The source of nitrous acid (HONO) and its importance in island or marine environments are poorly understood. Herein, based on comprehensive field measurements at a hilltop on Corsica Island, we find an inverse diel variation of HONO with higher concentrations during daytime. Night-time HONO budget analysis indicates significant HONO formation during air mass transport along the hillside. In the daytime, although photosensitized NO2 uptake on the ground and NO + OH make considerable contributions (26% and 5%, respectively), a large part of HONO formation (67%, 320 pptv h(-1)) still cannot be explained with state-of-the-art parametrization. Nevertheless, photosensitized heterogeneous NO2 reactions are likely to account for the missing source, due to underestimation of the source by typical parametrizations at low NO2 levels. Furthermore, we demonstrate a significant role of HONO formation as a OH primary source at this island site, with a OH production rate exceeding one-fourth of that of O-3 photolysis. Our findings underscore a potential role of heterogeneous surface reactions in the oxidizing capacity of the island environments.

<p>In the polar regions, the usual OH radical formation pathway (ozone photolysis and reaction of O(<sup>1</sup>D) with H<sub>2</sub>O) is limited by the low water vapour concentration. However, gases emitted from the snowpack can be pre-cursors of HO<em><sub>x</sub></em> radicals and ozone, thereby controlling the oxidising capacity of the lower atmosphere above remote snow-covered regions.</p> <p>Snowpack photolysis of nitrate and the resulting emissions of the reactive nitrogen species NO<em><sub>x</sub></em> and HONO can lead to OH production through rapid cycling of RO<sub>2</sub> → HO<sub>2</sub> → OH and photolysis of HONO. Research into reactive nitrogen species in polar environments has focused on NO<em><sub>x</sub></em>, with far fewer investigations into HONO. Previous studies of HONO in the polar boundary layer and snowpack interstitial air suggest a photolytic snowpack source but the exact mechanism for HONO production is poorly understood; photochemical models of HONO sources and sinks often cannot be reconciled with the measured HONO concentrations.</p> <p>A LOng Path Absorption Photometer (LOPAP) was used to investigate the net HONO flux density above snow in the Clean Air Sector at Halley VI Research Station in coastal Antarctica during Austral summer 2021/22. We present amount fraction measurements of HONO in ambient air, as well as measurements of the HONO flux density between the snow and atmosphere by the flux-gradient method. The potential snowpack reactions driving this HONO release are discussed, as well as the implications of these measurements for the HO<em><sub>x</sub></em> budget. These findings help further our understanding of the atmospheric budget of reactive nitrogen and highlight the significant effects snow surfaces can have on the atmospheric chemistry in the boundary layer above.</p>

AbstractIn this chapter, we focus on aspects of analysis of typical simulation chamber experiments and recommend best practices in term of data analysis of simulation chamber results relevant for both gas phase and particulate phase atmospheric chemistry. The first two sections look at common gas-phase measurements of relative rates and product yields. The simple yield expressions are extended to account for product removal. In the next two sections, we examine aspects of particulate phase chemistry looking firstly at secondary organic aerosol (SOA) yields including correction for wall losses, and secondly at new particle formation using a variety of methods. Simulations of VOC oxidation processes are important components of chamber work and one wants to present methods that lead to fundamental chemistry and not to specific aspects of the chamber that the experiment was carried out in. We investigate how one can analyse the results of a simulation experiment on a well-characterized chemical system (ethene oxidation) to determine the chamber-specific corrections. Finally, we look at methods of analysing photocatalysis experiments, some with a particular focus on NOx reduction by TiO2-doped surfaces. In such systems, overall reactivity is controlled by both chemical processes and transport. Chambers can provide useful practical information, but care needs to be taken in extrapolating results to other conditions. The wider impact of surfaces on photosmog formation is also considered.

Nitrogen oxides (NOx = NO + NO2) 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 NOx to nitrate (NO3−) 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 NOx 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 NOx mitigation from the atmosphere by transforming it into nitrate, and simultaneously provides an energy efficient source of essential fertilizer to agriculture.

Trace gases of nitrogen (N), such as NOx (nitric oxide, NO + nitrogen dioxide, NO2) have a negative impact on human health and the environment. Although NOx are naturally produced in volcanic eruptions, forest fires and biotic nitrification and denitrification in soils, human activity is a major source of these contaminants via e.g. the combustion of fossil fuels. Additionally, N fertilization in agricultural soils is also an important source of NOx emissions. These emissions involve a loss of soil N to the atmosphere and have a negative impact in air quality. The abiotic part of the N cycle in terrestrial ecosystems has not received as much attention as the biotic part and certain abiotic reactions could play a key role in regulating NOx emissions. Photocatalysis is an example as this is used to abate NOx gases in urban and industrial areas. This reaction requires the presence of a catalyst (e.g. titanium oxide), oxygen, water, and energy from the sun (UV-visible light) to transform NO from the atmosphere into innocuous inorganic N forms (mainly nitrate, NO3-). There is a continuous investment in the production of catalysts by the industry. However, a variety of soil minerals such as anatase or rutile (titanium oxides), hematite and goethite (iron oxides), are found in soils and they could act as catalysts; however, the occurrence of photocatalysis in soils has not been evaluated so far. In this study, we assess (i) the potential of a selection of soils with different mineralogy and a wide variety of soil properties to fix or emit NOx through photocatalysis, and (ii) the possible alterations in the fixation or emission of other N gases from the soil, i.e., nitrous oxide (N2O) and ammonia (NH3), when photocatalysis is induced. Around thirty agricultural soils were selected to meet the first objective and irradiated for 1 hour with UV-visible light under a constant flux of air and NO (100 ppm). Similar experiments were carried out with a selection of soils, whose potential to fix NO was different and tested in the previous experiment, to satisfy the second objective. However, only air (without NO) was pumped within the soil chamber in this case and the soils were previously fertilized with different N fertilisers (urea or KNO3-) and rates (0 to 250 mg N kg-1 soil). Our experiments show that weathered soils (with a high content in titanium and iron oxides) were able to fix more atmospheric NO through photocatalysis (objective i), and that NO and NH3 fixation and emissions after N fertilization depended not only on the N fertilizer and rate but also on soil properties, mainly soil pH and N content (objective ii). Soil mineralogy and properties play a key role in soil photocatalysis, and this abiotic reaction should be considered in order to design more sustainable strategies for agriculture.

Measurements of atmospheric nitrous acid (HONO) amount fraction and flux density above snow were carried out using a long-path absorption photometer at Halley station in coastal Antarctica between 22 January and 3 February 2022. The mean ±1σ HONO amount fraction was (2.1 ± 1.5) pmol mol−1 and showed a diurnal cycle (range of 1.0–3.2 pmol mol−1) with a maximum at solar noon. These HONO amount fractions are generally lower than have been observed at other Antarctic locations. The flux density of HONO from the snow, measured between 31 January and 1 February 2022, was between 0.5 and 3.4×1012 m-2s-1 and showed a decrease during the night. The measured flux density is close to the calculated HONO production rate from photolysis of nitrate present in the snow. A simple box model of HONO sources and sinks showed that the flux of HONO from the snow makes a >10 times larger contribution to the HONO budget than its formation through the reaction of OH and NO. Ratios of these HONO amount fractions to NOx measurements made in summer 2005 are low (0.15–0.35), which we take as an indication of our measurements being comparatively free from interferences. Further calculations suggest that HONO photolysis could produce up to 12 pmolmol-1h-1 of OH, approximately half that produced by ozone photolysis, which highlights the importance of HONO snow emissions as an OH source in the atmospheric boundary layer above Antarctic snowpacks.

Air pollution is still among the biggest environmental health threats for humans in Europe. Traffic, industry, and agriculture are the main responsible sources that are emitting air pollutants, that is, nitrogen oxides (NO×), sulfur dioxide (SO2), carbon monoxide (CO), and particulate matter (PM). Moreover, indoor volatile organic compounds (VOCs), such as aromatics, aldehydes, and alcohols, can be emitted by building materials, consumer products, and other sources. The reduction of these pollutants cannot only be mitigated by active reduction of the emissions but also needs to be mitigated by novel technologies or materials. During the last two decades, novel multifunctional building materials have been designed by embedding photocatalysts employed to reduce (oxidize) the pollutants via photocatalytic reaction (PCR). Photocatalysts, for example, TiO2, are using UV light to convert adsorbed water and oxygen into highly reactive radicals (OH, O2−), which oxidize (clean) pollutants. In this context, the present work reports recent developments and future trends on experimental and numerical research of employing a multifunctional (highly porous) concrete foam, produced at the Institute of Construction and Building Materials of the Technische Universität Darmstadt, and enhanced with embedded TiO2. A wide range of experimentally analyzed thermal energy storage outputs, combined with the study of the photocatalytic activity, measured at the Bergische Universität Wuppertal, are presented to demonstrate the promising and outstanding multifunctionality of TiO2 foams.

A comprehensive field campaign, with measurements of HONO and related parameters, was conducted in summer 2018 at the foot (150 m a.s.l.) and the summit (1534 m a.s.l.) of Mt. Tai (Shandong province, China). At the summit station, high HONO mixing ratios were observed (mean ± 1σ: 133 ± 106 pptv, maximum: 880 pptv), with a diurnal noontime peak (mean ± 1σ: 133 ± 72 pptv at 12:30 local time). Constraints on the kinetics of aerosol-derived HONO sources (NO2 uptake on the aerosol surface and particulate nitrate photolysis) were performed and discussed, which enables a better understanding of the interaction of HONO and aerosols, especially in the polluted North China Plain. Various evidence of air mass transport from the ground to the summit level was provided. Furthermore, daytime HONO formation from different paths and its role in radical production were quantified and discussed. We found that the homogeneous reaction NO + OH could only explain 8.0 % of the daytime HONO formation, resulting in strong unknown sources (Pun). Campaigned-averaged Pun was about 290 ± 280 pptv h−1, with a maximum of about 1800 pptv h−1. Aerosol-derived HONO formation mechanisms were not the major sources of Pun at the summit station. Their contributions to daytime HONO formation varied from negligible to moderate (similar to NO + OH), depending on the chemical kinetic parameters used. Coupled with sensitivity tests on the kinetic parameters used, the NO2 uptake on the aerosol surface and particulate nitrate photolysis contributed 1.5 %–19 % and 0.6 %–9.6 % of the observed Pun, respectively. Based on synchronous measurements at the foot and the summit station, an amount of field evidence was proposed to support the finding that the remaining majority (70 %–98 %) of Pun was dominated by the rapid vertical transport from the ground to the summit level and heterogeneous formation on the mountain surfaces during transport. HONO photolysis at the summit level initialized daytime photochemistry and still represented an essential OH source in the daytime, with a contribution of about one-quarter of O3. We provided evidence that ground-derived HONO played a significant role in the oxidizing capacity of the upper boundary layer through the enhanced vertical air mass exchange driven by mountain winds. The follow-up impacts should be considered in regional chemistry transport models.

Since road traffic and particularly diesel vehicles are still the major source of nitrogen oxides in Germany, it is currently discussed to ban diesel vehicles from many city centres to force NO2 concentrations below the limit value. However, there is also a debate in the public whether roadside measurement stations were positioned in Germany in agreement with the EU guidelines for monitoring stations. In the present study, NOx and CO2 emissions were investigated in the city centre of Wuppertal at three different locations during 2018 to estimate the contribution of buses to the NOx emission. From the measured data daily averaged values NOx were calculated showing that on March, 21, 2018 (no strike) NOx was about 87% higher than on the day when public services were on strike. The final results of the study indicate a pronounced impact of public buses on the measured concentrations, in particular the NO2 concentration. However, a significant impact of a single emission source/category on the measured concentrations at a monitoring station is in contradiction to rules in the corresponding EU Guidelines. [1].

ABSTRACT Since road traffic and particularly diesel vehicles are still the major source of nitrogen oxides in Germany, it is currently discussed to ban diesel vehicles from many city centres to force NO2 concentrations below the limit value. However, there is also a debate in the public whether roadside measurement stations were positioned in Germany in agreement with the EU guidelines for monitoring stations. In the present study, NOx and CO2 emissions were investigated in the city centre of Wuppertal at three different locations during 2018 to estimate the contribution of buses to the NOx emission. From the measured data daily averaged values NOx were calculated showing that on March, 21, 2018 (no strike) NOx was about 87% higher than on the day when public services were on strike. The final results of the study indicate a pronounced impact of public buses on the measured concentrations, in particular the NO2 concentration. However, a significant impact of a single emission source/category on the measured concentrations at a monitoring station is in contradiction to rules in the corresponding EU Guidelines. [1].

Small scale bed flow photoreactor experiments were performed to assess the photocatalytic performance of cement-based TiO2-containing materials for NOx reduction through the determination of kinetic parameters under variation of the experimental conditions (relative humidity, flow rate, mixing ratio and light intensity) and monitoring of potential reaction products in the gas phase and the aqueous extract of the surface. The results clearly demonstrated the general potential of the tested material to photocatalytically remediate gas phase NOx by conversion into nitrite and nitrate as identified reaction products at the surface. The measured uptake coefficients for NO and NO2 under atmospheric relevant conditions were in the range of 5 x 10-5 with a corresponding surface deposition velocity of about 0.5 cm s-1. However, it became also clear that the photocatalytic activity is in part significantly dependent on the experimental conditions. The relative humidity and the mixing ratio of the air pollutant were identified as the most important parameters. In addition, under certain conditions, a renoxificadon process can occur. The comprehensive results of the present study are discussed in detail to develop recommendations for a possible future application of this technique to improve urban air quality.

In several previous studies emission of ozone (O3) during fireworks has been reported, which was attributed to either photolysis of molecular oxygen (O2) or nitrogen dioxide (NO2) by short/near UV radiation emitted during the high-temperature combustion of fireworks. In contrast, in the present study no O3 formation was observed using a selective O3-LOPAP instrument during the combustion of pyrotechnical material in the laboratory, while a standard O3 monitor using UV absorption showed extremely high O3 signals. The artificial O3 response of the standard O3 monitor was caused by known interferences associated with high levels of co-emitted VOCs and could also be confirmed in field measurements during New Year's Eve in the city of Wuppertal, Germany. The present results help to explain unreasonably high ozone levels documented during ambient fireworks, which are in contradiction to the fast titration of O3 by nitrogen monoxide (NO) in the night-time atmosphere.

Abstract. The Dead Sea water budget is no longer in equilibrium. The lake level decline exceeds 1 m a−1 and causes severe environmental problems, such as a shifting of the fresh/saline groundwater interface and climatic changes. As the Dead Sea is a terminal lake, located in an arid environment, evaporation is the key component of the Dead Sea water budget and accounts for the main loss of water. However, the actual amount of evaporation as well as the governing factors are unknown. Therefore, for the first time, long-term eddy covariance measurements were performed for a period of one year, starting in March 2014. The total annual amount measured at this location was 994 ± 81 mm a−1. The median daily evaporation rate reaches 4.3 mm d−1 in July and only 1.1 mm d−1 in December. The wind velocity and vapour pressure deficit were identified as the main governing factors of evaporation throughout the year. Consequently, the local wind systems define the diurnal evaporation cycle. In the evening, strong downslope winds govern the wind field and cause evaporation amounts which are up to 100 % higher than during daytime, and also during the night evaporation rates are accelerated compared to daytime evaporation, due to strong northerly along-valley flows. Furthermore, a robust and reliable regression model is presented to calculate sub-daily and multiday evaporation values with a linear function of wind velocity and vapour pressure deficit. An overall correlation coefficient of 0.8 is achieved and the cross validation results in a prediction error of 4.8 %. Finally, indirect evaporation approaches were tested for their applicability for the Dead Sea and compared to the measurements. The aerodynamic approach is applicable for sub-daily and multi-day calculations and attains correlation coefficients between 0.85 and 0.99. For the application of the Bowen-Ratio-Energy-Balance (BREB) method and the Priestley–Taylor method, measurements of the heat storage term are inevitable to calculate evaporation on time scales up to one month. Without the heat storage term, the equations yield strong seasonal biases and over- or underestimate daily evaporation rates by up 100 %. The usage of an empirically gained linear function or a hysteresis model depending on the net radiation to estimate the heat storage term was not accurate enough to provide reliable evaporation amounts. The Penman equation was adapted to calculate realistic evaporation amounts, by using an empirically gained linear function for the heat storage term. The correlation coefficients are above 0.9, the daily mean difference is only 0.5 mm d−1 and the estimated annual amount is within the range of the measurement uncertainties. In summary, this study provides the first directly measured amounts of Dead Sea evaporation and applicable methods to calculate evaporation.

Particulate matter (PM) and nitrogen oxides NOx (NOx = NO2 + NO) are key species for urban air quality in Europe and are emitted by mobile sources. According to European recommendations, a significant fraction of road freight should be shifted to waterborne transport in the future. In order to better consider this changed emission pattern in future emission inventories, in the present study, inland water transport emissions of NOx, CO2 and PM were investigated under 10 real world conditions at the river Rhine, Germany in 2013. An average NO2/NOx emission ratio of 0.08 ± 0.02 was obtained, which is indicative of ship diesel engines without after-treatment systems. For all measured motor ship types and operation conditions overall weighted average emission indices of EINOx = 54  4 g kg -1 and a lower limit EIPM1 =  2.0  0.3 g kg -1 were obtained. EIs for NOx and PM1 were found to be in the range of 20–161 g kg -1 and  0.2–8.1 g kg -1 , respectively. A comparison with threshold values of national German guidelines shows that the NOx emissions of all investigated motor ship 15 types are above the threshold values, while the obtained lower limit PM1 emissions just within. To reduce NOx emissions to acceptable values, implementation of after-treatment systems is recommended.

AbstractEin europäisches Forschungsprojekt hat untersucht, ob photokatalytische Beschichtungen an Straßen Schadstoffe wie Stickoxide und Kohlenwasserstoffe merklich abbauen und so die Luft reinigen. Messorte waren nicht nur das Labor, sondern auch ein Fahrzeugtunnel und eine Modellstraßenschlucht. Die Photokatalyse funktioniert zwar grundsätzlich, kann aber alleine nicht verhindern, dass die Schadstoffkonzentrationen in der Luft die Grenzwerte überschreiten.