F. Wittrock

GOME observations of tropospheric BrO in northern hemispheric spring and summer 1997

Measurements from the Global Ozone Monitoring Experiment GOME have been analysed for tropospheric BrO in the northern hemispheric spring and summer 1997. Tropospheric excess columns have been determined by subtracting measurements from a longitude range which is assumed to represent background conditions. From February until the end of May enhanced tropospheric BrO columns are observed over the Hudson Bay area and parts of the Canadian Arctic. This large and persistent event has not been reported before and can only be explained by a large local source of bromine. In addition, from March to May other smaller and shorter tropospheric BrO events are detectable along the coast lines of the Arctic Sea and over the polar ice. They correspond to the ground-based observations of enhanced tropospheric BrO reported from several stations in the high Arctic.

Simultaneous global observations of glyoxal and formaldehyde from space

[1] The first global simultaneous observations of glyoxal (CHOCHO) and formaldehyde (HCHO) columns retrieved from measurements by the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) satellite instrument are presented and compared to model calculations. The global pattern of the distribution of CHOCHO is similar to that of HCHO. High values are observed over areas with large biogenic isoprene emissions (Central Africa, parts of South America, and Indonesia). Also regions with biomass burning and anthropogenic pollution exhibit elevated levels of CHOCHO. The ratio of the columns of CHOCHO to HCHO is generally of the order of 0.05 in regions having biogenic emissions, which is in reasonable agreement with the current understanding of the oxidation of hydrocarbons emitted by the biosphere. However and in contrast to our model, high values of both HCHO and CHOCHO are also observed over areas of the tropical oceans. This is tentatively attributed to outflow from the continents and local oceanic biogenic sources of the precursors of HCHO and CHOCHO. Citation: Wittrock, F., A. Richter, H. Oetjen, J. P. Burrows, M. Kanakidou, S. Myriokefalitakis, R. Volkamer, S. Beirle, U. Platt, and T. Wagner (2006), Simultaneous global observations of glyoxal and formaldehyde from space, Geophys. Res. Lett., 33, L16804, doi:10.1029/2006GL026310.

Validation of Ozone Monitoring Instrument nitrogen dioxide columns

[1] We review the standard nitrogen dioxide (NO2) data product (Version 1.0.), which is based on measurements made in the spectral region 415–465 nm by the Ozone Monitoring Instrument (OMI) on the NASA Earth Observing System-Aura satellite. A number of ground- and aircraft-based measurements have been used to validate the data product’s three principal quantities: stratospheric, tropospheric, and total NO2 column densities under nearly or completely cloud-free conditions. The validation of OMI NO2 is complicated by a number of factors, the greatest of which is that the OMI observations effectively average the NO2 over its field of view (minimum 340 km 2 ), while a ground-based instrument samples at a single point. The tropospheric NO2 field is often very inhomogeneous, varying significantly over tens to hundreds of meters, and ranges from 10 16 cm � 2 over urban and industrial areas. Because of OMI’s areal averaging, when validation measurements are made near NO2 sources the OMI measurements are expected to underestimate the ground-based, and this is indeed seen. Further, we use several different instruments, both new and mature, which might give inconsistent NO2 amounts; the correlations between nearby instruments is 0.8–0.9. Finally, many of the validation data sets are quite small and span a very short length of time; this limits the statistical conclusions that can be drawn from them. Despite these factors, good agreement is generally seen between the OMI and ground-based measurements, with OMI stratospheric NO2 underestimated by about 14% and total and tropospheric columns underestimated by 15–30%. Typical correlations between OMI NO2 and ground-based measurements are generally >0.6.

GOME MEASUREMENTS OF STRATOSPHERIC AND TROPOSPHERIC BrO

Measurements from the Global Ozone Monitoring Experiment (GOME) have been analysed for BrC absorption using the Differential Optical Absorption (DOAS) method. By introducing a correction for a small angle dependency of the diffisor used for the direct sun measurements in the GOME instrument, the overall consistency of the BrO data set could be improved significantly. Evidence is found for large tropospheric contributions to the BrO columns measured by GOME, both from BrO in the polar boundary layer in spring and a global BrO background, probably located in the free troposphere and present throughout the year. The latter has been further investigated by comparing BrO and 0, columns above the remote Pacific, resulting in an estimate of 0.5 - 2ppt of uniformly mixed BrO in the troposphere, in agreement with previous studies. 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

Intercomparison of BrO measurements from ERS-2 GOME, ground-based and balloon platforms

The consistency of BrO column amounts derived from GOME spectra and from correlative ground-based and balloon measurements performed in 1998-1999 during the Third European Stratospheric Experiment on Ozone (THESEO) has been investigated. The study relies on W-visible observations at several mid- and high latitude ground-based stations in both hemispheres, complemented by balloon-borne solar occultation profile measurements and 3D chemical transport model simulations. Previous investigations have reported GOME BrO columns systematically larger than those deduced from balloon, suggesting BrO being present, possibly ubiquitously, in the free troposphere. The robustness of this hypothesis has been further tested based on the presently available correlative data set. It is shown that when accounting for the BrO diurnal variation and the solar zenith angle dependency of the sensitivity of correlative data to the troposphere, measurements from all platforms are consistent with the presence of a tropospheric BrO background of 1-3 ~10’~ mole&m’ extending over mid- and high

Testing and improving OMI DOMINO tropospheric NO2 using observations from the DANDELIONS and INTEX‐B validation campaigns

We present a sensitivity analysis of the tropospheric NO2 retrieval from the Ozone Monitoring Instrument (OMI) using measurements from the Dutch Aerosol and Nitrogen Dioxide Experiments for Validation of OMI and SCIAMACHY (DANDELIONS) and Intercontinental Chemical Transport Experiment-B (INTEX-B) campaigns held in 2006. These unique campaigns covered a wide range of pollution conditions and provided detailed information on the vertical distribution of NO2. During the DANDELIONS campaign, tropospheric NO2 profiles were measured with a lidar in a highly polluted region of the Netherlands. During the INTEX-B campaign, NO2 profiles were measured using laser-induced fluorescence onboard an aircraft in a range of meteorological and polluted conditions over the Gulf of Mexico and the east Pacific. We present a comparison of measured profiles with a priori profiles used in the OMI tropospheric NO2 retrieval algorithm. We examine how improvements in surface albedo estimates improve the OMI NO2 retrieval. From these comparisons we find that the absolute average change in tropospheric columns retrieved with measured profiles and improved surface albedos is 23% with a standard deviation of 27% and no trend in the improved being larger or smaller than the original. We show that these changes occur in case studies related to pollution in the southeastern United States and pollution outflow in the Gulf of Mexico. We also examine the effects of using improved Mexico City terrain heights on the OMI NO2 product.

Comparison of measurements and model calculations of stratospheric bromine monoxide

Ground-based zenith sky UV-visible measurements of stratospheric bromine monoxide (BrO) slant column densities are compared with simulations from the SLIMCAT three-dimensional chemical transport model. The observations have been obtained from a network of 11 sites, covering high and midlatitudes of both hemispheres. This data set gives for the first time a near-global picture of the distribution of stratospheric BrO from ground-based observations and is used to test our current understanding of stratospheric bromine chemistry. In order to allow a direct comparison between observations and model calculations, a radiative transfer model has been coupled to the chemical model to calculate simulated slant column densities. The model reproduces the observations in general very well. The absolute amount of the BrO slant columns is consistent with a total stratospheric bromine loading of 20 ± 4 ppt for the period 1998-2000, in agreement with previous estimates. The seasonal and latitudinal variations of BrO are well reproduced by the model. In particular, the good agreement between the observed and modeled diurnal variation provides strong evidence that the BrO-related bromine chemistry is correctly modeled. A discrepancy between observed and modeled BrO at high latitudes during events of chlorine activation can be resolved by increasing the rate constant for the reaction BrO + ClO → BrCl + O 2 to the upper limit of current recommendations. However, other possible causes of the discrepancy at high latitudes cannot be ruled out.

Multiwavelength lidar measurements of stratospheric aerosols above Spitsbergen during winter 1992/93

Within the period of December 1992 to March 1993 lidar investigations of stratospheric aerosols were performed at Ny-Alesund, Spitsbergen (79°N, 12°E). Backscatter signals at wavelengths of 353, 532 and 1064 nm and depolarization signals at 532 nm in altitudes ranging from the tropopause up to 30 km were analyzed. Throughout the whole measurement period we observed an aerosol layer of volcanic origin in the lower stratosphere. Depolarization profiles suggest that the volcanic aerosol layer consisted mainly of liquid droplets. Comparison with model calculations indicate median particle radii between 0.1 and 0.2 µm. Surface densities exceeded approximately 40 µm²/cm³ in the lower part of the layer around 12 km. In January 1993 polar stratospheric clouds (PSCs) were frequently observed at altitudes up to 22 km. We analyzed the backscatter and depolarization data with respect to the temperature history of backward trajectories reaching Ny-Alesund. Signatures for micron sized crystalline PSC particles appear for cooling rates of −1 to −3 K day−1. Larger cooling rates of −4 to −10 K day −1 produced submicron sized aerosols, presumably supercooled droplets, characterized by enhanced backscatter ratios and reduced depolarization.

Exploring the missing source of glyoxal (CHOCHO) over China

[1] Recent comparisons between satellite observed and global model simulated glyoxal (CHOCHO) have consistently revealed a large unknown source of CHOCHO over China. We examine this missing CHOCHO source by analyzing SCIAMACHY observed CHOCHO vertical column densities (VCDs) using a Regional chEmical trAnsport Model (REAM). This missing source is first quantified by the difference between SCIAMACHY observed and REAM simulated CHOCHO VCDs (DCCHOCHO), which have little overlap with high biogenic isoprene emissions but are collocated with dense population and high anthropogenic NOx and VOC emissions. We then apply inverse modeling to constrain CHOCHO precursor emissions based on SCIAMACHY CHOCHO and find that this missing source is most likely caused by substantially underestimated aromatics emissions (by a factor of 4–10, varying spatially) in the VOC emission inventories over China used in current regional and global models. Comparison with in situ observations in Beijing, Shanghai, and a site in the Pearl River Delta shows that the large model biases in aromatics concentrations are greatly reduced after the inversion. The top-down estimated aromatics emission is 13.4 Tg yr � 1 in total, about 6 times the bottom-up estimate (2.4 Tg yr � 1 ). The resulting impact on regional oxidant levels is large (e.g., � 100% increase of PAN in the afternoon). Furthermore, since aromatics are important precursors of secondary organic aerosol (SOA), such an increase of aromatics could lead to � 50% increase of global aromatic SOA production and thereby help to reduce the low bias of simulated organic aerosols over the region in previous modeling studies. Citation: Liu, Z., et al. (2012), Exploring the missing source of glyoxal (CHOCHO) over China, Geophys. Res. Lett., 39, L10812, doi:10.1029/2012GL051645.

Measurements of iodine monoxide (IO) above Spitsbergen

Zenithskymeasurementsofscatteredlighthave been made at Ny-Alesund, Spitsbergen (79N, 12E) since thebeginningof1995. Adierentialopticalabsorptionspec- troscopy (DOAS)algorithm hasbeenusedtoretrievedier- ential slant column densities (DSCD) of IO for the observa- tionsfrom1995-1998. IOwasdetectedeveryyearforseveral days within each month. Model simulations result in an es- timatedstratosphericIOmixingratiointherangefrom0.65 to 0.80 (0:2) pptv in polar spring 1997. In addition, indi- cations for the presence of tropospheric IO have been found in late spring and summer of each year.