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We present a simulation of the global composition of the troposphere which includes the chemistry of halogens (Cl, Br, I). Building on previous work within the GEOS-Chem model we include emissions of inorganic iodine from the oceans, anthropogenic and biogenic sources of halogenated gases, gas phase chemistry, and a parameterised approach to heterogeneous halogen chemistry. Consistent with Schmidt et al. (2016) we do not include sea-salt de-bromination. Observations of halogen radicals (BrO, IO) are sparse but the model has some skill in reproducing these. IO shows both high and low biases in different datasets, BrO concentrations though appear to be modelled low. Comparisons to the very sparse observations dataset of reactive Cl species suggests the model represents a lower limit on impacts due to likely underestimates in emissions and therefore burdens. Inclusion of Cl, Br, I results in a general improvement in simulation of ozone (O3) concentrations, except in polar regions where the model now underestimates O3 concentrations. Halogen chemistry reduces the global tropospheric O3 burden by ∼ 15 %, with the O3 lifetime reducing from 26 days to 22 days. Global mean OH concentrations of 1.34 ×106 molecules cm−3 are 4.5 % lower than in a simulation without halogens, leading to an increase in the CH4 lifetime (6.5 %) due to OH oxidation from 7.48 years to 7.96 years. Oxidation of CH4 by Cl is small (∼ 1 %) but Cl oxidation of other VOCs (ethane, acetone, and propane) can be significant (∼ 9–18 %). Oxidation of VOCs by Br is smaller, representing 2.1 % of the loss of acetaldehyde and 0.6 % of the loss of formaldehyde. [ABSTRACT FROM AUTHOR]

This recommended practice enables the quantification of volatile compounds in flavourings to be made by gas chromatography with flame-ionization detection, without having authentic compounds available, and also in many cases it can avoid time-consuming calibration procedures. The relative-response factors (RRF) can be predicted from the molecular formula of the compound, and this approach can be applied to compounds containing the atoms C, H, O, N, S, F, Cl, Br, I, and Si, providing that the molecular formula and number of benzene rings in the analytes are known. The purity of chemically-defined flavouring substances or chromatographic standards can also be estimated using these predicted RRF, and this procedure can also be used to quantify (poly)hydroxylated compounds, after their derivatization into trimethylsilyl ethers or esters. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We present a global simulation of tropospheric iodine chemistry within the GEOSChem chemical transport model. This includes organic and inorganic iodine sources, standard gas-phase iodine chemistry and simplified higher iodine oxide (I2OX, X = 2, 3,4) chemistry, photolysis, deposition and parametrised heterogeneous reactions. In comparisons with recent Iodine Oxide (IO) observations the iodine simulation shows an average bias of ~ +66% available surface observations in the marine boundary layer (outside of polar regions), and of ~ +73% within the free troposphere (350< hPa <900) over the eastern Pacific. Iodine emissions (3.8 Tgyr-1) are overwhelmingly dominated by the inorganic ocean source, with 76% of this emission from Hypoiodous acid (HOI). HOI is also found to be the dominant iodine species in terms of global tropospheric Iγ burden (contributing up to 70 %). The iodine chemistry leads to a significant global tropospheric O3 burden decrease (9.0 %) compared to standard GEOS-Chem (v9-2). The iodine-driven OX loss rate (748 TgOX yr-1) is by photolysis of HOI (78 %), photolysis of OIO (21 %), and reaction of IO and BrO (1 %). Increases in global mean OH concentrations (1.8 %) by increased conversion of hydroperoxy radicals exceeds the decrease in OH primary production from the reduced O3 concentration. We perform sensitivity studies on a range parameters and conclude that the simulation is sensitive to choices in parameterisation of heterogeneous uptake, ocean surface iodide, and I2OX (X = 2, 3,4) photolysis. The new iodine chemistry combines with previously implemented bromine chemistry to yield a total bromine and iodine driven tropospheric O3 burden decrease of 14.4% compared to a simulation without iodine and bromine chemistry in the model. This is a significant impact and so halogen chemistry needs to be considered in climate and air quality models. [ABSTRACT FROM AUTHOR]

We present an innovative instrument telescope and describe a retrieval method to probe three-dimensional (3-D) distributions of atmospheric trace gases that are relevant to air pollution and tropospheric chemistry. The University of Colorado (CU) two-dimensional (2-D) multi-axis differential optical absorption spectroscopy (CU 2-D-MAX-DOAS) instrument measures nitrogen dioxide (NO2), formaldehyde (HCHO), glyoxal (CHOCHO), oxygen dimer (O2-O2, or O4), and water vapor (H2O); nitrous acid (HONO), bromine monoxide (BrO), and iodine monoxide (IO) are among other gases that can in principle be measured. Information about aerosols is derived through coupling with a radiative transfer model (RTM). The 2-D telescope has three modes of operation: mode 1 measures solar scattered photons from any pair of elevation angle (-20° < EA < +90° or zenith; zero is to the horizon) and azimuth angle (-180° < AA < +180°; zero being north); mode 2 measures any set of azimuth angles (AAs) at constant elevation angle (EA) (almucantar scans); and mode 3 tracks the direct solar beam via a separate view port. Vertical profiles of trace gases are measured and used to estimate mixing layer height (MLH). Horizontal distributions are then derived using MLH and parameterization of RTM (Sinreich et al., 2013). NO2 is evaluated at different wavelengths (350, 450, and 560 nm), exploiting the fact that the effective path length varies systematically with wavelength. The area probed is constrained by O4 observations at nearby wavelengths and has a diurnal mean effective radius of 7.0 to 25 km around the instrument location; i.e., up to 1960 km2 can be sampled with high time resolution. The instrument was deployed as part of the Multi-Axis DOAS Comparison campaign for Aerosols and Trace gases (MADCAT) in Mainz, Germany, from 7 June to 6 July 2013. We present first measurements (modes 1 and 2 only) and describe a four-step retrieval to derive (a) boundary layer vertical profiles and MLH of NO2; (b) near-surface horizontal distributions of NO2; (c) range-resolved NO2 horizontal distribution measurements using an "onion-peeling" approach; and (d) the ratios HCHO to NO2 (RFN), CHOCHO to NO2 (RGN), and CHOCHO to HCHO (RGF) at 14 pre-set azimuth angles distributed over a 360° view. Three-dimensional distribution measurements with 2-D-MAX-DOAS provide an innovative, regional perspective of trace gases as well as their spatial and temporal concentration gradients, and they maximize information to compare near-surface observations with atmospheric models and satellites. [ABSTRACT FROM AUTHOR]