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Iodine's impact on tropospheric oxidants: A global model study in GEOS-Chem

AutorSherwen, T.; Evans, M.J.; Carpenter, L.J.; Andrews, S.J.; Lidster, R.T.; Dix, B.; Koenig, T.K.; Sinreich, R.; Ortega, I.; Volkamer, R.; Saiz-Lopez, A. ; Prados-Roman, C.; Mahajan, A.S.; Ordóñez, C.
Fecha de publicación2016
EditorEuropean Geophysical Society
CitaciónAtmospheric Chemistry and Physics 16: 1161- 1186 (2016)
ResumenWe present a global simulation of tropospheric iodine chemistry within the GEOS-Chem chemical transport model. This includes organic and inorganic iodine sources, standard gas-phase iodine chemistry, and simplified higher iodine oxide (I2O<i>X</i>, <i>X</i> Combining double low line 2, 3, 4) chemistry, photolysis, deposition, and parametrized heterogeneous reactions. In comparisons with recent iodine oxide (IO) observations, the simulation shows an average bias of g1/4 +90g€¯% with available surface observations in the marine boundary layer (outside of polar regions), and of g1/4 +73g€¯% within the free troposphere (350g€¯hPag€¯ < g€¯<i>p</i>g€¯ < g€¯900g€¯hPa) over the eastern Pacific. Iodine emissions (3.8g€¯Tg yr−1) are overwhelmingly dominated by the inorganic ocean source, with 76g€¯% of this emission from hypoiodous acid (HOI). HOI is also found to be the dominant iodine species in terms of global tropospheric I<i>Y</i> burden (contributing up to 70g€¯%). The iodine chemistry leads to a significant global tropospheric O3 burden decrease (9.0g€¯%) compared to standard GEOS-Chem (v9-2). The iodine-driven O<i>X</i> loss rate1 (748g€¯Tgg€¯O<i>X</i>g€¯yrg'1) is due to photolysis of HOI (78g€¯%), photolysis of OIO (21g€¯%), and reaction between IO and BrO (1g€¯%). Increases in global mean OH concentrations (1.8g€¯%) 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 of parameters and conclude that the simulation is sensitive to choices in parametrization of heterogeneous uptake, ocean surface iodide, and I2O<i>X</i> (<i>X</i> Combining double low line 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 14.4g€¯% compared to a simulation without iodine and bromine chemistry in the model, and a small increase in OH (1.8g€¯%). This is a significant impact and so halogen chemistry needs to be considered in both climate and air quality models.<br><br> 1 Here O<i>X</i> is defined as O3 + NO2 + 2NO3 + PAN + PMN+PPN + HNO4 + 3N2O5 + HNO3 + BrO + HOBr + BrNO2+2BrNO3 + MPN + IO + HOI + INO2 + 2INO3 + 2OIO+2I2O2 + 3I2O3 + 4I2O4, where PANg€¯ Combining double low line g€¯peroxyacetyl nitrate, PPNg€¯ Combining double low line g€¯peroxypropionyl nitrate, MPNg€¯ Combining double low line g€¯methyl peroxy nitrate, and MPNg€¯ Combining double low line g€¯peroxymethacryloyl nitrate.
Identificadoresdoi: 10.5194/acp-16-1161-2016
issn: 1680-7324
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