebral perfusion pressure (CPP), mean arterial pressure (MAP), peripheral oxygen saturation (SpO 2 ), and heart rate (HR) were monitored continuously every minute. Primary outcome parameter was an increase of ICP during PT (ICP > 20 mmHg). Influencing factors were evaluated by the means of logistic regression analysis: Body mass index (BMI), age, gender, physician performing the procedure (neurologist vs. neurosurgeon), duration of the procedure, underlying disease, duration of mechanical ventilation, and baseline ICP value before the procedure., Results: A total of 175 PTs were performed during the observation period between 2010 and 2013. Of these, 54 received ICP monitoring and were included into this study. Median initial ICP value was 10.4 mmHg and rose significantly to a median value of 18.4 mmHg (p < 0.05). In 21 patients (38,9%) an increase of median ICP above 20 mmHg was seen during at least one interval. Comparing patients with and without pathological ICP increase a significant difference between the two groups was only observed for patients with an increased baseline ICP above 15 mmHg. All other factors had no significant influence on the development of a pathological ICP peaks during PT., Conclusion: Percutaneous tracheostomies in patients with cerebral injury leads to a significant increase of ICP during the procedure. Patients with a baseline ICP > 15 mmHg are at risk to develop harmful ICP crises., (Copyright © 2020 Elsevier B.V. All rights reserved.)

In their comment to our recent paper about low HONO and NO2 formation by photolysis of adsorbed HNO3 Sullivan et al. confirmed their former results of HNO3 adsorption on silica under dry conditions using a quartz crystal microbalance. The authors concluded that the differences between their results and our conclusions are caused by the different experimental conditions, i.e. adsorption under very dry conditions compared to our experiments at 50% r.h. While we agree that adsorption of the highly water soluble HNO3 will strongly depend on humidity, there is still the conflict in the photolysis frequency of adsorbed HNO3 under atmospheric conditions to which the authors referred in their previous publications (see their atmospheric implication sections) and to which also our paper refers. If their results on both the adsorption cross sections of HNO3 (two to three orders of magnitude larger compared to the gas phase) and the quantum yield for NO2 formation (close to unity) are applicable under conditions prevailing in the atmosphere, then the photolytic lifetime of HNO3 on surfaces would be only ∼5 min for atmospheric solar flux (0° SZA), which is highly unlikely.

Background: Resident education has to adapt to a changing health care environment. Although aspects such as working hours and attrition rates have been studied in detail, data about the residents' perspective, especially in European countries, are underrepresented in the scientific literature. The aim of this study was to assess and report aspects of neurosurgical education in German-speaking countries and to identify risk factors for quitting or changing the neurosurgical residency program., Methods: We conducted a nonanonymous online survey among neurosurgical residents in Germany, Austria, and Switzerland. Log-binomial regression models were calculated to further assess risk factors., Results: Of 201 residents who responded to the survey, 37.3% (n = 75) dropped out of neurosurgical training programs, including 20 residents (10%) who ultimately quit neurosurgery and changed to another specialty. Only female gender (relative risk, 2.97; 95% confidence interval, 1.3-6.78) and starting residency in a city one studied or grew up in (2.38; 1.01-5.62) were significant risk factors. Residents who had close supervision at work (0.39; 0.17-0.89), who observed the residency program for >3 days before applying (0.54; 0.31-0.95), who had well-defined guidelines within the program (0.57; 0.35-0.92), and who were working in a university hospital (0.41; 0.26-0.64) were significantly less likely to quit or change their program., Conclusions: The high attrition rate, especially among female residents, in Germany, Austria, and Switzerland should encourage program directors to specifically address the issues reported by this survey during interviews and to further improve their residency program accordingly., (Copyright © 2018 Elsevier Inc. All rights reserved.)

Background  Studies investigating multimodal cerebral monitoring including partial brain tissue oxygen monitoring (ptiO2) in neuro-intensive care patients during physiotherapy are completely lacking in the literature. Materials and Methods  We performed a post hoc analysis of prospectively collected data of patients on multimodal cerebral monitoring by intracranial pressure (ICP) and cerebral perfusion pressure (CPP) measurement as well as ptiO2. Patients with severe brain diseases were treated with passive range of motion (PROM). We recorded ICP, CPP, and ptiO2 continuously every minute at baseline (15 minutes), during treatment (26 minutes), and 15 minutes after treatment with PROM. Results  Overall, 25 treatment units with PROM in 10 patients with combined ICP/CPP and ptiO2 monitoring were evaluated. Median ICP, CPP, and ptiO2 at baseline were 12 ± 6.1 mm Hg, 86 ± 17.1 mm Hg, and 27 ± 14.3 mm Hg, respectively. Values for ICP, CPP, and ptiO2 did not change significantly when comparing mean values before, during, and after therapy. Conclusions  Based on ptiO2 measurements, our data provide new information about the feasibility and safety of physiotherapy in patients with severe brain diseases., (Georg Thieme Verlag KG Stuttgart · New York.)

Air pollution is the environmental factor with the greatest impact on human health in Europe. Understanding the key processes driving air quality across the relevant spatial scales, especially during pollution exceedances and episodes, is essential to provide effective predictions for both policymakers and the public. It is particularly important for policy regulators to understand the drivers of local air quality that can be regulated by national policies versus the contribution from regional pollution transported from mainland Europe or elsewhere. One of the main objectives of the Coupled Urban and Regional processes: Effects on AIR quality (CUREAIR) project is to determine local and regional contributions to ozone events. A detailed zero-dimensional (0-D) box model run with the Master Chemical Mechanism (MCMv3.2) is used as the benchmark model against which the less explicit chemistry mechanisms of the Generic Reaction Set (GRS) and the Common Representative Intermediates (CRIv2-R5) schemes are evaluated. GRS and CRI are used by the Atmospheric Dispersion Modelling System (ADMS-Urban) and the regional chemistry transport model EMEP4UK, respectively. The MCM model uses a near-explicit chemical scheme for the oxidation of volatile organic compounds (VOCs) and is constrained to observations of VOCs, NOx, CO, HONO (nitrous acid), photolysis frequencies and meteorological parameters measured during the ClearfLo (Clean Air for London) campaign. The sensitivity of the less explicit chemistry schemes to different model inputs has been investigated: Constraining GRS to the total VOC observed during ClearfLo as opposed to VOC derived from ADMS-Urban dispersion calculations, including emissions and background concentrations, led to a significant increase (674% during winter) in modelled ozone. The inclusion of HONO chemistry in this mechanism, particularly during wintertime when other radical sources are limited, led to substantial increases in the ozone levels predicted (223%). When the GRS and CRIv2-R5 schemes are run with the equivalent model constraints to the MCM, they are able to reproduce the level of ozone predicted by the near-explicit MCM to within 40% and 20% respectively for the majority of the time. An exception to this trend was observed during pollution episodes experienced in the summer, when anticyclonic conditions favoured increased temperatures and elevated O3. The in situ O3 predicted by the MCM was heavily influenced by biogenic VOCs during these conditions and the low GRS [O3] : MCM [O3] ratio (and low CRIv2-R5 [O3] : MCM [O3] ratio) demonstrates that these less explicit schemes under-represent the full O3 creation potential of these VOCs. To fully assess the influence of the in situ O3 generated from local emissions versus O3 generated upwind of London and advected in, the time since emission (and, hence, how far the real atmosphere is from steady state) must be determined. From estimates of the mean transport time determined from the NOx : NOy ratio observed at North Kensington during the summer and comparison of the O3 predicted by the MCM model after this time, ∼60% of the median observed [O3] could be generated from local emissions. During the warmer conditions experienced during the easterly flows, however, the observed [O3] may be even more heavily influenced by London's emissions.

During the last few decades, nitrous acid (HONO) has attracted significant attention as a major source of the OH radical, the detergent of the atmosphere. However, the different daytime sources identified in the laboratory are still the subject of controversial discussion. In the present study, one of these postulated HONO sources, the heterogeneous photolysis of nitric acid (HNO3), was studied on quartz glass surfaces in a photo flow-reactor under atmospherically relevant conditions. In contrast to other investigations, a very low HNO3 photolysis frequency for HONO formation of J(HNO3→ HONO) = 2.4 × 10(-7) s(-1) (0° SZA, 50% r.h.) was determined. If these results can be translated to atmospheric surfaces, HNO3 photolysis cannot explain the significant HONO levels in the daytime atmosphere. In addition, it is demonstrated that even the small measured yields of HONO did not result from the direct photolysis of HNO3 but rather from the consecutive heterogeneous conversion of the primary photolysis product NO2 on the humid surfaces. The secondary NO2 conversion was not photo-enhanced on pure quartz glass surfaces in good agreement with former studies. A photolysis frequency for the primary reaction product NO2 of J(HNO3→ NO2) = 1.1 × 10(-6) s(-1) has been calculated (0° SZA, 50% r.h.), which indicates that renoxification by photolysis of adsorbed HNO3 on non-reactive surfaces is also a minor process in the atmosphere.

Background Context: The hemostatic properties of hydrogen peroxide (H2O2) are often used in neurosurgical practice., Case Report: We present the case of an 81-year-old woman who underwent lumbar spinal surgery (microsurgical decompression) in an external hospital. H2O2 was used during the procedure. The patient was transferred to our hospital. She remained unconscious postoperatively, with progressive loss of brainstem reflexes. Computed tomography showed intra- and extradurally trapped air ascending from the operated lumbar segment up to frontal lobe. Magnetic resonance imaging demonstrated severe brainstem lesions on T2- and diffusion-weighted series. The patient died 10 days after surgery. Autopsy was not performed., Conclusions: Our case demonstrates a fatal complication with ischemic brainstem lesions and pneumocephalus after the use of hydrogen peroxide. Therefore, H2O2 should only be used in cases without any signs of dural injury., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Purpose: The objective of our trial was to obtain more comprehensive data on the risks and benefits of kinetic therapy in intensive care patients with intracerebral pathology., Methods: Standardized data of prone positioning in our NeuroIntensive Care Unit were collected from 2007 onward. A post hoc analysis of all available data was undertaken, with special consideration given to values of intracranial pressure (ICP), cerebral perfusion pressure (CPP) and oxygenation in correlation to prone (PP), or supine positioning (SP) of patients. Cases were considered eligible if kinetic therapy and ICP were documented. Prone positioning was performed in a 135° position for 8 h per treatment unit., Results: A total of 115 patients treated with prone positioning from 2007 to 2013 were identified in our medical records. Of these, 29 patients received ICP monitoring. Overall, 119 treatment units of prone positioning with a mean duration of 2.5 days per patient were performed. The mean baseline ICP in SP was 9.5 ± 5.9 mmHg and was increased significantly during PP (p < 0.0001). There was no significant difference between CPP in SP (82 ± 14.5 mmHg) compared to PP (p > 0.05). ICP values >20 mmHg occurred more often during PP than SP (p < 0.0001) and were associated with significantly more episodes of decreased CPP <70 mmHg (p < 0.0022). The mean paO(2)/FiO(2) ratio (P/F ratio) was increased significantly in prone positioning of patients (p < 0.0001)., Conclusions: The analyzed data allow a more precise understanding of changes in ICP and oxygenation during prone positioning in patients with acute brain injury and almost normal baseline ICP. Our study shows a moderate, yet significant elevation of ICP during prone positioning. However, the achieved increase of oxygenation by far exceeded the changes in ICP. It is evident that continuous monitoring of cerebral pressure is required in this patient group.

Photochemical reactions in snow can have an important impact on the composition of the atmosphere over snow-covered areas as well as on the composition of the snow itself. One of the major photochemical processes is the photolysis of nitrate leading to the formation of volatile nitrogen compounds. We report nitrite concentrations determined together with nitrate and hydrogen peroxide in surface snow collected at the coastal site of Barrow, Alaska. The results demonstrate that nitrite likely plays a significant role as a precursor for reactive hydroxyl radicals as well as volatile nitrogen oxides in the snow. Pollution events leading to high concentrations of nitrous acid in the atmosphere contributed to an observed increase in nitrite in the surface snow layer during nighttime. Observed daytime nitrite concentrations are much higher than values predicted from steady-state concentrations based on photolysis of nitrate and nitrite indicating that we do not fully understand the production of nitrite and nitrous acid in snow. The discrepancy between observed and expected nitrite concentrations is probably due to a combination of factors, including an incomplete understanding of the reactive environment and chemical processes in snow, and a lack of consideration of the vertical structure of snow.

Background: Physiotherapy plays an important role in the therapy of patients with acute cerebral diseases. Studies concerning the effects of physiotherapy on intracerebral pressure (ICP) and cerebral perfusion pressure (CPP) are, however, rare., Methods: An observational study was performed on critically ill patients who were receiving ICP measurements and who were treated with passive range of motion (PROM) on our neuro-intensive care unit. ICP, CPP, mean arterial pressure (MAP) and heart rate were recorded continuously every minute, beginning 15 min before, during (26 min) and 15 min after treatment with PROM. Patients with mean ICP <15 mmHg (Group 1) and patients with mean ICP ≥15 mmHg (Group 2) before physiotherapy were analyzed separately., Results: Overall there were 84 patients (f:m = 1:1) with 298 treatments units, 224 in Group 1 and 74 in Group 2, respectively. Mean ICP before treatment was 11.5 ± 5.1 mmHg, with a significant decrease of 1 mmHg during therapy (p = 2.0e-10). This was also true for Group 1 (baseline ICP 9.4 ± 3.7 mmHg, decrease of 0.7 mmHg, p = 3.8e-6) and Group 2 (baseline ICP 18.1 ± 2.7 mmHg, decrease of 2 mmHg, p = 3.7e-6). However, a persistent ICP reduction after therapy was seen only in Group 2. There were no significant differences between mean CPP and MAP comparing ICP before and after PROM in all groups. No adverse side effects of PROM were observed., Conclusions: Physiotherapy with PROM can be used safely in patients with acute neurological diseases, even if ICP is elevated before therapy.

Background: Bedside percutaneous tracheostomy (PT) is very commonly used for patients who require prolonged mechanical ventilation. The effect of tracheostomy on intracranial pressure (ICP) is currently a subject of controversy. The aim of our study is to clarify the relation between PT and its effect on ICP and cerebral perfusion pressure., Methods: 38 patients on our intensive care unit were included prospectively in an observational study. We examined mean values of HF, SpO(2), ICP, CPP, and MAP for changes over five different phases of the procedure using paired Mann-Whitney U tests. A p value of <0.05 was considered significant. p values were Bonferroni corrected for multiple testing., Results: PT was performed on 38 patients (f = 19, m = 19; mean = 56 years). Median ICP before intervention was 9 mmHg. During positioning of the patient, ICP had risen to 14, during bronchoscopy to 16, and during tracheostomy to 18 mmHg, all being significantly higher than baseline level. Monitoring of MAP showed a significant increase to 101 mmHg only during tracheostomy. SpO(2) and HF did not show any significant changes. Mean duration of positioning, bronchoscopy and tracheostomy was 19, 10, and 17 min. 8 patients received osmotherapy due to a rise of ICP of more than 30 mmHg., Conclusion: PT only leads to a significant rise of ICP during the procedure. Nevertheless, therapy of ICP is necessary in some patients. From our point of view, therefore, tracheostomy should only be performed under continuous monitoring of ICP and CPP in patients with severe cerebral dysfunctions and critically elevated ICP.

Herpes encephalitis (HSE) is an acute illness. Imaging is an important part of the diagnostic work-up of HSE. In early stages one can see a manifestation in the insular cortex as well as in the fornix. These MRI findings are rather typical for HSE. We present three cases treated in our hospital over the past ten years. In these cases the clinical symptoms and signs as well as the morphological findings were compatible with the HSE diagnostic criteria. Later on, a glioblastoma multiforme was found in all three cases., (© Georg Thieme Verlag KG Stuttgart · New York.)

Nitrous acid (HONO) is an important precursor of the hydroxyl radical (OH), the self-cleaning agent of the atmosphere and a key species in the formation of harmful photooxidants during summer smog. Recent field measurements using very sensitive HONO instruments have shown that daytime HONO concentrations are much higher than has been assumed previously and that the contribution of HONO to the radical formation was underestimated in the past. A strong photochemical HONO source has been proposed, which contributes to the primary OH radical production up to 56 %. These exciting results initiated new laboratory studies, in which new sources of HONO have been identified. It is demonstrated that HONO is photochemically formed 1) on surfaces treated with nitric acid, 2) by reduction of NO(2) on photosensitized organic surfaces like humic acids and c) in the gas phase photolysis of ortho-substituted nitroaromatics. Although significant uncertainties still exist on the exact mechanisms, these additional sources might explain daytime observations in the atmosphere and demonstrate that HONO should be generally measured in field campaigns, besides other radical sources.

Nitrous acid is a significant photochemical precursor of the hydroxyl radical, the key oxidant in the degradation of most air pollutants in the troposphere. The sources of nitrous acid in the troposphere, however, are still poorly understood. Recent atmospheric measurements revealed a strongly enhanced formation of nitrous acid during daytime via unknown mechanisms. Here we expose humic acid films to nitrogen dioxide in an irradiated tubular gas flow reactor and find that reduction of nitrogen dioxide on light-activated humic acids is an important source of gaseous nitrous acid. Our findings indicate that soil and other surfaces containing humic acid exhibit an organic surface photochemistry that produces reductive surface species, which react selectively with nitrogen dioxide. The observed rate of nitrous acid formation could explain the recently observed high daytime concentrations of nitrous acid in the boundary layer, the photolysis of which accounts for up to 60 per cent of the integrated hydroxyl radical source strengths. We suggest that this photo-induced nitrous acid production on humic acid could have a potentially significant impact on the chemistry of the lowermost troposphere.