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Published on February 20, 2008

Author: Rinald

Source: authorstream.com

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2003 SOLAS Summer School Introduction to Marine and Atmospheric Chemistry Eric S. Saltzman Earth System Science Univ. of CA, Irvine :  2003 SOLAS Summer School Introduction to Marine and Atmospheric Chemistry Eric S. Saltzman Earth System Science Univ. of CA, Irvine Slide2:  The question: What are the mechanisms by which ocean chemistry impacts atmospheric chemistry and climate? The approach: Understand the atmospheric photochemical system and its relationship to climate Understand natural oceanic emissions/uptake and what controls them Identify critical areas where the atmospheric chemistry system is sensitive to perturbation by oceanic emissions Find the feedbacks between the ocean and atmosphere Composition of the atmosphere:  Composition of the atmosphere Dynamic range in mixing ratio of important chemicals…~1014!!!:  Dynamic range in mixing ratio of important chemicals…~1014!!! Global mean radiative forcing of climate for year 2000 relative to 1750:  Global mean radiative forcing of climate for year 2000 relative to 1750 How does ocean/atmosphere chemical exchange impact radiative forcing?:  How does ocean/atmosphere chemical exchange impact radiative forcing? perturb the tropospheric photochemical system (O3, OH) VOC’s, OVOC’s, RONO2, Cl, Br, I lifetimes of CH4 and other GHG’s lifetimes of O3 depleting substances change the stratospheric ozone layer N2O, CH3Br, CH3Cl, CHBr3?... shortwave absorption stratospheric dynamics alter chemistry, distribution, and optical properties of aerosols seasalt, DMS, I? direct/indirect radiative effects reactive sites for heterogeneous chemistry basics of tropospheric photochemistry...:  basics of tropospheric photochemistry... hydroxyl radical, one of the keys to tropospheric chemistry! 106 cm-3 ,  = ~1 sec what’s the fate of OH?:  what’s the fate of OH? one major pathway... ___________________________ net reaction: HO2  hydroperoxy radical the role of organics is similar ... radical propagation conversion to peroxy radicals H abstraction organoperoxy radical CO and organics, like alkanes, are the fuel for tropospheric photochemistry Tropospheric OH distribution:  Tropospheric OH distribution ·  small dynamic range - the whole range of variability is about 1 order of magnitude! Why? ·   strong OH maximum in the tropics - most photochemically active region of the atmosphere. ·   hemispheric asymmetry - OH higher in N hemisphere because of NOx and O3 distributions. What’s next...depends on NOx levels? :  What’s next...depends on NOx levels? if there’s enough NOx, HO2 regenerates OH and NO2 – “spinning up” to make more O3. NOX is the catalyst. (...if there’s REALLY a lot of NOx it can suppress OH altogether...) if there’s not enough NOx, the radical chemistry destroys HOx and O3 - “spinning down” Slide11:  Tropospheric photochemistry (much abridged) Energy Fuel Catalyst Impact of NOx on tropospheric ozone “tendency”:  Impact of NOx on tropospheric ozone “tendency” Liu et al., 1987;1992 Total column tropospheric ozone :  Total column tropospheric ozone Total column tropospheric ozone shows a maximum in the high latitudes of the summertime hemisphere. This effect is much more distinct in the northern hemisphere than in the southern hemisphere. This seasonality reflects both photochemical production and the dynamics of stratosphere/troposphere exchange. Tropospheric ozone budget:  Tropospheric ozone budget strat/trop exchange 400 Tg/yr net photochemistry: 420 Tg/yr production: 4100 destruction: -3680 dry deposition -820 Tg/yr Wang et al., 1998 note: the photochemical term is the small difference of two large numbers! Tropospheric ozone, cont’d.:  Tropospheric ozone, cont’d. Low MBL O3 ITCZ uplift S/T exchange Biomass burning Alkyl nitrates... an oceanic source of reactive nitrogen to the remote marine atmosphere...:  Alkyl nitrates... an oceanic source of reactive nitrogen to the remote marine atmosphere... Atlantic (Chuck et al. 2002) MeONO2 EtONO2 Pacific (Blake et al., 1999) Acetone: upper tropospheric HOx precursor with an oceanic source?:  Acetone: upper tropospheric HOx precursor with an oceanic source? Zhou and Mopper, 1997 Jaegle et al., 2001 Jacob et al., 2002 photolysis microbial uptake? air/sea exchange convective transport also: H2O peroxides aldehydes Slide18:  Arctic tropospheric ozone depletion Spicer et al., 2002 total depletion of surface ozone over sea ice at polar sunrise correlated with elevated Br2, BrCl “bromine explosion” elevated BrO in Arctic free troposphere? _______________ net: catalytic ozone destruction: Tropospheric halogen chemistry...:  Tropospheric halogen chemistry... sea spray generation mechanisms of halogen activation autocatalysis acid catalysis surface enrichment acid displacement does catalytic ozone destruction occur in the MBL at large? are halogen atoms important oxidants? indirect evidence: hydrocarbon ratios morning ozone loss aerosol halide depletion direct evidence: coastal Cl2 observations Stratospheric ozone...:  Stratospheric ozone... 1 DU = ozone column thickness at STP (273K, 1 atm), in units of 10-2 mm total ozone typically 300 DU (3 mm thick at STP) the troposphere is a small part of total column ozone note units! Stratospheric ozone distribution:  ozone minimum in tropics ozone maximum in high lat NH during spring. SH has mid-latitude maximum during austral spring strong minimum at high lat SH during spring overall NH ozone > SH ozone Stratospheric ozone distribution Stratospheric ozone:  Stratospheric ozone “Chapman” reactions (1930) oxygen-only chemistry Stratospheric ozone:  Stratospheric ozone That might be expected in the lower stratosphere, where the lifetime of ozone is long, but in the upper stratosphere the disagreement indicates that there must be another loss mechanism for ozone. the Chapman reactions do a fairly good job of reproducing the shape of the ozone profile, with a 20 – 30 km maximum. the Chapman reactions significantly overestimate stratospheric ozone levels What’s missing?:  What’s missing? stratospheric HOx sources: catalytic O3 destruction, e.g.: more catalytic cycles...:  more catalytic cycles... stratospheric NOx sources: catalytic O3 destruction, e.g.: more catalytic cycles...:  more catalytic cycles... stratospheric halogen sources: natural – CH3Cl, CH3Br ... short-lived CHxXy species, like CH3I, CH2Br2, CHBr3 man made – CFC’s -11 (CFCl3), -12 (CF2Cl2)... catalytic O3 destruction, e.g.: Br is 40-100x more effective than Cl: The net result is that much less bromine is tied up in reservoir species and more is available for the ozone-destroying cycles. Antarctic ozone hole...:  Antarctic ozone hole... “denoxification” – conversion of NOx to HNO3 liberation of reactive Cl from reservoir species “denitrification” - removal of HNO3 by gravitational settling of PSC’s heterogeneous chemistry on PSC’s: Methyl bromide...:  Production: phytoplankton bacteria? fungi? CH3Br (~2 pM) stratosphere troposphere Loss: ~9 pptv,  = 0.7 yrs Methyl bromide... Global distribution of CH3Br saturation anomaly:  Global distribution of CH3Br saturation anomaly Nine cruises of CH3Br Dg data with a map showing cruise tracks where Dg data was collected: BLAST I, BLAST II, BLAST III, GasEx98, RB-99-06, CLIVAR SR3, GM98A, GM98P, and G99 (a), CH3Br saturation Dg data from the nine cruises plotted against SST and separated by season with spring/summer (b) and fall winter (c). Quadratic fits (─) are used to describe the data. Figure and data from King et al. [2002] and references therein. Slide30:  Modes of interaction with the photochemical system: fuel CO, CH4, NMHC’s ... catalysts – “spin up” or “spin down” troposphere: RONO2, Clx, Brx, Ix, OVOC’s stratosphere: N2O, Clx, Brx, Ix sites for heterogeneous chemistry seasalt, DMS, Ix Clearly, there are many ways for ocean chemistry to impact the atmospheric photochemical system and climate How does the atmosphere impact ocean chemistry...?

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