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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.

Existing studies suggest various potential daytime sources of atmospheric nitrous acid (HONO), including photolysis surface reactions and photo-enhanced NO2 conversion on organic surfaces. However, the understanding of daytime HONO sources is still inadequate. In this study, we report the HONO formation on asphalt surfaces under various NO2, VOCs (toluene and hexane), and UV irradiance conditions using a continuous flow chamber. Although no HONO formation was found without light exposure, the light threshold for HONO formation on the asphalt surface was very low, with a total UV (TUV) of 0.7 W m−2. HONO formation on the asphalt surface was linearly dependent on NO2 up to 300 ppb in the presence of VOCs, but no HONO formation was observed with humified air and NO2. HONO production was saturated at high hydrocarbon concentrations and light intensities. The calculated first-order NO2 conversion rate to HONO on the asphalt surface was 1.2 × 10−4 s −1. The observed mean HONO emission flux was 1.3 × 109 molecules cm−2 s −1 with a similar range of those on other urban covered surfaces. The calculated vertical HONO profile using the measured HONO emission flux and 1-D steady state model revealed that the asphalt surface may account for 13% of daytime HONO in the elevated on-road pollutant concentrations in Seoul. However, we show that its HONO contribution could be much higher on real-life road surfaces directly exposed to much higher NO2 emissions from vehicle exhaust.

International audience; Nitrous acid (HONO) photolysis is an important atmospheric reaction that leads to the formation of hydroxyl radicals (OH), the main diurnal atmospheric oxidants. The process of HONO formation remains unclear, and comparisons between field measurements and model results have highlighted the presence of unknown HONO sources. HONO production on plant surfaces was recently suggested to contribute to atmospheric HONO formation, but there is limited information on the quantification of HONO production and uptake by plants. To address this gap in the existing knowledge, the current study investigated HONO exchange on living Zea mays plants. Experiments were conducted in growth chambers under controlled experimental conditions (temperature, relative humidity, NO2 mixing ratio, light intensity, CO2 mixing ratio) at temperatures ranging between 283 and 299 K. To investigate the effect of drought on HONO plant-atmosphere exchanges, experiments were carried out on two sets of Zea mays plants exposed to two different water supply conditions during their growth: optimal watering (70% of the field capacity) and water stress (30% of the field capacity). Results indicated that the uptake of HONO by control Zea mays plants increased linearly with ambient temperature, and was correlated with CO2 assimilation for temperatures ranging from 283 to 299 K. At 299 K, HONO production on the leaves offset this uptake and Zea mays plants were a source of HONO, with a net production rate of 27 ± 7 ppt h-1. Deposition velocities were higher for HONO than CO2, suggesting a higher mesophyll resistance for CO2 than HONO. As water stress reduced the stomatal opening, it also decreased plant-atmosphere gas exchange. Thus, climate change, which may limit the availability of water, will have an impact on HONO exchange between plants and the atmosphere.

Nitrogen dioxide (NO2) is an air pollutant discharged from combustion of human activities. Nitrous acid (HONO), measured as NO2, is thought to impact respiratory function more than NO2. HONO and NO2 have an equilibrium relationship, and their reaction is affected by climate conditions. This study was conducted to discuss the extent of HONO contained in NO2, depending on the level of urbanization. Whether climate conditions that promote HONO production enhanced the level of NO2 measured was investigated using time series analysis. Climate and outdoor air pollution data measured in April 2009&ndash, March 2017 in urban (Tokyo, Osaka, and Aichi) and rural (Yamanashi) areas in Japan were used for the analysis. Air temperature had a trend of negative associations with NO2, which might indicate the decomposition of HONO in the equilibrium between HONO and NO2. The associations of relative humidity with NO2 did not have consistent trends by prefecture: humidity only in Yamanashi was positively associated with NO2. In high relative humidity conditions, the equilibrium goes towards HONO production, which was observed in Yamanashi, suggesting the proportion of HONO in NO2 might be low/high in urban/rural areas.

30 pags., 13 figs., 4 tabs., We present the inter-comparison of delta slant column densities (SCDs) and vertical profiles of nitrous acid (HONO) derived from measurements of different multiaxis differential optical absorption spectroscopy (MAXDOAS) instruments and using different inversion algorithms during the Second Cabauw Inter-comparison campaign for Nitrogen Dioxide measuring Instruments (CINDI- 2) in September 2016 at Cabauw, the Netherlands (51.97° N, 4.93° E). The HONO vertical profiles, vertical column densities (VCDs), and near-surface volume mixing ratios are compared between different MAX-DOAS instruments and profile inversion algorithms for the first time. Systematic and random discrepancies of the HONO results are derived from the comparisons of all data sets against their median values. Systematic discrepancies of HONO delta SCDs are observed in the range of ±0:3×1015 molec. cm2, which is half of the typical random discrepancy of 0:6× 1015 molec. cm2. For a typical high HONO delta SCD of 2×1015 molec. cm2, the relative systematic and random discrepancies are about 15% and 30 %, respectively. The inter-comparison of HONO profiles shows that both systematic and random discrepancies of HONO VCDs and nearsurface volume mixing ratios (VMRs) are mostly in the range of ∼ ±0:5×1014 molec. cm2 and ∼ ±0:1 ppb (typically ∼ 20 %). Further we find that the discrepancies of the retrieved HONO profiles are dominated by discrepancies of the HONO delta SCDs. The profile retrievals only contribute to the discrepancies of the HONO profiles by ∼ 5 %. However, some data sets with substantially larger discrepancies than the typical values indicate that inappropriate implementations of profile inversion algorithms and configurations of radiative transfer models in the profile retrievals can also be an important uncertainty source. In addition, estimations of measurement uncertainties of HONO dSCDs, which can significantly impact profile retrievals using the optimal estimation method, need to consider not only DOAS fit errors, but also atmospheric variability, especially for an instrument with a DOAS fit error lower than ∼ 3×1014 molec. cm2. The MAX-DOAS results during the CINDI-2 campaign indicate that the peak HONO levels (e.g. near-surface VMRs of ∼ 0:4 ppb) often appeared in the early morning and below 0.2 km. The near-surface VMRs retrieved from the MAXDOAS observations are compared with those measured using a co-located long-path DOAS instrument. The systematic differences are smaller than 0.15 and 0.07 ppb during early morning and around noon, respectively. Since true HONO values at high altitudes are not known in the absence of real measurements, in order to evaluate the abilities of profile inversion algorithms to respond to different HONO profile shapes, we performed sensitivity studies using synthetic HONO delta SCDs simulated by a radiative transfer model with assumed HONO profiles. The tests indicate that the profile inversion algorithms based on the optimal estimation method with proper configurations can reproduce the different HONO profile shapes well. Therefore we conclude that the features of HONO accumulated near the surface derived from MAX-DOAS measurements., Funding for this study was provided by ESA through the CINDI-2 (ESA contract no. 4000118533/16/I-Sbo) and FRM4DOAS (ESA contract no. 4000118181/16/I-EF) projects, by the NSFC (grant no. 41805027), the Russian Foundation for Basic Research (grant no. 18-35-00682), the Russian Academy of Sciences (grant nos. 0150-2018-0052 and 0129-2019- 0002), NASA’s Atmospheric Composition Program (grant no. NASA-16-NUP2016-0001), the US National Science Foundation (AGS-1620530 award), and the European Union’s Horizon 2020 research and innovation programme through the ACTRIS-2 transnational access programme (grant no. 654109). The AIOFM group is grateful for the support by the NSFC (grant no. 41530644). The article processing charges for this open-access publication were covered by the Max Planck Society

International audience; The heterogeneous degradation of nitrogen dioxide (NO2) on five samples of natural Icelandic volcanic particles has been investigated. Laboratory experiments were carried out under simulated atmospheric conditions using a coated wall flow tube (CWFT). The CWFT reactor was coupled to a blue light nitrogen oxides analyzer (NOx analyzer), and a long path absorption photometer (LOPAP) to monitor in real time the concentrations of NO2, NO and HONO, respectively. Under dark and ambient relative humidity conditions, the steady state uptake coefficients of NO2 varied significantly between the volcanic samples probably due to differences in magma composition and morphological variation related with the density of surface OH groups. The irradiation of the surface with simulated sunlight enhanced the uptake coefficients by a factor of three indicating that photo-induced processes on the surface of the dust occur. Furthermore, the product yields of NO and HONO were determined under both dark and simulated sunlight conditions. The relative humidity was found to influence the distribution of gaseous products, promoting the formation of gaseous HONO. A detailed reaction mechanism is proposed that supports our experimental observations. Regarding the atmospheric implications, our results suggest that the NO2 degradation on volcanic particles and the corresponding formation of HONO is expected to be significant during volcanic dust storms or after a volcanic eruption.

The Amazon rainforest presents a unique, natural laboratory for the study of surface–atmosphere interactions. Its alternation between a near-pristine marine-influenced atmosphere during the wet season and a vulnerable system affected by periodic intrusions of anthropogenic pollution during the dry season provides an opportunity to investigate some fundamental aspects of boundary-layer chemical processes. This study presents the first simultaneous hourly measurements of concentrations, fluxes, and deposition velocities of the inorganic trace gases NH3, HCl, HONO, HNO3, and SO2 as well as their water-soluble aerosol counterparts NH4+, Cl−, NO2-, NO3- and SO42- over the Amazon. Species concentrations were measured in the dry season (from 6 October to 5 November 2017), at the Amazon Tall Tower Observatory (ATTO) in Brazil, using a two-point gradient wet-chemistry instrument (GRadient of AErosols and Gases Online Registration, GRAEGOR) sampling at 42 and 60 m. Fluxes and deposition velocities were derived from the concentration gradients using a modified form of the aerodynamic gradient method corrected for measurement within the roughness sub-layer. Findings from this campaign include observations of elevated concentrations of NH3 and SO2 partially driven by long-range transport (LRT) episodes of pollution and the substantial influence of coarse Cl− and NO3- particulate on overall aerosol mass burdens. From the flux measurements, the dry season budget of total reactive nitrogen dry deposition at the ATTO site was estimated as −2.9 kg N ha-1a-1. HNO3 and HCl were deposited continuously at a rate close to the aerodynamic limit. SO2 was deposited with an average daytime surface resistance (Rc) of 28 s m−1, whilst aerosol components showed average surface deposition velocities of 2.8 and 2.7 mm s−1 for SO42- and NH4+, respectively. Deposition rates of NO3- and Cl− were higher at 7.1 and 7.8 mm s−1, respectively, reflecting their larger average size. The exchange of NH3 and HONO was bidirectional, with NH3 showing emission episodes in the afternoon and HONO in the early morning hours. This work provides a unique dataset to test and improve dry deposition schemes for these compounds for tropical rainforest, which have typically been developed by interpolation from conditions in temperate environments. A future campaign should focus on making similar measurements in the wet season in order to provide a complete view of the annual pattern of inorganic trace gas and coarse aerosol biosphere–atmosphere exchange over tropical rainforest.

Production of hydroxyl (OH) radicals is frequently dominated by the photolysis of tropospheric ozone (O3). However, photolysis of nocturnal radical reservoirs, such as nitrous acid (HONO) and nitryl chloride (ClNO2), also produces radicals (OH and Cl atoms) that contribute to the oxidising capacity of the local atmosphere, and initiate many radical-chain reactions that lead to the formation of harmful secondary pollutants. Photolysis of nitric acid (HNO3) is also a minor radical production mechanism. In this paper, locally representative photolysis rate constants (j-values) for these molecules are shown to be critical for quantifying and understanding the rate of radical production in a local atmosphere. The first long-term 4-π filter radiometer dataset in the UK (21 November 2018–20 November 2019) available for direct atmospheric model validation is reported. Measurements were made at Auchencorth Moss, a Scottish rural background site, and j(NO2) is used to generate a measurement-driven adjustment factor (MDAF) for calculated j-values that accounts for local changes in meteorological variables without significantly increasing computational cost. Modelled clear-sky j-values and actinic flux for Auchencorth Moss were generated using the Tropospheric Ultraviolet and Visible radiation model (TUV; v.5.3.1). Applying the MDAF metric resulted in the calculated photolytic production rate of OH radicals, from all sources considered, being ~40 % lower over the year. Photolysis of HONO resulted in an increased rate of OH production compared to that from O3 in low-light conditions, such as sunrise and sunset (Solar Zenith Angle > 80°). Hydroxyl radical production from HONO photolysis exceeded that from O3 consistently throughout the day during the winter and autumn (by a factor of 5 and 2.1, respectively). Radical production rates from HONO and ClNO2 reached maximum values during the early morning hours of summer (06:00–09:00 UTC), with OH produced at a rate of 1.06 × 106 OH radicals cm−3 s−1, and Cl radicals at 3.20 × 104 Cl radicals cm−3 s−1, with the MDAF metric applied. This first application of the MDAF j-values demonstrates an efficient measurement and computational approach to improve modelling of the local atmospheric photochemistry that drives NO2, O3 and PM pollution levels. The incorporation of local radiation measurements in measurement networks, and the consequent greater spatial resolution of locally-relevant photolysis coefficients in model photolysis parameterisations, will improve the accuracy of assessment of air pollution and policy-intervention impacts.

Gaseous sulfuric acid (H2SO4) is known as one of the key precursors for atmospheric new particle formation (NPF) processes, but its measurement remains challenging. Therefore, a proxy method that is able to derive gaseous sulfuric acid concentrations from parameters that can be measured relatively easily and accurately is highly desirable for the atmospheric chemistry community. Although such methods are available for clean atmospheric environments, a proxy that works well in a polluted atmosphere, such as that found in Chinese megacities, is yet to be developed. In this study, the gaseous sulfuric acid concentration was measured in February–March 2018, in urban Beijing using a nitrate based – long time-of-flight chemical ionization mass spectrometer (LToF-CIMS). A number of atmospheric parameters were recorded concurrently including the ultraviolet radiation B (UVB) intensity, the concentrations of O3, NOx (sum of NO and NO2), SO2, and HONO, and aerosol particle number size distributions. A proxy for atmospheric daytime gaseous sulfuric acid concentration was derived via a statistical analysis method using the UVB intensity, [SO2], the condensation sink (CS), [O3], and [HONO] (or [NOx]) as the predictor variables, where square brackets denote the concentrations of the corresponding species. In this proxy method, we considered the formation of gaseous sulfuric acid from reactions of SO2 and OH radicals during the daytime, and the loss of gaseous sulfuric acid due to its condensation onto the preexisting particles. In addition, we explored the formation of OH radicals from the conventional gas-phase photochemistry using O3 as a proxy and from the photolysis of HONO using HONO (and subsequently NOx) as a proxy. Our results showed that the UVB intensity and [SO2] are dominant factors in the production of gaseous sulfuric acid, and that the simplest proxy could be constructed with the UVB intensity and [SO2] alone. When the OH radical production from both homogenously and heterogeneously formed precursors were considered, the relative errors were reduced by up to 20 %.

In recent years, high fine particulate (PM2.5) pollution episodes with high ozone (O3) levels have been observed in Shanghai from time to time. However, their occurrence and characteristics remain poorly understood. Meanwhile, as a major precursor of tropospheric hydroxyl radical (OH) that initiates the formation of hydroperoxyl and organic peroxy radicals, HONO would inevitably affect the formation of O3, but its role in the formation of O3 during the double high-level PM2.5 and O3 pollution episodes remains unclear. In this study, the characteristics of the double high pollution episodes and the role of HONO in O3 formation in these episodes were investigated based on field observation in urban Shanghai from 2014 to 2016. Results showed that high PM2.5 pollution and high O3 pollution could occur simultaneously. The cases with data of double high O3 and PM2.5 concentrations accounted for about 1.0% of the whole sampling period. During the double high pollution episodes, there still existed active photochemical processes, while the active photochemical processes at high PM2.5 concentration were conductive to the production and accumulation of O3 under a VOC-limited regime and a calm atmospheric condition including high temperature, moderately high relative humidity, and low wind speed, which in turn enhanced the conversions of SO2 and NO2 and the formation and accumulation of secondary sulfate and nitrate aerosols and further promoted the increase of PM2.5 concentration and the deterioration of air pollution. Further analysis indicated that the daytime HONO concentration could be strongly negatively correlated with O3 concentration in most of the double high pollution episodes, revealing the dominant role of HONO in O3 formation during these pollution episodes. This study provides important field measurement-based evidence for understanding the significant contribution of daytime HONO to O3 formation, and helps to clarify the formation and coexistence mechanisms of the double high-level O3 and PM2.5 pollution episodes.

International audience; This work introduces a newly developed reaction mechanism for the oxidation of ammonia in freely propagating and burner-stabilized premixed flames as well as shock-tube, jet-stirred reactor, and plug-flow reactor experiments. The paper mainly focuses on pure ammonia and ammonia–hydrogen fuel blends. The reaction mechanism also considers the formation of nitrogen oxides as well as the reduction of nitrogen oxides depending upon the conditions of the surrounding gas phase. Doping of the fuel blend with NO2 can result in acceleration of H2 autoignition via the reaction NO2 + HO2 ⇋ HONO + O2, followed by the thermal decomposition of HONO, or deceleration of H2 oxidation via NO2 + OH ⇋ NO + HO2. The concentration of HO2 is decisive for the active reaction pathway. The formation of NO in burner-stabilized premixed flames is shown to demonstrate the capability of the mechanism to be integrated into a mechanism for hydrocarbon oxidation.

The interaction of HONO with UV irradiated TiO2 solid films was studied using a low pressure flow reactor (1–10 Torr of Helium) combined with a modulated molecular beam mass spectrometer for monitoring of the gaseous species involved. The reactive uptake of HONO to TiO2 was studied in absence of O2 in the reactor as a function of HONO concentration ([HONO]0 = (0.5–5.0) × 1012 molecule cm−3), relative humidity (RH = 0.001–60%) and temperature (T = 275–320 K). The measured initial uptake coefficient (γ0) of HONO on TiO2 surface was independent of the HONO concentration and showed slight inverse dependence on temperature (activation factor = −1390 ± 150 K) and relative humidity: γ0 = 6.9 × 10−4 (RH)−0.3 (at T = 280 K, calculated using BET surface area, 30% uncertainty). NO2 and NO were observed as products of the HONO reaction with UV irradiated TiO2 surface with sum of their yields corresponding to nearly 100% of the nitrogen mass balance. The yields of the NO and NO2 products were found to be (48 ± 7) and (52 ± 8) %, respectively, independent of relative humidity, temperature and concentration of HONO under experimental conditions used. The HONO loss on mineral aerosol during daytime (calculated with uptake data for HONO on TiO2 surface in presence of UV irradiation) appears to be of minor importance compared with HONO photolysis in the atmosphere.

Self-cleaning surfaces containing TiO2 nanoparticles have been postulated to efficiently remove NOx from the atmosphere. However, UV irradiation of NOx adsorbed on TiO2 also was shown to form harmful gas-phase byproducts such as HONO and N2O that may limit their depolluting potential. Ambient pressure XPS was used to study surface and gas-phase species formed during adsorption of NO2 on TiO2 and subsequent UV irradiation at λ = 365 nm. It is shown here that NO3 -, adsorbed on TiO2 as a byproduct of NO2 disproportionation, was quantitatively converted to surface NO2 and other reduced nitrogenated species under UV irradiation in the absence of moisture. When water vapor was present, a faster NO 3 - conversion occurred, leading to a net loss of surface-bound nitrogenated species. Strongly adsorbed NO3 - in the vicinity of coadsorbed K+ cations was stable under UV light, leading to an efficient capture of nitrogenated compounds. © 2013 American Chemical Society. Fil: Rosseler, Olivier. Chemical Sciences Division and Advanced Light Source; Estados Unidos. Université de Strasbourg; Francia Fil: Sleiman, Mohamad. Chemical Sciences Division and Advanced Light Source; Estados Unidos Fil: Montesinos, Victor Nahuel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Comisión Nacional de Energía Atómica; Argentina Fil: Shavorskiy, Andrey. Chemical Sciences Division and Advanced Light Source; Estados Unidos Fil: Keller, Valerie. Université de Strasbourg; Francia Fil: Keller, Nicolas. Université de Strasbourg; Francia Fil: Litter, Marta Irene. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica; Argentina Fil: Bluhm, Hendrik. Chemical Sciences Division and Advanced Light Source; Estados Unidos Fil: Salmeron, Miquel. Chemical Sciences Division and Advanced Light Source; Estados Unidos. University of California at Berkeley; Estados Unidos Fil: Destaillats, Hugo. Chemical Sciences Division and Advanced Light Source; Estados Unidos

Kinetics and products of the interaction of HONO with solid films of Al 2 O 3 were investigated under dark and UV irradiation conditions using a low pressure flow reactor (1–10 Torr) combined with a modulated molecular beam mass spectrometer for monitoring of the gaseous species involved. The reactive uptake of HONO to Al 2 O 3 was studied as a function of HONO concentration ([HONO] 0 = (0.6–3.5) × 10 12 molecule cm −3 ), relative humidity (RH = 1.4 × 10 −4 to 35.4%), temperature ( T = 275–320 K) and UV irradiation intensity ( J NO 2 = 0.002 – 0.012 s − 1 ). The measured reactive uptake coefficient was independent of the HONO concentration and temperature. In contrast, the relative humidity (RH) was found to have a strong impact on the uptake coefficient: γ = 4.8 × 10 −6 (RH) −0.61 and γ = 1.7 × 10 −5 (RH) −0.44 under dark conditions and on irradiated surface ( J NO 2 = 0.012 s − 1 ), respectively ( γ calculated with BET surface area, 30% conservative uncertainty). NO 2 and NO were observed as products of the HONO reaction with Al 2 O 3 surface with yields of 40 ± 6 and 60 ± 9%, respectively, independent of relative humidity, temperature, concentration of HONO and UV irradiation intensity under experimental conditions used. The HONO uptake on mineral aerosol (calculated with uptake data for HONO on Al 2 O 3 surface) appears to be of minor importance compared with other HONO loss processes in the boundary layer of the earth atmosphere.

An open top chamber experiment was carried out in the summer of 2003 to examine the effect of nitrous acid (HONO) gas on the physiological status of Scots pine saplings (Pinus sylvestris). Four-year-old pine trees were exposed to two different levels of HONO gas (at ca. 2.5 ppb and 5.0 ppb) and a control (filtered air) from early evening to early morning (18:00-6:00), in duplicate open top chambers. Significant decreases in the ratios of chlorophylls a to b, an increase in the carbon to nitrogen (C/N) ratio, and a reduction of maximum yield of PS II (F(v)/F(m)) in pine needles were also observed after the 2 months' fumigation. Cation contents of pine needles were also decreased by the fumigation with HONO gas. The results could be explained by the harmful effects of OH radicals, generated from photolysis of HONO gas, and/or aqueous phase HONO (NO(2)(-)/HONO), on the photosynthetic capacity of pine needles.

[1] The photolysis of nitrous acid (HONO) in the early morning hours is an important source of OH radicals, the most important daytime oxidizing species. Although the importance of this mechanism has been recognized for many years, no accurate quantification of this OH source is available, and the role of HONO photolysis is often underestimated. We present measurements of HONO and its precursor NO2 by Differential Optical Absorption Spectroscopy (DOAS) during the Berliner Ozonexperiment (BERLIOZ) field campaign in July/August 1998 at Pabstthum near Berlin, Germany. HONO concentrations, formation rates, and simultaneously measured HONO photolysis frequencies are used to calculate the total amount of OH formed by HONO photolysis during a full diurnal cycle. A comparison with the OH formation by photolysis of O3 and HCHO and by the reaction of alkenes with ozone shows that HONO photolysis contributed up to 20% of the total OH formed in a 24 hour period during this campaign. In the morning hours, HONO photolysis was by far the most important OH source during BERLIOZ. INDEX TERMS: 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0394 Atmospheric Composition and Structure: Instruments and techniques; KEYWORDS: HONO, photochemistry, OH, photolysis, HCHO, ozone

A new in situ instrument (LOPAP: long path absorption photometer) to measure gaseous nitrous acid (HONO) using wet chemical sampling and photometric detection has been developed. This instrument is aimed to overcome the known problems with current HONO measurement techniques and was designed to be a cheap, sensitive, compact, and continuouslyworking HONO monitorfor ambient air measurements in the troposphere or for measurements of higher concentrations e.g. in smog chambers, in exhaust gases, and in indoor environments. Laboratory investigations were carried outto characterize the instrument components with respect to collection efficiency, optimum dye formation, optimum detection, and interfering species. Detection limits ranging from approximately 3 to 50 pptV have been obtained with response times from 4 to 1.5 min, respectively, using different instrument parameters. The accuracy of the measurements is in the range between +/-(10-15)%. The validation of the instrument was performed in the laboratory for HONO concentrations of 3 and 30 ppbV using ion chromatography and with a DOAS (differential optical absorption spectrometer) instrument in a large outdoor smog chamber in the range from 0.1 to 20 ppbV. The deviations were well within the errors of the measurements; however, when comparing the data with the DOAS instrument systematically higher values were found with the LOPAP instrument.