Aircraft and satellite observations indicate the presence of ppt (ppt ≡ pmol/mol) levels of BrO in the free troposphere with important implications for the tropospheric budgets of ozone, OH, and mercury. We can reproduce these observations with the GEOS-Chem global tropospheric chemistry model by including a broader consideration of multiphase halogen (Br-Cl) chemistry than has been done in the past. Important reactions for regenerating BrO from its nonradical reservoirs include HOBr + Br − /Cl − in both aerosols and clouds, and oxidation of Br − by ClNO 3 and ozone. Most tropospheric BrO in the model is in the free troposphere, consistent with observations and originates mainly from the photolysis and oxidation of ocean-emitted CHBr 3 . Stratospheric input is also important in the upper troposphere. Including production of gas phase inorganic bromine from debromination of acidified sea salt aerosol increases free tropospheric Br y by about 30%. We find HOBr to be the dominant gas-phase reservoir of inorganic bromine. Halogen (Br-Cl) radical chemistry as implemented here in GEOS-Chem drives 14% and 11% decreases in the global burdens of tropospheric ozone and OH, respectively, a 16% increase in the atmospheric lifetime of methane, and an atmospheric lifetime of 6 months for elemental mercury. The dominant mechanism for the Br-Cl driven tropospheric ozone decrease is oxidation of NO x by formation and hydrolysis of BrNO 3 and ClNO 3 .

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Abstract. This overview paper highlights the successes of the Ozone Monitoring Instrument (OMI) on board the Aura satellite spanning a period of nearly 14 years. Data from OMI has been used in a wide range of applications and research resulting in many new findings. Due to its unprecedented spatial resolution, in combination with daily global coverage, OMI plays a unique role in measuring trace gases important for the ozone layer, air quality, and climate change. With the operational very fast delivery (VFD; direct readout) and near real-time (NRT) availability of the data, OMI also plays an important role in the development of operational services in the atmospheric chemistry domain.

We present a self-consistent empirical model for several plasma parameters of a polar coronal hole near solar minimum, derived from observations with the Solar and Heliospheric Observatory Ultraviolet Coronagraph Spectrometer. The model describes the radial distribution of density for electrons, H , and O and the outflow of O are also significantly larger than the corresponding velocities of H . We discuss the constraints and 5ϩ 0 implications on various theoretical models of coronal heating and acceleration.

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Abstract. We present and discuss the Smithsonian Astrophysical Observatory (SAO) formaldehyde (H 2 CO) retrieval algorithm for the Ozone Monitoring Instrument (OMI) which is the operational retrieval for NASA OMI H 2 CO. The version of the algorithm described here includes relevant changes with respect to the operational one, including differences in the reference spectra for H 2 CO, the fit of O 2 -O 2 collisional complex, updates in the high-resolution solar reference spectrum, the use of a model reference sector over the remote Pacific Ocean to normalize the retrievals, an updated air mass factor (AMF) calculation scheme, and the inclusion of scattering weights and vertical H 2 CO profile in the level 2 products. The setup of the retrieval is discussed in detail. We compare the results of the updated retrieval with the results from the previous SAO H 2 CO retrieval. The improvement in the slant column fit increases the temporal stability of the retrieval and slightly reduces the noise. The change in the AMF calculation has increased the AMFs by 20 %, mainly due to the consideration of the radiative cloud fraction. Typical values for retrieved vertical columns are between 4 × 10 15 and 4 × 10 16 molecules cm −2 , with typical fitting uncertainties ranging between 45 and 100 %. In high-concentration regions the errors are usually reduced to 30 %. The detection limit is estimated at 1 × 10 16 molecules cm −2 .

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