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Hydroxyl radicals (OH) are the major oxidizing species in the troposphere. Because of their central importance, absolute measurements of their concentrations are needed to validate chemical mechanisms of atmospheric models. The extremely low and highly variable concentrations in the troposphere, however, make measurements of OH difficult. Three techniques are currently used worldwide for tropospheric observations of OH after about 30~years of technical developments: Differential Optical Laser Absorption Spectroscopy (DOAS), Laser-Induced Fluorescence Spectroscopy (LIF), and Chemical Ionisation Mass Spectrometry (CIMS). Even though many measurement campaigns with OH data were published, the question of accuracy and precision is still under discussion. Here, we report results of the first formal, blind intercomparison of these techniques. Six OH instruments (4~LIF, 1~CIMS, 1~DOAS) participated successfully in the ground-based, international HOxComp campaign carried out in Jülich, Germany, in summer 2005. Comparisons were performed for three days in ambient air (3~LIF, 1 CIMS) and for six days in the atmosphere simulation chamber SAPHIR (3~LIF, 1~DOAS). All instruments were found to measure tropospheric OH concentrations with high sensitivity and good time resolution. The pairwise correlations between different data sets were linear and yielded high correlation coefficients (r2=0.75−0.96). Excellent absolute agreement was observed for the instruments at the SAPHIR chamber, yielding slopes between 1.01 and 1.13 in the linear regressions. In ambient air, the slopes deviated from unity by factors of 1.06 to 1.69, which can partly be explained by the stated instrumental accuracies. In addition, sampling inhomogeneities and calibration problems have apparently contributed to the discrepancies. The absolute intercepts of the linear regressions did not exceed 0.6×106 cm−3, mostly being insignificant and of minor importance for daytime observations of OH. No relevant interferences with respect to ozone, water vapour, NOx and peroxy radicals could be detected. The HOxComp campaign has demonstrated that OH can be measured reasonably well by current instruments, but also that there is still room for improvement of calibrations.

International audience; Continuous measurements of concentrations of reactive gases, radiation, and meteorological parameters are carried out at the Meteorological Observatory Hohenpeissenberg (MOHp) as part of the Global Atmosphere Watch (GAW) Program. NO, NO2, O3 and JNO2 data from a four-year period (March 1999-December 2002) are evaluated for consistency with photochemical steady state (PSS, ?=1) conditions. The extent of deviation from PSS reveals a strong dependence on wind direction at the station. Median values of ? in the south sector are in the range of 2.5-5.7 and show a high variability. In contrast, values for the other directions show a relatively low variability around a median level of 2. When taking into account peroxy radical concentrations (?ext=1) PSS was reached in 13-32% of all cases for the years 1999-2002. The differences in wind sectors can be explained by local effects. It is shown that the height of the sample inlet line, its distance to the forest and the surrounding topography has a strong impact on both the absolute and relative deviations from PSS. Global irradiance and thus, photolysis of NO2 is reduced within the dense forest. Since the reaction of NO with O3 is still proceeding under these conditions, increased NO2/NO ratios are produced locally in air which is transported through the forest and advected to the MOHp site. Estimates of the peroxy radical concentration (RO2) inferred from PSS are compared with peroxy radical measurements made at the site in June 2000 during a three-week campaign. The PSS derived RO2 levels were higher than corresponding measured levels by at least a factor of 2-3. This analysis was made for a wind sector with minimal local effects on PSS. Thus the corresponding ? median of 2 can be regarded as an upper limit for a deviation from PSS due to chemical reactions, i.e. by peroxy radicals and possible other oxidants converting additional NO to NO2.

Decesari, S. ... et. al.-- 24 pages, 15 figures, 4 tables, the supplement related to this article is available online https://doi.org/10.5194/acp-14-12109-2014-supplement, The use of co-located multiple spectroscopic techniques can provide detailed information on the atmospheric processes regulating aerosol chemical composition and mixing state. So far, field campaigns heavily equipped with aerosol mass spectrometers have been carried out mainly in large conurbations and in areas directly affected by their outflow, whereas lesser efforts have been dedicated to continental areas characterised by a less dense urbanisation. We present here the results obtained at a background site in the Po Valley, Italy, in summer 2009. For the first time in Europe, six state-of-the-art spectrometric techniques were used in parallel: aerosol time-of-flight mass spectrometer (ATOFMS), two aerosol mass spectrometers (high-resolution time-of-flight aerosol mass spectrometer - HR-ToF-AMS and soot particle aerosol mass spectrometer - SP-AMS), thermal desorption aerosol gas chromatography (TAG), chemical ionisation mass spectrometry (CIMS) and (offline) proton nuclear magnetic resonance (1H-NMR) spectroscopy. The results indicate that, under high-pressure conditions, atmospheric stratification at night and early morning hours led to the accumulation of aerosols produced by anthropogenic sources distributed over the Po Valley plain. Such aerosols include primary components such as black carbon (BC), secondary semivolatile compounds such as ammonium nitrate and amines and a class of monocarboxylic acids which correspond to the AMS cooking organic aerosol (COA) already identified in urban areas. In daytime, the entrainment of aged air masses in the mixing layer is responsible for the accumulation of low-volatility oxygenated organic aerosol (LV-OOA) and also for the recycling of non-volatile primary species such as black carbon. According to organic aerosol source apportionment, anthropogenic aerosols accumulating in the lower layers overnight accounted for 38% of organic aerosol mass on average, another 21% was accounted for by aerosols recirculated in residual layers but still originating in northern Italy, while a substantial fraction (41 %) was due to the most aged aerosols imported from transalpine areas. The different meteorological regimes also affected the BC mixing state: in periods of enhanced stagnation and recirculation of pollutants, the number fraction of the BC-containing particles determined by ATOFMS was 75% of the total, while in the days of enhanced ventilation of the planetary boundary layer (PBL), such fraction was significantly lower (50 %) because of the relative greater influence of non-BC-containing aerosol local sources in the Po Valley. Overall, a full internal mixing between BC and the nonrefractory aerosol chemical components was not observed during the experiment in this environment. © Author(s) 2014, This work was funded by European integrated project on aerosol cloud climate and air quality interactions (no. 036833-2, EUCAARI). The ERA-Interim data were kindly provided by Silvio Davolio (CNR-ISAC). Data analysis was co-funded by the project PEGASOS (EC FP7-ENV-2010-265148) and by the project SUPERSITO of Region Emilia-Romagna. ACCENTC is also gratefully acknowledged. Finally, Emanuela Finessi (CNR-ISAC, now at University of York) is also gratefully acknowledged for the precious work in aerosol filter collection in the field. Manuel Dall’Osto and Roy M. Harrison thank the UK National Centre for Atmospheric Science for financial support