scholarly journals Technical Note: Intercomparison of formaldehyde measurements at the atmosphere simulation chamber SAPHIR

2008 ◽  
Vol 8 (8) ◽  
pp. 2189-2200 ◽  
Author(s):  
A. Wisthaler ◽  
E. C. Apel ◽  
J. Bossmeyer ◽  
A. Hansel ◽  
W. Junkermann ◽  
...  
Keyword(s):  
Ambient Air ◽  
Proton Transfer Reaction ◽  
Technical Note ◽  
Research Centre ◽  
Mixing Ratios ◽  
Analytical Instruments ◽  
Chamber Volume ◽  
Set Up ◽  
Atmosphere Simulation Chamber ◽  
Simulation Chamber

Abstract. The atmosphere simulation chamber SAPHIR at the Research Centre Jülich was used to test the suitability of state-of-the-art analytical instruments for the measurement of gas-phase formaldehyde (HCHO) in air. Five analyzers based on four different sensing principles were deployed: a differential optical absorption spectrometer (DOAS), cartridges for 2,4-dinitrophenylhydrazine (DNPH) derivatization followed by off-line high pressure liquid chromatography (HPLC) analysis, two different types of commercially available wet chemical sensors based on Hantzsch fluorimetry, and a proton-transfer-reaction mass spectrometer (PTR-MS). A new optimized mode of operation was used for the PTR-MS instrument which significantly enhanced its performance for online HCHO detection at low absolute humidities. The instruments were challenged with typical ambient levels of HCHO ranging from zero to several ppb. Synthetic air of high purity and particulate-filtered ambient air were used as sample matrices in the atmosphere simulation chamber onto which HCHO was spiked under varying levels of humidity and ozone. Measurements were compared to mixing ratios calculated from the chamber volume and the known amount of HCHO injected into the chamber; measurements were also compared between the different instruments. The formal and blind intercomparison exercise was conducted under the control of an independent referee. A number of analytical problems associated with the experimental set-up and with individual instruments were identified, the overall agreement between the methods was fair.

scholarly journals Technical Note: Intercomparison of formaldehyde measurements at the atmosphere simulation chamber SAPHIR

2007 ◽  
Vol 7 (6) ◽  
pp. 15619-15650 ◽  
Author(s):  
A. Wisthaler ◽  
E.C. Apel ◽  
J. Bossmeyer ◽  
A. Hansel ◽  
W. Junkermann ◽  
...  
Keyword(s):  
Ambient Air ◽  
Proton Transfer Reaction ◽  
Technical Note ◽  
Research Centre ◽  
Mixing Ratios ◽  
Analytical Instruments ◽  
Chamber Volume ◽  
Set Up ◽  
Atmosphere Simulation Chamber ◽  
Simulation Chamber

Abstract. The atmosphere simulation chamber SAPHIR at the Research Centre Jülich was used to test the suitability of state-of-the-art analytical instruments for the measurement of gas-phase formaldehyde (HCHO) in air. Five analyzers based on four different sensing principles were deployed: a differential optical absorption spectrometer (DOAS), cartridges for 2,4-dinitro-phenyl-hydrazine (DNPH) derivatization followed by off-line high pressure liquid chromatography (HPLC) analysis, two different types of commercially available wet chemical sensors based on Hantzsch fluorimetry, and a proton-transfer-reaction mass spectrometer (PTR-MS). A new optimized mode of operation was used for the PTR-MS instrument which significantly enhanced its performance for on-line HCHO detection at low absolute humidities. The instruments were challenged with typical ambient levels of HCHO ranging from zero to several ppb. Synthetic air of high purity and particulate-filtered ambient air were used as sample matrices in the atmosphere simulation chamber onto which HCHO was spiked under varying levels of humidity and ozone. Measurements were compared to mixing ratios calculated from the chamber volume and the known amount of HCHO injected into the chamber; measurements were also compared between the different instruments. The formal and blind intercomparison exercise was conducted under the control of an independent referee. A number of analytical problems associated with the experimental set-up and with individual instruments were identified, the overall agreement between the methods was good.

scholarly journals Technical Note: Formal blind intercomparison of HO<sub>2</sub> measurements in the atmosphere simulation chamber SAPHIR during the HOxComp campaign

2010 ◽  
Vol 10 (24) ◽  
pp. 12233-12250 ◽  
Author(s):  
H. Fuchs ◽  
T. Brauers ◽  
H.-P. Dorn ◽  
H. Harder ◽  
R. Häseler ◽  
...  
Keyword(s):  
Water Vapor ◽  
Correlation Coefficients ◽  
Chemical Conversion ◽  
Ambient Air ◽  
Technical Note ◽  
Data Set ◽  
Air Masses ◽  
Regression Parameters ◽  
Atmosphere Simulation Chamber ◽  
Simulation Chamber

Abstract. Hydroperoxy radical (HO2) concentrations were measured during the formal blind intercomparison campaign HOxComp carried out in Jülich, Germany, in 2005. Three instruments detected HO2 via chemical conversion to hydroxyl radicals (OH) and subsequent detection of the sum of OH and HO2 by laser induced fluorescence (LIF). All instruments were based on the same detection and calibration scheme. Because measurements by a MIESR instrument failed during the campaign, no absolute reference measurement was available, so that the accuracy of individual instruments could not be addressed. Instruments sampled ambient air for three days and were attached to the atmosphere simulation chamber SAPHIR during the second part of the campaign. Six experiments of one day each were conducted in SAPHIR, where air masses are homogeneously mixed, in order to investigate the performance of instruments and to determine potential interferences of measurements under well-controlled conditions. Linear correlation coefficients (R2) between measurements of the LIF instruments are generally high and range from 0.82 to 0.98. However, the agreement between measurements is variable. The regression analysis of the entire data set of measurements in SAPHIR yields slopes between 0.69 to 1.26 and intercepts are smaller than typical atmospheric daytime concentrations (less than 1 pptv). The quality of fit parameters improves significantly, when data are grouped into data subsets of similar water vapor concentrations. Because measurements of LIF instruments were corrected for a well-characterized water dependence of their sensitivities, this indicates that an unknown factor related to water vapor affected measurements in SAPHIR. Measurements in ambient air are also well-correlated, but regression parameters differ from results obtained from SAPHIR experiments. This could have been caused by differences in HO2 concentrations in the sampled air at the slightly different locations of instruments.

scholarly journals Technical Note: Formal blind intercomparison of HO<sub>2</sub> measurements in the atmosphere simulation chamber SAPHIR during the HOxComp campaign

2010 ◽  
Vol 10 (9) ◽  
pp. 21189-21235 ◽  
Author(s):  
H. Fuchs ◽  
T. Brauers ◽  
H.-P. Dorn ◽  
H. Harder ◽  
R. Häseler ◽  
...  
Keyword(s):  
Water Vapor ◽  
Correlation Coefficients ◽  
Chemical Conversion ◽  
Ambient Air ◽  
Technical Note ◽  
Data Set ◽  
Air Masses ◽  
Regression Parameters ◽  
Atmosphere Simulation Chamber ◽  
Simulation Chamber

Abstract. Hydroperoxy radical (HO2) concentrations were measured during the formal blind intercomparison campaign HOxComp carried out in Jülich, Germany. Three instruments detected HO2 via chemical conversion to hydroxyl radicals (OH) and subsequent detection of the sum of OH and HO2 by laser induced fluorescence (LIF). Instruments sampled ambient air for three days and were attached to the atmosphere simulation chamber SAPHIR during the second part of the campaign. Six experiments of one day each were conducted in SAPHIR, where air masses were homogeneously mixed, in order to investigate the performance of instruments and to determine potential interferences of measurements under well-controlled conditions. Linear correlation coefficients between measurements of the LIF instruments are generally high and range from 0.82 to 0.98. However, the agreement between measurements is variable. The regression analysis of the entire data set of measurements in SAPHIR yields slopes between 0.69 to 1.26 and intercepts are smaller than typical atmospheric daytime concentrations (less than 1 pptv). The quality of fit parameters improves significantly, when data are grouped into data subsets of similar water vapor concentrations. Because measurements of LIF instruments were corrected for a well-characterized water dependence of their sensitivities, this indicates that an unknown factor related to water vapor affected measurements in SAPHIR. Measurements in ambient air are also well correlated, but regression parameters differ from results obtained from SAPHIR experiments. This is most likely caused by sampling different air masses at the slightly distant locations of instruments.

scholarly journals Technical Note: Determination of formaldehyde mixing ratios in polluted air with PTR-MS: laboratory experiments and field measurements

2007 ◽  
Vol 7 (5) ◽  
pp. 12845-12876
Author(s):  
S. Inomata ◽  
H. Tanimoto ◽  
S. Kameyama ◽  
U. Tsunogai ◽  
H. Irie ◽  
...  
Keyword(s):  
Proton Transfer ◽  
Field Measurements ◽  
Proton Transfer Reaction ◽  
Technical Note ◽  
Detection Sensitivity ◽  
Optical Absorption Spectroscopy ◽  
High Time Resolution ◽  
Mixing Ratios ◽  
Proton Transfer Reactions ◽  
Humidity Dependence

Abstract. Formaldehyde (HCHO), the most abundant carbonyl compound in the atmosphere, is generated as an intermediate product in the oxidation of nonmethane hydrocarbons. Proton transfer reaction mass spectrometry (PTR-MS) has the capability to detect HCHO from ion signals at m/z 31 with high time-resolution. However, the detection sensitivity is low compared to other detectable species, and is considerably affected by humidity, due to back reactions between protonated HCHO and water vapor prior to analysis. We performed a laboratory calibration of HCHO by PTR-MS and examined the detection sensitivity and humidity dependence at various field strengths. Subsequently, we deployed the PTR-MS instrument in a field campaign at Mount Tai in China in June 2006 to measure HCHO in various meteorological and photochemical conditions; we also conducted intercomparison measurements by Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS). Correction of interference in the m/z 31 signals by fragments from proton transfer reactions with methyl hydroperoxide, methanol, and ethanol greatly improves agreement between the two methods, giving the correlation [HCHO]MAX-DOAS = (0.99±0.16) [HCHO]PTR-MS + (0.02±0.38), where error limits represent 95% confidence levels.

scholarly journals An intercomparison of GC-FID and PTR-MS toluene measurements in ambient air under conditions of enhanced monoterpene loading

2010 ◽  
Vol 3 (1) ◽  
pp. 1-54
Author(s):  
J. L. Ambrose ◽  
K. Haase ◽  
R. S. Russo ◽  
Y. Zhou ◽  
M. L. White ◽  
...  
Keyword(s):  
Drift Tube ◽  
Ambient Air ◽  
Proton Transfer Reaction ◽  
Quantitative Agreement ◽  
Operating Conditions ◽  
Oxidation Products ◽  
Maximum Enhancement ◽  
Mixing Ratios ◽  
Flame Ionization ◽  
Two Systems

Abstract. Toluene was measured using both a gas chromatographic system (GC), with a flame ionization detector (FID), and a proton transfer reaction-mass spectrometer (PTR-MS) at the AIRMAP atmospheric monitoring station Thompson Farm (THF) in rural Durham, NH during the summer of 2004. Simultaneous measurements of monoterpenes, including α- and β-pinene, camphene, Δ3-carene, and d-limonene, by GC-FID demonstrated large enhancements in monoterpene mixing ratios relative to toluene, with median and maximum enhancement ratios of ~2 and ~30, respectively. A detailed intercomparison among the GC-FID and PTR-MS toluene measurements was conducted to test the specificity of PTR-MS for atmospheric toluene measurements under conditions often dominated by biogenic emissions. We derived quantitative estimates of potential interferences in the PTR-MS toluene measurements related to sampling and analysis of monoterpenes, including fragmentation of the monoterpenes and some of their primary carbonyl oxidation products via reactions with H3O+, O2+ and NO+ in the PTR-MS drift tube. The PTR-MS and GC-FID toluene measurements were in good quantitative agreement and the two systems tracked one another well from the instrumental limits of detection to maximum mixing ratios of ~0.5 ppbv. Discrepancies in the measured mixing ratios were not well correlated with enhancements in the monoterpenes. Better quantitative agreement between the two systems was obtained by correcting the PTR-MS measurements for contributions from monoterpene fragmentation in the PTR-MS drift tube; however, the improvement was minor. Interferences in the PTR-MS measurements from fragmentation of the monoterpene oxidation products pinonaldehyde, caronaldehyde and α-pinene oxide were also likely negligible. The results from THF suggest that toluene can be reliably quantified by PTR-MS using our operating conditions under the ambient compositions probed. This work extends the range of field conditions under which PTR-MS validation studies have been conducted.

scholarly journals The Comparative Reactivity Method – a new tool to measure total OH Reactivity in ambient air

2008 ◽  
Vol 8 (8) ◽  
pp. 2213-2227 ◽  
Author(s):  
V. Sinha ◽  
J. Williams ◽  
J. N. Crowley ◽  
J. Lelieveld
Keyword(s):  
Dynamic Range ◽  
Field Tests ◽  
Ambient Air ◽  
Proton Transfer Reaction ◽  
Vital Role ◽  
Oh Radicals ◽  
Current Configuration ◽  
Sink Term ◽  
On Line ◽  
Set Up

Abstract. Hydroxyl (OH) radicals play a vital role in maintaining the oxidizing capacity of the atmosphere. To understand variations in OH radicals both source and sink terms must be understood. Currently the overall sink term, or the total atmospheric reactivity to OH, is poorly constrained. Here, we present a new on-line method to directly measure the total OH reactivity (i.e.~total loss rate of OH radicals) in a sampled air mass. In this method, a reactive molecule (X), not normally present in air, is passed through a glass reactor and its concentration is monitored with a suitable detector. OH radicals are then introduced in the glass reactor at a constant rate to react with X, first in the presence of zero air and then in the presence of ambient air containing VOCs and other OH reactive species. Comparing the amount of X exiting the reactor with and without the ambient air allows the air reactivity to be determined. In our existing set up, X is pyrrole and the detector used is a proton transfer reaction mass spectrometer. The present dynamic range for ambient air reactivity is about 6 to 300 s−1, with an overall maximum uncertainty of 25% above 8 s−1 and up to 50% between 6–8 s−1. The system has been tested and calibrated with different single and mixed hydrocarbon standards showing excellent linearity and accountability with the reactivity of the standards. Potential interferences such as high NO in ambient air, varying relative humidity and photolysis of pyrrole within the setup have also been investigated. While interferences due changing humidity and photolysis of pyrrole are easily overcome by ensuring that humidity in the set up does not change drastically and the photolytic loss of pyrrole is measured and taken into account, respectively, NO>10 ppb in ambient air remains a significant interference for the current configuration of the instrument. Field tests in the tropical rainforest of Suriname (~53 s

scholarly journals Intercomparison of NO<sub>3</sub> radical detection instruments in the atmosphere simulation chamber SAPHIR

2013 ◽  
Vol 6 (1) ◽  
pp. 303-379 ◽  
Author(s):  
H.-P. Dorn ◽  
R. L. Apodaca ◽  
S. M. Ball ◽  
T. Brauers ◽  
S. S. Brown ◽  
...  
Keyword(s):  
Water Vapour ◽  
Absorption Spectroscopy ◽  
Organic Aerosol ◽  
Optical Absorption Spectroscopy ◽  
Acquisition Time ◽  
Data Set ◽  
Mixing Ratios ◽  
Enhanced Absorption ◽  
Atmosphere Simulation Chamber ◽  
Simulation Chamber

Abstract. The detection of atmospheric NO3 radicals is still challenging owing to its low mixing ratios (≈ 1 to 300 pptv) in the troposphere. While long-path differential optical absorption spectroscopy (DOAS) is a well established NO3 detection approach for over 25 yr, newly sensitive techniques have been developed in the past decade. This publication outlines the results of the first comprehensive intercomparison of seven instruments developed for the spectroscopic detection of tropospheric NO3. Four instruments were based on cavity ring-down spectroscopy (CRDS), two utilised open-path cavity enhanced absorption spectroscopy (CEAS), and one applied "classical" long-path DOAS. The intercomparison campaign "NO3Comp" was held at the atmosphere simulation chamber SAPHIR in Jülich (Germany) in June 2007. Twelve experiments were performed in the well mixed chamber for variable concentrations of NO3, N2O5, NO2, hydrocarbons, and water vapour, in the absence and in the presence of inorganic or organic aerosol. The overall precision of the cavity instruments varied between 0.5 and 5 pptv for integration times of 1 s to 5 min; that of the DOAS instrument was 9 pptv for an acquisition time of 1 min. The NO3 data of all instruments correlated excellently with the NOAA-CRDS instrument, which was selected as the common reference because of its superb sensitivity, high time resolution, and most comprehensive data coverage. The median of the coefficient of determination (r2) over all experiments of the campaign (60 correlations) is r2 = 0.981 (25th/75th percentiles: 0.949/0.994; min/max: 0.540/0.999). The linear regression analysis of the campaign data set yielded very small intercepts (1.2 ± 5.3 pptv) and the average slope of the regression lines was close to unity (1.02, min: 0.72, max: 1.36). The deviation of individual regression slopes from unity was always within the combined accuracies of each instrument pair. The very good correspondence between the NO3 measurements by all instruments for aerosol-free experiments indicates that the losses of NO3 in the inlet of the instruments were determined reliably by the participants for the corresponding conditions. In the presence of inorganic or organic aerosol, however, differences in the measured NO3 mixing ratios were detectable among the instruments. In individual experiments the discrepancies increased with time, pointing to additional NO3 radical losses by aerosol deposited onto the inlet walls of the instruments. Instruments using DOAS analyses showed no significant effect of aerosol on the detection of NO3. No hint of a cross interference of NO2 was found. The effect of non-Lambert–Beer behaviour of water vapour absorption lines on the accuracy of the NO3 detection by broadband techniques was small and well controlled. The NO3Comp campaign demonstrated the high quality, reliability and robustness of performance of current state-of-the-art instrumentation for NO3 detection.

scholarly journals Technical Note: Quantitative long-term measurements of VOC concentrations by PTR-MS – measurement, calibration, and volume mixing ratio calculation methods

2008 ◽  
Vol 8 (22) ◽  
pp. 6681-6698 ◽  
Author(s):  
R. Taipale ◽  
T. M. Ruuskanen ◽  
J. Rinne ◽  
M. K. Kajos ◽  
H. Hakola ◽  
...  
Keyword(s):  
Field Measurements ◽  
Proton Transfer Reaction ◽  
Technical Note ◽  
Transmission Curve ◽  
Mixing Ratio ◽  
Calculation Methods ◽  
Mixing Ratios ◽  
Ratio Calculation ◽  
Gas Standard

Abstract. Proton transfer reaction mass spectrometry (PTR-MS) is a technique for online measurements of atmospheric concentrations, or volume mixing ratios, of volatile organic compounds (VOCs). This paper gives a detailed description of our measurement, calibration, and volume mixing ratio calculation methods, which have been designed for long-term stand-alone field measurements by PTR-MS. The PTR-MS instrument has to be calibrated regularly with a gas standard to ensure the accuracy needed in atmospheric VOC measurements. We introduce a novel method for determining an instrument specific relative transmission curve using information obtained from a calibration. This curve enables consistent mixing ratio calculation for VOCs not present in a calibration gas standard. Our method proved to be practical, systematic, and sensitive enough to capture changes in the transmission over time. We also propose a new approach to considering the abundance of H3O+H2O ions in mixing ratio calculation. The approach takes into account the difference in the transmission efficiencies for H3O+ and H3O+H2O ions. To illustrate the functionality of our measurement, calibration, and calculation methods, we present a one-month period of ambient mixing ratio data measured in a boreal forest ecosystem at the SMEAR II station in southern Finland. During the measurement period 27 March–26 April 2007, the hourly averages of the mixing ratios were 0.051–0.57 ppbv for formaldehyde, 0.19–3.1 ppbv for methanol, 0.038–0.39 ppbv for benzene, and 0.020–1.3 ppbv for monoterpenes. The detection limits for the hourly averages were 0.020, 0.060, 0.0036, and 0.0092 ppbv, respectively.

scholarly journals Intercomparison of NO<sub>3</sub> radical detection instruments in the atmosphere simulation chamber SAPHIR

2013 ◽  
Vol 6 (5) ◽  
pp. 1111-1140 ◽  
Author(s):  
H.-P. Dorn ◽  
R. L. Apodaca ◽  
S. M. Ball ◽  
T. Brauers ◽  
S. S. Brown ◽  
...  
Keyword(s):  
Water Vapour ◽  
Absorption Spectroscopy ◽  
Organic Aerosol ◽  
Optical Absorption Spectroscopy ◽  
Acquisition Time ◽  
Data Set ◽  
Mixing Ratios ◽  
Enhanced Absorption ◽  
Atmosphere Simulation Chamber ◽  
Simulation Chamber

Abstract. The detection of atmospheric NO3 radicals is still challenging owing to its low mixing ratios (≈ 1 to 300 pptv) in the troposphere. While long-path differential optical absorption spectroscopy (DOAS) has been a well-established NO3 detection approach for over 25 yr, newly sensitive techniques have been developed in the past decade. This publication outlines the results of the first comprehensive intercomparison of seven instruments developed for the spectroscopic detection of tropospheric NO3. Four instruments were based on cavity ring-down spectroscopy (CRDS), two utilised open-path cavity-enhanced absorption spectroscopy (CEAS), and one applied "classical" long-path DOAS. The intercomparison campaign "NO3Comp" was held at the atmosphere simulation chamber SAPHIR in Jülich (Germany) in June 2007. Twelve experiments were performed in the well-mixed chamber for variable concentrations of NO3, N2O5, NO2, hydrocarbons, and water vapour, in the absence and in the presence of inorganic or organic aerosol. The overall precision of the cavity instruments varied between 0.5 and 5 pptv for integration times of 1 s to 5 min; that of the DOAS instrument was 9 pptv for an acquisition time of 1 min. The NO3 data of all instruments correlated excellently with the NOAA-CRDS instrument, which was selected as the common reference because of its superb sensitivity, high time resolution, and most comprehensive data coverage. The median of the coefficient of determination (r2) over all experiments of the campaign (60 correlations) is r2 = 0.981 (quartile 1 (Q1): 0.949; quartile 3 (Q3): 0.994; min/max: 0.540/0.999). The linear regression analysis of the campaign data set yielded very small intercepts (median: 1.1 pptv; Q1/Q3: −1.1/2.6 pptv; min/max: −14.1/28.0 pptv), and the slopes of the regression lines were close to unity (median: 1.01; Q1/Q3: 0.92/1.10; min/max: 0.72/1.36). The deviation of individual regression slopes from unity was always within the combined accuracies of each instrument pair. The very good correspondence between the NO3 measurements by all instruments for aerosol-free experiments indicates that the losses of NO3 in the inlet of the instruments were determined reliably by the participants for the corresponding conditions. In the presence of inorganic or organic aerosol, however, differences in the measured NO3 mixing ratios were detectable among the instruments. In individual experiments the discrepancies increased with time, pointing to additional NO3 radical losses by aerosol deposited onto the filters or on the inlet walls of the instruments. Instruments using DOAS analyses showed no significant effect of aerosol on the detection of NO3. No hint of a cross interference of NO2 was found. The effect of non-Lambert–Beer behaviour of water vapour absorption lines on the accuracy of the NO3 detection by broadband techniques was small and well controlled. The NO3Comp campaign demonstrated the high quality, reliability and robustness of performance of current state-of-the-art instrumentation for NO3 detection.

scholarly journals Characterisation of the photolytic HONO-source in the atmosphere simulation chamber SAPHIR

2004 ◽  
Vol 4 (6) ◽  
pp. 7881-7915 ◽  
Author(s):  
F. Rohrer ◽  
B. Bohn ◽  
T. Brauers ◽  
D. Brüning ◽  
F.-J. Johnen ◽  
...  
Keyword(s):  
Adsorbed Water ◽  
Ambient Air ◽  
Solar Irradiation ◽  
Oh Radical ◽  
Nox Formation ◽  
Empirical Function ◽  
Glass Surfaces ◽  
Atmospheric Concentrations ◽  
Atmosphere Simulation Chamber ◽  
Simulation Chamber

Abstract. HONO formation has been proposed as an important OH radical source in simulation chambers for more than two decades. Besides the heterogeneous HONO formation by the dark reaction of NO2 and adsorbed water, a photolytic source has been proposed to explain the elevated reactivity in simulation chamber experiments. However, the mechanism of the photolytic process is not well understood so far. As expected, production of HONO and NOx was also observed inside the new atmosphere simulation chamber SAPHIR under solar irradiation. This photolytic HONO and NOx formation was studied with a sensitive HONO instrument under reproducible controlled conditions at atmospheric concentrations of other trace gases. It is shown that the photolytic HONO source in the SAPHIR chamber is not caused by NO2 reactions and that it is the only direct NOy source under illuminated conditions. In addition, the photolysis of nitrate which was recently postulated for the observed photolytic HONO formation on snow, ground, and glass surfaces, can be excluded in the chamber. A photolytic HONO source at the surface of the chamber is proposed which is strongly dependent on humidity, on light intensity, and on temperature. An empirical function of the form S(HONO)=a1,2×J(NO2)×(1+(RH/RH0)2)×exp(−T0/T) describes these dependencies and reproduces the observed HONO formation rates to within 10%. It is shown that the photolysis of HONO represents the dominant radical source in the SAPHIR chamber for typical tropospheric O3/H2O concentrations. For these conditions, the HONO concentrations inside SAPHIR are similar to recent observations in ambient air.

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