A first direct intercomparison of aerosol vertical profiles from Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations, performed during the Cabauw Intercomparison Campaign of Nitrogen Dioxide measuring Instruments (CINDI) in summer 2009, is presented. Five out of 14 participants of the CINDI campaign reported aerosol extinction profiles and aerosol optical thickness (AOT) as deduced from observations of differential slant column densities of the oxygen collision complex (O4) at different elevation angles. Aerosol extinction vertical profiles and AOT are compared to backscatter profiles from a ceilometer instrument and to sun photometer measurements, respectively. Furthermore, the near-surface aerosol extinction coefficient is compared to in situ measurements of a humidity-controlled nephelometer and dry aerosol absorption measurements. The participants of this intercomparison exercise use different approaches for the retrieval of aerosol information, including the retrieval of the full vertical profile using optimal estimation and a parametrised approach with a prescribed profile shape. Despite these large conceptual differences, and also differences in the wavelength of the observed O4 absorption band, good agreement in terms of the vertical structure of aerosols within the boundary layer is achieved between the aerosol extinction profiles retrieved by the different groups and the backscatter profiles observed by the ceilometer instrument. AOTs from MAX-DOAS and sun photometer show a good correlation (R>0.8), but all participants systematically underestimate the AOT. Substantial differences between the near-surface aerosol extinction from MAX-DOAS and from the humidified nephelometer remain largely unresolved.

Ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements of sulfur dioxide (SO2) have been performed at the Xianghe station (39.8° N, 117.0° E) located at ~ 50 km southeast of Beijing from March 2010 to February 2013. Tropospheric SO2 vertical profiles and corresponding vertical column densities (VCDs), retrieved by applying the optimal estimation method to the MAX-DOAS observations, have been used to study the seasonal and diurnal cycles of SO2, in combination with correlative measurements from in situ instruments, as well as meteorological data. A marked seasonality was observed in both SO2 VCD and surface concentration, with a maximum in winter (February) and a minimum in summer (July). This can be explained by the larger emissions in winter due to the domestic heating and, in case of surface concentration, by more favorable meteorological conditions for the accumulation of SO2 close to the ground during this period. Wind speed and direction are also found to be two key factors in controlling the level of the SO2-related pollution at Xianghe. In the case of east or southwest wind, the SO2 concentration does not change significantly with the wind speed, since the city of Tangshan and heavy polluting industries are located to the east and southwest of the station, respectively. In contrast, when wind comes from other directions, the stronger the wind, the less SO2 is observed due to a more effective dispersion. Regarding the diurnal cycle, the SO2 amount is larger in the early morning and late evening and lower at noon, in line with the diurnal variation of pollutant emissions and atmospheric stability. A strong correlation with correlation coefficients between 0.6 and 0.9 is also found between SO2 and aerosols in winter, suggesting that anthropogenic SO2, through the formation of sulfate aerosols, contributes significantly to the total aerosol content during this season. The observed diurnal cycles of MAX-DOAS SO2 surface concentration are also in very good agreement (correlation coefficient close to 0.9) with those from collocated in situ data, indicating the good reliability and robustness of our retrieval.

We present a detailed analysis and interpretation of the high-mass binary V380 Cyg, based on high-precision space photometry gathered with the Kepler space mission as well as high-resolution ground-based spectroscopy obtained with the HERMES spectrograph attached to the 1.2m Mercator telescope. We derive a precise orbital solution and the full physical properties of the system, including dynamical component mass estimates of 11.43+/-0.19 and 7.00+/-0.14 solar masses for the primary and secondary, respectively. Our frequency analysis reveals the rotation frequency of the primary in both the photometric and spectroscopic data and additional low-amplitude stochastic variability at low frequency in the space photometry with characteristics that are compatible with recent theoretical predictions for gravity-mode oscillations excited either by the convective core or by sub-surface convective layers. Doppler Imaging analysis of the silicon lines of the primary suggests the presence of two high-contrast stellar surface abundance spots which are located either at the same latitude or longitude. Comparison of the observed properties of the binary with present-day single-star evolutionary models shows that the latter are inadequate and lack a serious amount of near-core mixing.

Retrievals of tropospheric nitrogen dioxide (NO2) from the Ozone Monitoring Instrument (OMI) are subject to errors in the treatments of aerosols, surface reflectance anisotropy, and vertical profile of NO2. Here we quantify the influences over China via an improved retrieval process. We explicitly account for aerosol optical effects (simulated by nested GEOS-Chem at 0.667 degrees long. x 0.5 degrees lat. and constrained by aerosol measurements), surface reflectance anisotropy, and high-resolution vertical profiles of NO2 (simulated by GEOS-Chem). Prior to the NO2 retrieval, we derive the cloud information using consistent ancillary assumptions.We compare our retrieval to the widely used DOMINO v2 product, using MAX-DOAS measurements at three urban/suburban sites in East China as reference and focusing the analysis on the 127 OMI pixels (in 30 days) closest to the MAX-DOAS sites. We find that our retrieval reduces the interference of aerosols on the retrieved cloud properties, thus enhancing the number of valid OMI pixels by about 25 %. Compared to DOMINO v2, our retrieval better captures the day-to-day variability in MAX-DOAS NO2 data (R-2 = 0.96 versus 0.72), due to pixel-specific radiative transfer calculations rather than the use of a look-up table, explicit inclusion of aerosols, and consideration of surface reflectance anisotropy. Our retrieved NO2 columns are 54% of the MAX-DOAS data on average, reflecting the inevitable spatial inconsistency between the two types of measurement, errors in MAX-DOAS data, and uncertainties in our OMI retrieval related to aerosols and vertical profile of NO2.Sensitivity tests show that excluding aerosol optical effects can either increase or decrease the retrieved NO2 for individual OMI pixels with an average increase by 14 %. Excluding aerosols also complexly affects the retrievals of cloud fraction and particularly cloud pressure. Employing various surface albedo data sets slightly affects the retrieved NO2 on average (within 10 %). The retrieved NO2 columns increase when the NO2 profiles are taken from MAX-DOAS retrievals (by 19% on average) or TM4 simulations (by 13 %) instead of GEOS-Chem simulations. Our findings are also relevant to retrievals of other pollutants (e. g., sulfur dioxide, formaldehyde, glyoxal) from UV-visible backscatter satellite instruments.

Ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements of nitrous acid (HONO) and its precursor NO2 (nitrogen dioxide) as well as aerosols have been performed daily in Beijing city centre (39.98° N, 116.38° E) from July 2008 to April 2009 and at the suburban site of Xianghe (39.75° N, 116.96° E) located ~60 km east of Beijing from March 2010 to December 2012. This extensive dataset allowed for the first time the investigation of the seasonal cycle of HONO as well as its diurnal variation in and in the vicinity of a megacity. Our study was focused on the HONO and NO2 near-surface concentrations (0–200 m layer) and total vertical column densities (VCDs) and also aerosol optical depths (AODs) and extinction coefficients retrieved by applying the Optimal Estimation Method to the MAX-DOAS observations. Monthly averaged HONO near-surface concentrations at local noon display a strong seasonal cycle with a maximum in late fall/winter (~0.8 and 0.7 ppb at Beijing and Xianghe, respectively) and a minimum in summer (~0.1 ppb at Beijing and 0.03 ppb at Xianghe). The seasonal cycles of HONO and NO2 appear to be highly correlated, with correlation coefficients in the 0.7–0.9 and 0.5–0.8 ranges at Beijing and Xianghe, respectively. The stronger correlation of HONO with NO2 and also with aerosols observed in Beijing suggests possibly larger role of NO2 conversion into HONO in the Beijing city center than at Xianghe. The observed diurnal cycle of HONO near-surface concentration shows a maximum in the early morning (about 1 ppb at both sites) likely resulting from night-time accumulation, followed by a decrease to values of about 0.1–0.4 ppb around local noon. The HONO / NO2 ratio shows a similar pattern with a maximum in the early morning (values up to 0.08) and a decrease to ~0.01–0.02 around local noon. The seasonal and diurnal cycles of the HONO near-surface concentration are found to be similar in shape and in relative amplitude to the corresponding cycles of the HONO total VCD and are therefore likely driven mainly by the balance between HONO sources and the photolytic sink, whereas dilution effects appear to play only a minor role. The estimation of OH radical production from HONO and O3 photolysis based on retrieved HONO near-surface concentrations and calculated photolysis rates indicate that in the 0–200 m altitude range, HONO is by far the largest source of OH radicals in winter as well as in the early morning at all seasons, while the contribution of O3 dominates in summer from mid-morning until mid-afternoon.

Abstract. Retrievals of tropospheric nitrogen dioxide (NO2) from the Ozone Monitoring Instrument (OMI) are subject to errors in the treatments of aerosols, surface reflectance anisotropy, and vertical profile of NO2. Here we quantify the influences over China via an improved retrieval process. We explicitly account for aerosol optical effects (simulated by nested GEOS-Chem at 0.667° lon × 0.5° lat and constrained by aerosol measurements), surface reflectance anisotropy, and high-resolution vertical profiles of NO2 (simulated by GEOS-Chem). Prior to the NO2 retrieval, we derive the cloud information using consistent ancillary assumptions. We compare our retrieval to the widely used DOMINO v2 product, using as reference MAX-DOAS measurements at three urban/suburban sites in East China and focusing the analysis on the 127 OMI pixels (in 30 days) closest to the MAX-DOAS sites. We find that our retrieval reduces the interference of aerosols on the retrieved cloud properties, thus enhancing the number of valid OMI pixels by about 25%. Compared to DOMINO v2, our retrieval improves the correlation with the MAX-DOAS data in the day-to-day variability of NO2 (R2 = 0.96 vs. 0.72). Our retrieved NO2 columns are about 50% of the MAX-DOAS data on average. This reflects the inevitable spatial inconsistency between the two types of measurement, uncertainties in MAX-DOAS data, and residual uncertainties in our OMI retrievals related to aerosols and vertical profile of NO2. Through a series of tests, we find that excluding aerosol scattering/absorption can either increase or decrease the retrieved NO2, with a mean absolute difference by about 20%. Concentrating aerosols at the boundary layer top enhances the retrieved NO2 by 8% on average with a mean absolute difference by 23%. The aerosol perturbations also affect nonlinearly the retrieved cloud fraction and particularly cloud pressure. Employing various surface albedo datasets alters the retrieved NO2 by 0–7% on average. The retrieved NO2 columns increase when the NO2 profiles are taken from MAX-DOAS retrievals (by 20% on average) or TM4 simulations (by 10%) instead of GEOS-Chem simulations. Our findings are also relevant to retrievals of other pollutants (e.g., sulfur dioxide, formaldehyde, glyoxal) from UV-vis backscatter satellite instruments.

We present intercomparison results for formaldehyde (HCHO) slant column measurements performed during the Cabauw Intercomparison campaign of Nitrogen Dioxide measuring Instruments (CINDI) that took place in Cabauw, the Netherlands, in summer 2009. During two months, nine atmospheric research groups simultaneously operated MAX-DOAS (MultiAXis Differential Optical Absorption Spectroscopy) instruments of various designs to record UV-visible spectra of scattered sunlight at different elevation angles that were analysed using common retrieval settings. The resulting HCHO data set was found to be highly consistent, the mean difference between instruments generally not exceeding 15% or 7.5 × 1015 molec cm−2, for all viewing elevation angles. Furthermore, a sensitivity analysis was performed to investigate the uncertainties in the HCHO slant column retrieval when varying key input parameters such as the molecular absorption cross sections, correction terms for the Ring effect or the width and position of the fitting interval. This study led to the identification of potentially important sources of errors associated with cross-correlation effects involving the Ring effect, O4, HCHO and BrO cross sections and the DOAS closure polynomial. As a result, a set of updated recommendations was formulated for HCHO slant column retrieval in the 336.5–359 nm wavelength range. To conclude, an error budget is proposed which distinguishes between systematic and random uncertainties. The total systematic error is estimated to be of the order of 20% and is dominated by uncertainties in absorption cross sections and related spectral cross-correlation effects. For a typical integration time of one minute, random uncertainties range between 5 and 30%, depending on the noise level of individual instruments.

1BIRA-IASB – Belgian Institute for Space Aeronomy, Brussels, Belgium 2Department of Physics, University of Toronto, Ontario, Canada 3Max Planck Institute for Chemistry, Mainz, Germany 4NASA/Goddard Space Flight Center, GSFC, Greenbelt, MD, USA 5Institute of Environmental Physics, University of Heidelberg, Heidelberg, Germany 6National Institute for Aerospace technology, INTA, Madrid, Spain 7Research Institute for Global Change, JAMSTEC, Yokohama, Japan 8Institute of Environmental Physics, University of Bremen, Bremen, Germany 9Royal Netherlands Meteorological Institute, KNMI, De Bilt, The Netherlands 10Laboratory for Atmospheric Research, Washington State University, Pullman, WA, USA 11Energy, Environment and Water Research Center, The Cyprus Institute, Nicosia, Cyprus * now at: Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Leuven, Belgium ** now at: Center for Environmental Remote Sensing, Chiba University, Chiba, Japan *** now at: Department of Earth System Science, Faculty of Science, Fukuoka University, Fukuoka, Japan

In the framework of EUMETSAT’s Satellite Application Facility on Ozone and Atmospheric Chemistry Monitoring (O3M-SAF), the second phase of the Continuous Development and Operations Project (CDOP-2) started in March 2012 with the aim to deliver operational GOME-2 and IASI data products of well characterized precision, accuracy and stability matching the needs of a large community of Users. The validation of new data products and the continuous monitoring of the quality of operational data sets are essential activities in the O3M-SAF. For minor trace gases, validation and Quality Assessment (QA) activities are coordinated at BIRA-IASB. During the first phase of CDOP, an end-toend validation approach has been designed and demonstrated for GOME-2 NO2 total and tropospheric column measurements. In compliance with international QA/QC standards, the idea is to evaluate independently all critical individual components of the level-1-to-2 retrieval chain. Evaluations are carried out by means of a suite of correlative observations performed by complementary ground-based and satellite instruments supported by radiative transfer and chemical-transport modelling tools. For CDOP-2, this precursor trace gas validation system will be significantly enhanced to cover a number of additional gases measured by the GOME-2 and IASI sensors (NO2, BrO, H2CO, SO2, glyoxal, HNO3 and OClO) on board of the three EUMETSAT MetOp platforms. A validation web portal will be set up, to report on the validation of new data products and on the continuous QA of operational products. Validation and QA will be based on a number of sources of ground-based correlative measurements, including remote-sensing zenith-sky, direct sun and MAXDOAS spectrometers, and FTIR instruments from selected NDACC stations. In addition, validation activities will include satellite-to-satellite comparisons relying on well-documented data sets from the GOME, SCIAMACHY and OMI instruments.

From June to July 2009 more than thirty different in-situ and remote sensing instruments from all over the world participated in the Cabauw Intercomparison campaign for Nitrogen Dioxide measuring Instruments (CINDI). The campaign took place at KNMI’s Cabauw Experimental Site for Atmospheric Research (CESAR) in the Netherlands. Its main objectives were to determine the accuracy of state-ofthe-art ground-based measurement techniques for the detection of atmospheric nitrogen dioxide (both in-situ and remote sensing), and to investigate their usability in satellite data validation. The expected outcomes are recommendations regarding the operation and calibration of such instruments, retrieval settings, and observation strategies for the use in ground-based networks for air quality monitoring and satellite data validation. Twenty-four optical spectrometers participated in the campaign, of which twenty-one had the capability to scan different elevation angles consecutively, the so-called Multi-axis DOAS systems, thereby collecting vertical profile information, in particular for nitrogen dioxide and aerosol. Various in-situ samplers and lidar instruments simultaneously characterized the variability of atmospheric trace gases and the physical properties of aerosol particles. A large data set of continuous measurements of these atmospheric constituents has been collected under various meteorological conditions and air pollution levels. Together with the permanent measurement capability at the CESAR site characterizing the meteorological state of the atmosphere, the CINDI campaign provided a comprehensive observational data set of atmospheric constituents in a highly polluted region of the world during summertime. First detailed comparisons performed with the CINDI data show that slant column measurements of NO 2, O4 and HCHO with MAX-DOAS agree within 5 to 15 %, vertical profiles of NO2 derived from several independent instruments agree within 25 % of one another, and MAX-DOAS aerosol optical thickness agrees within 20–30 % with AERONET data. For the in-situ NO2 instrument using a molybdenum converter, a bias was found as large as 5 ppbv during day time, when compared to the other in-situ instruments using photolytic converters.

SSD Science for a Sustainable Development – Atmosphere 0 M. DE MAZIERE, H. DE BACKER, M. CARLEER, E. MAHIEU, P. DEMOULIN, P. DUCHATELET, A. CHEYMOL, K. CLEMER, P.F. COHEUR, B. DILS, S. FALLY, F. HENDRICK, C.HERMANS, M. KRUGLANSKI, A. MANGOLD, J. VANDER AUWERA, R. VAN MALDEREN, M. VAN ROOZENDAEL, C. VIGOUROUX ADVANCED EXPLOITATION OF GROUND-BASED MEASUREMENTS FOR ATMOSPHERIC CHEMISTRY AND CLIMATE APPLICATIONS

In the field, aerosol in-situ measurements are often performed under dry conditions (relative humidity RH<30–40%). Since ambient aerosol particles experience hygroscopic growth at enhanced RH, their microphysical and optical properties – especially the aerosol light scattering – are also strongly dependent on RH. The knowledge of this RH effect is of crucial importance for climate forcing calculations or for the comparison of remote sensing with in-situ measurements. Here, we will present results from a four-month campaign which took place in summer 2009 in Cabauw, The Netherlands. The aerosol scattering coefficient σsp(λ) was measured dry and at various, predefined RH conditions between 20 and 95% with a humidified nephelometer. The scattering enhancement factor f(RH,λ) is the key parameter to describe the effect of RH on σsp(λ) and is defined as σsp(RH,λ) measured at a certain RH divided by the dry σsp(dry,λ). The measurement of f(RH,λ) together with the dry absorption measurement (assumed not to change with RH) allows the determination of the actual extinction coefficient σep(RH,λ) at ambient RH. In addition, a wide range of other aerosol properties were measured in parallel. The measurements were used to characterize the effects of RH on the aerosol optical properties. A closure study showed the consistency of the aerosol in-situ measurements. Due to the large variability of air mass origin (and thus aerosol composition) a simple parameterization of f(RH,λ) could not be established. If f(RH,λ) needs to be predicted, the chemical composition and size distribution need to be known. Measurements of four MAX-DOAS (multi-axis differential optical absorption spectroscopy) instruments were used to retrieve vertical profiles of σep(λ). The values of the lowest layer were compared to the in-situ values after conversion of the latter ones to ambient RH. The comparison showed a good correlation of R2 = 0.62–0.78, but the extinction coefficients from MAX-DOAS were a factor of 1.5–3.4 larger than the in-situ values. Best agreement is achieved for a few cases characterized by low aerosol optical depths and low planetary boundary layer heights. Differences were shown to be dependent on the applied MAX-DOAS retrieval algorithm. The comparison of the in-situ extinction data to a Raman LIDAR (light detection and ranging) showed a good correlation and higher values measured by the LIDAR (R2 = 0.82−0.85, slope of 1.69–1.76) if the Raman retrieved profile was used to extrapolate the directly measured extinction coefficient to the ground. The comparison improved if only nighttime measurements were used in the comparison (R2 = 0.96, slope of 1.12).

In June 2009, 22 spectrometers from 14 institutes measured tropospheric and stratospheric NO2 from the ground for more than 11 days during the Cabauw Intercomparison Campaign of Nitrogen Dioxide measuring Instruments (CINDI), at Cabauw, NL (51.97° N, 4.93° E). All visible instruments used a common wavelength range and set of cross sections for the spectral analysis. Most of the instruments were of the multi-axis design with analysis by differential spectroscopy software (MAX-DOAS), whose non-zenith slant columns were compared by examining slopes of their least-squares straight line fits to mean values of a selection of instruments, after taking 30-min averages. Zenith slant columns near twilight were compared by fits to interpolated values of a reference instrument, then normalised by the mean of the slopes of the best instruments. For visible MAX-DOAS instruments, the means of the fitted slopes for NO2 and O4 of all except one instrument were within 10% of unity at almost all non-zenith elevations, and most were within 5%. Values for UV MAX-DOAS instruments were almost as good, being 12% and 7%, respectively. For visible instruments at zenith near twilight, the means of the fitted slopes of all instruments were within 5% of unity. This level of agreement is as good as that of previous intercomparisons, despite the site not being ideal for zenith twilight measurements. It bodes well for the future of measurements of tropospheric NO2, as previous intercomparisons were only for zenith instruments focussing on stratospheric NO2, with their longer heritage.

P. Zieger, E. Weingartner, J. Henzing, M. Moerman, G. de Leeuw, J. Mikkilä, M. Ehn, T. Petäjä, K. Clémer, M. van Roozendael, S. Yilmaz, U. Frieß, H. Irie, T. Wagner, R. Shaiganfar, S. Beirle, A. Apituley, K. Wilson, and U. Baltensperger Paul Scherrer Institut, Laboratory of Atmospheric Chemistry, 5232 Villigen, Switzerland Netherlands Organization for Applied Scientific Research TNO, Princetonlaan 6, 3508 Utrecht, The Netherlands Finnish Meteorological Institute, Climate Change Unit, Erik Palmenin Aukio 1, 00101 Helsinki, Finland University of Helsinki, Department of Physics, Gustaf Hällströmin katu 2, 00014 Helsinki, Finland

Over the last years, ground-based multi-axis differential absorption spectroscopy (MAXDOAS) has been proved to be a very promising tool for the automated retrieval of tropospheric pollutants. The MAXDOAS instruments are designed to allow for the quasi simultaneous observation of the scattered sun light in a range of different line-of-sight (LOS) directions from the horizon to the zenith, resulting in an increased sensitivity towards atmospheric absorbers present close to the surface.

(1) Paul Scherrer Institut, Laboratory of Atmospheric Chemistry, Villigen-PSI, Switzerland (paul.zieger@psi.ch, +41563104525), (2) Belgium Institute for Space Aeronomy, BIRA-IASB, Brussels, Belgium, (3) University of Heidelberg, Institute of Environmental Physics, Heidelberg, Germany, (4) Japan Agency for Marine-Earth Science and Technology, Research Institute for Global Change, Yokohama, Japan, (5) TNO, Utrecht, The Netherlands, (6) Finnish Meteorological Institute, Climate Change Unit, Helsinki, Finland, (7) University of Helsinki, Departement of Physics, Helsinki, Finland

We report on the retrieval of aerosol extinction profiles at four wavelengths from ground-based multi-axis differential absorption spectroscopy (MAXDOAS) measurements performed in Beijing, China. Measurements were made over a 10-month time period (June 2008 to April 2009) using a newly developed MAXDOAS instrument. A retrieval algorithm, based on an on-line implementation of the radiative transfer code LIDORT and the optimal estimation technique, has been designed to provide near real time information on aerosol extinction vertical profiles. The algorithm was applied to O-4 measurements at four wavelengths (360, 477, 577, and 630 nm). The total aerosol optical depths (AODs) calculated from the retrieved profiles exhibit higher values in spring and summer and lower values in autumn and winter. Comparison of the retrieved total AODs with values from a co-located CIMEL sunphotometer revealed a good correlation. The best results are obtained for the UV region with a correlation coefficient (R) of 0.91 and a slope of the linear regression fit of 1.1. At the longest wavelength, R drops down to 0.67 and the slope increases to 1.5. The results confirm that good quality O-4 slant column measurements are essential for the success of the retrievals. A method is presented to determine a correction factor to account for systematic errors. It is demonstrated that the algorithm is capable of reliably retrieving aerosol extinction profiles for a wide range of atmospheric conditions (total AODs at 360 nm ranging from about 0.1 to 3). The results open up new perspectives for the extension of the algorithm for the near real time retrieval of trace gas vertical profiles.