CHEM-5151_S05_Nitrogen

CHEM-5151_S05_Nitrogen - Chemistry of Inorganic Nitrogen...

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1 Chemistry of Inorganic Nitrogen Compounds Edward Dunlea, Jose-Luis Jimenez Atmospheric Chemistry CHEM-5151/ATOC-5151 Required Reading: Finlayson-Pitts and Pitts Chapter 7 Other Possible Reading: Seinfeld and Pandis Chapter 5 Outline Introduction Oxidation of NO to NO 2 and the Leighton Relationship Oxidation of NO 2 Atmospheric Chemistry of HONO Reactions of NO 3 and N 2 O 5 Atmospheric Chemistry of HNO 3 “Missing NOy” Ammonia Bonus Material
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2 Motivation • Inorganic nitrate compounds intimately involved in important chemical processes in both troposphere and stratosphere –NO 2 is key to formation of tropospheric ozone • Both in polluted and background troposphere –HNO 3 key ingredient in acid rain 3 is primary night time oxidant – Participate with halogens in O 3 destruction chemistry in stratosphere • Regulate chain length of O 3 -destroying reactions • Fascinating chemistry + unresolved issues Intro Diagram of NOy • Stable species in circles • Major pathways denoted by arrows • Will use this diagram throughout Intro
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3 Definitions • NOx = NO + NO 2 – These are the key nitrogen containing species – Rapid interconversion between NO & NO 2 in stratosphere and troposphere • NOy = NO + NO 2 + HNO 3 + PAN + HONO + NO 3 + 2N 2 O 5 + organic nitrates (RNO 3 ) + particulate nitrate (pNO 3 - ) + … – NOx + all of its reservoir species • NOz = NOy – NOx – Just the reservoir species Intro Breaking Down NOy • NOx = NO + NO 2 –Rap id interconversion • NOy = NOx + reservoir species • NOz = reservoir species Intro
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4 Sources and Sinks • Sources of NOy primarily directly emitted NO + NO 2 – Combustion sources • Mobile – planes, trains and automobiles • Stationary - power plants, industrial – Natural – Biomass burning, lightning • Sinks of NOy lost through HNO 3 – Washout of HNO 3 – Dry deposition – Incorporation into aerosols as nitrate (pNO 3 - ) followed by loss of particles Intro Typical NOy Concentrations Remote areas NOy ~ 1 ppb Rural areas NOy ~ 1-20 ppb Moderately polluted NOy ~ 0.02-0.2 ppm Heavily polluted NOy ~ 0.2-0.5 ppm Typical OH < 1 ppt Typical tropospheric O 3 ~ 50 ppb Mexico City NOy = 0.1-0.4 ppm Tropospheric CH 4 = 1.7 ppm Intro
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5 Overall Look at Oxidation Smog chamber experiment to simulate atmospheric oxidation Start with NO and NO 2 Over time oxidized to HNO 3 and PAN Ratio of PAN/HNO 3 depends on initial VOC/NOx ratio Some HNO 3 lost (in this case to chamber walls) Intro From Finlayson-Pitts and Pitts Oxidation of NO to NO 2 • First guess in old days: – Reaction 2 NO + O 2 Æ 2 NO 2 responsible for NO oxidation in polluted urban areas – Not the case – too slow at typical [NO] – Does occur in power plant plumes with very high [NO] • Now known: peroxy radicals responsible for NO oxidation –NO + HO 2 Æ OH + NO 2 + RO 2 Æ RO + NO 2 • Leighton relationship: – Assume hypothetical atmosphere – NO, NO 2 and air (no organics!) – Also called “photostationary state” – Continued on next slide… A
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6 “Photostationary State” •NO 2 + h ν Æ NO + O
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CHEM-5151_S05_Nitrogen - Chemistry of Inorganic Nitrogen...

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