U. Platt

Short‐lived alkyl iodides and bromides at Mace Head, Ireland: Links to biogenic sources and halogen oxide production

Automated in situ gas chromatograph/mass spectrometer (GC/MS) measurements of a range of predominantly biogenic alkyl halides in air, including CHBr3, CHBr2Cl, CH3Br, C2H5Br, CH3I, C2H5I, CH2ICl, CH2I2, and the hitherto unreported CH2IBr were made at Mace Head during a 3-week period in May 1997. C3H7I and CH3CHICH3 were monitored but not detected. Positive correlations were observed between the polyhalomethane pairs CHBr3/CHBr2Cl and CHBr3/CH2IBr and between the monohalomethane pair CH3I/C2H5I, which are interpreted in terms of common or linked marine sources. During periods when air masses were affected by emissions from local seaweed beds, the concentrations of CHBr3, CH2ICl, and CH2IBr not only showed remarkable correlation but also maximized at low water. These are the first field observations to provide evidence for a link between the tidal cycle, polyhalomethanes in air, and potential marine production. The calculated total flux of iodine atoms into the boundary layer at Mace Head from organic gaseous precursors was dominated by photolytic destruction of CH2I2. Photolysis of CH3I contributed less than 3%. The calculated peak flux of iodine atoms during the campaign coincided with the highest measured levels of iodine oxide radicals, as determined using Differential Optical Absorption Spectrometry (DOAS).

Spectroscopic measurement of bromine oxide and ozone in the high Arctic during Polar Sunrise Experiment 1992

Bromine oxide (BrO) is proposed to be an important agent for tropospheric ozone depletion, as observed in the high Arctic during springtime. In this paper we report measurements of bromine oxide and ozone by Long Path Differential Optical Absorption Spectroscopy (LPDOAS), 8.6-km light path), performed in April 1992 in Alert (82.3°N, 62.2°W). BrO mixing ratios were found between the detection limit of about 4 ppt to 17 ppt. Because of the frequently poor visibility conditions, especially during ozone depletion events, long-signal integration times (sometimes more than 24 hours) were needed, and short-time BrO-peaks might have escaped detection. A pure in situ chemical mechanism based on BrO-catalyzed ozone destruction cannot account for the observed complete depletion of ozone at the observed BrO mixing ratios. On the other hand, it can be argued that the maximum time for chemical ozone depletion (by any mechanism) may not be much longer than 1 day. A simple scenario involving a combination of advection, atmospheric dispersion, and BrO-catalyzed chemical ozone destruction is described, which could explain the observed ozone loss.

Iodine oxide in the marine boundary layer

A striking example of the influence of halogen chemistry on tropospheric ozone levels is the episodic destruction of boundary-layer ozone during the Arctic sunrise by reactive halogen species, . We detected iodine oxide in the boundary layer at Mace Head, Ireland (53°20′ N, 9°54′ W) during May 1997, which indicates that iodine chemistry is occurring in the troposphere.

Numerical analysis and estimation of the statistical error of differential optical absorption spectroscopy measurements with least-squares methods

Differential optical absorption spectroscopy (DOAS) has become a widely used method to measure trace gases in the atmosphere. Their concentration is retrieved by a numerical analysis of the atmospheric absorption spectra, which often are a combination of overlapping absorption structures of several trace gases. A new analysis procedure was developed, modeling atmospheric spectra with the absorption structures of the individual trace gases, to determine their concentrations. The procedure also corrects differences in the wavelength-pixel mapping of these spectra. A new method to estimate the error of the concentrations considers the uncertainty of this correction and the influence of random residual structures in the absorption spectra.

Detection of bromine monoxide in a volcanic plume

The emission of volcanic gases usually precedes eruptive activity, providing both a warning signal and an indication of the nature of the lava soon to be erupted. Additionally, volcanic emissions are a significant source of gases and particles to the atmosphere, influencing tropospheric and stratospheric trace-gas budgets. Despite some halogen species having been measured in volcanic plumes (mainly HCl and HF), little is known about bromine compounds and, in particular, gas-phase reactive bromine species. Such species are especially important in the stratosphere, as reactive bromine—despite being two orders of magnitude less abundant than chlorine—accounts for about one-third of halogen-catalysed ozone depletion. In the troposphere, bromine-catalysed complete ozone destruction has been observed to occur regularly during spring in the polar boundary layers as well as in the troposphere above the Dead Sea basin. Here we report observations of BrO and SO2 abundances in the plume of the Soufrière Hills volcano (Montserrat) in May 2002 by ground-based multi-axis differential optical absorption spectroscopy. Our estimate of BrO emission leads us to conclude that local ozone depletion and small ozone ‘holes’ may occur in the vicinity of active volcanoes, and that the amount of bromine emitted from volcanoes might be sufficiently large to play a role not only in the stratosphere, but also in tropospheric chemistry.

Satellite chartography of atmospheric methane from SCIAMACHY on board ENVISAT: 2. Evaluation based on inverse model simulations

We extend the analysis of a global CH_4 data set retrieved from SCIAMACHY (Frankenberg et al., 2006) by making a detailed comparison with inverse TM5 model simulations for 2003 that are optimized versus high accuracy CH_4 surface measurements from the NOAA ESRL network. The comparison of column averaged mixing ratios over remote continental and oceanic regions shows that major features of the atmospheric CH_4 distribution are consistent between SCIAMACHY observations and model simulations. However, the analysis suggests that SCIAMACHY CH_4 retrievals may have some bias that depends on latitude and season (up to ∼30 ppb). Large enhancements of column averaged CH_4 mixing ratios (∼50–100 ppb) are observed and modeled over India, Southeast Asia, and the tropical regions of South America, and Africa. We present a detailed comparison of observed spatial patterns and their seasonal evolution with TM5 1° × 1° zoom simulations over these regions. Application of a new wetland inventory leads to a significant improvement in the agreement between SCIAMACHY retrievals and model simulations over the Amazon basin during the first half of the year. Furthermore, we present an initial coupled inversion that simultaneously uses the surface and satellite observations and that allows the inverse system to compensate for the potential systematic bias. The results suggest significantly greater tropical emissions compared to either the a priori estimates or the inversion based on the surface measurements only. Emissions from rice paddies in India and Southeast Asia are relatively well constrained by the SCIAMACHY data and are slightly reduced by the inversion.

Measurement of nitrate radical concentrations in continental air

Nighttime profiles of the atmospheric concentrations of the nitrate radical (NO/sub 3/), NO/sub 2/, and O/sub 3/ have been obtained by using a mobile differential ultraviolet/visible absorption spectrometer over path lengths from 3 to 17 km. Measurements were carried out at four semiarid/desert sites in the southern California desert (Death Valley, Edwards Air Force Base, Phelan, and Whitewater), and nitrate radicals were observed at all four locations at typical concentrations of approx. 10-100 parts per trillion. A decrease in the calculated NO/sub 3/ lifetime with increasing relative humidity (RH) was observed with lifetimes up to approx. 60 min for less than or equal to50% RH, but less than or equal to10 min for greater than or equal to50% RH. This suggests the involvement of water in the loss process for NO/sub 3/, either in the gas phase or in the aqueous state at the ground or on aerosol surfaces. These experimental field data support our earlier hypothesis that this loss process of NO/sub 3/ may constitute both a major sink for atmospheric NO/sub x/ and a significant formation route for nitric acid, a key component of acid rain and fog.

Observations of nitrous acid in the Los Angeles atmosphere and implications for predictions of ozone-precursor relationships.

Direct measurements of nitrous acid (HONO) were made in downtown Los Angeles and Riverside, CA, during night and early morning hours of July/August 1980 using a long-path differential optical absorption spectrometer. Up to 8 ppb of HONO were observed in Los Angeles, approximately twice the maximum levels previously measured in Riverside during the summer of 1979. Possible sources of the observed HONO are discussed. If the observed HONO levels are included in initial NO, concentration, EKMA isopleth calculations predict that more rigorous control of NO, emissions (especially a t low HC/NO, levels) or of hydrocarbons emissions is necessary to reduce ozone maxima by a given amount compared with predictions based on calculations neglecting initial HONO. Moreover, including HONO in the starting NO, leads to predictions of accelerated rates of oxidant production which results in much larger predicted O3 doses at elevated O3 levels. For example, the predicted O3 dosage at levels above 0.3 ppm ozone in the case of NMHC = 1 ppm and [NO,], = 0.12 ppm is increased by over 250% when 10 ppb of HONO is taken to be initially present.

Satellite mapping of enhanced BrO concentrations in the troposphere

Reactive bromine species contribute significantly to the destruction of ozone in the polar stratosphere. Reactive halogen compounds can have a strong effect not only on the chemistry of the stratosphere but also on that of the underlying troposphere. For example, severe ozone depletion events that are less persistent than those in the stratosphere occur in the Arctic and Antarctic boundary layer during springtime and are also associated with enhanced BrO abundances,. Observations of BrO (and ClO, which is less important) at ground level during these ozone depletion events have revealed halogen oxide mixing ratios of up to 30 parts per trillion—sufficient to destroy within one to two days the 30–40 parts per billion of ozone typically present in the boundary layer. The catalytic mechanism leading to so-called ‘tropospheric ozone holes’ is well established,, but the origin of the increased BrO concentrations and the spatial and temporal extent of these events remains poorly understood. Here we present satellite observations showing that tropospheric air masses enriched in BrO are always situated close to sea ice and typically extend over areas of about 300–2,000 km. The BrO abundances remain enhanced forperiods of 1 to 3 days. These observations support the suggestion,, that autocatalytic release of bromine from sea salt gives rise to significant BrO formation which, in turn, initiates ozone depletion in the polar troposphere.

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.