Tropospheric Ozone and Air Quality AOSC 433/633 & CHEM 433/633 Ross Salawitch. Why do we care?

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1 Tropospheric Ozone and Air Quality AOSC 433/633 & CHEM 433/633 Ross Salawitch Class Web Site: Today: Tropospheric ozone production mechanism (CO, NO x, and VOCs) Recent improvements of air quality Coupling of meteorology, and perhaps climate change, to air quality Lecture March Why do we care? Many thousands of deaths attributed to London Smog of 1952: 2

2 Why do we care? Today, epidemiologists relate many thousands of deaths (annually) to air pollution Table 2. Decreases in ozone (the population-weighted annual average 8-h daily maximum) and premature mortalities when European emissions are removed, for eight NH regions. Duncan et al., Atmos. Chem. Phys., Air Quality Standards and Why We Care Year Averaging Period EPA Surface Ozone Standard hr 125 ppb hr 85 ppb hr * 75 ppb 2013 #?????? * The 8 hr standard is met when the 3-yr average of the annual 4 th highest daily maximum 8 hr O 3 is less than 75 ppb. Increased risk of premature mortality for even low levels of surface O 3 ; further reductions will benefit public health Bell et al., # In 2011Obama directed EPA to postpone revising surface O 3 standard so regulation would not interfere with economic recovery: On 6 March 2013, Obama nominated Gina McCarthy to be EPA administrator: 4

3 Tropospheric Ozone & Air Quality Criteria Pollutants From Chapter 1 Chemistry in Context Criteria pollutant: common-place and detrimental to human welfare (i.e., ubiquitous pollutant) 5 Tropospheric Ozone Production OH + CO CO 2 + H H + O 2 + M HO 2 + M NO + HO 2 NO 2 + OH NO 2 + hν NO + O O + O 2 + M O 3 + M Net: CO + 2 O 2 CO 2 + O 3 NO & NO 2 : Emitted by fossil fuel combustion & biomass burning N 2 + O High T 2 2 NO CO: Emitted by fossil fuel combustion & biomass burning Complete combustion: 2 C 8 H O 2 16 CO H 2 O Extreme, incomplete combustion: 2 C 8 H O 2 16 CO + 18 H 2 O OH:???? 6

4 Tropospheric Ozone Production Suppose NO is converted to NO 2 by reaction with O 3 : OH + CO CO 2 + H H + O 2 +M HO 2 + M NO + O 3 NO 2 + O 2 NO 2 + hν NO + O O + O 2 + M O 3 + M Net:???????????????? 7 Tropospheric Ozone Production OH + CO CO 2 + H H + O 2 + M HO 2 + M HO 2 + NO OH + NO 2 NO 2 + hν NO + O O + O 2 + M O 3 + M Net: CO + 2 O 2 CO 2 + O 3 Chain Mechanism for production of ozone Chemical Initiation: H 2 O+O( 1 D) 2OH & human emission of NO, CO Since method for conversion of NO to NO 2 is crucial for whether O 3 is produced by this chain mechanism, chemists consider production of tropospheric ozone to be limited by k[ho 2 ][NO] 8

5 Tropospheric Ozone Production CO + OH CO 2 + H H + O 2 +M HO 2 + M HO 2 + NO OH + NO 2 NO 2 + hν NO +O O+ O 2 + M O 3 + M Net: CO + 2 O 2 CO 2 + O 3 RH + OH R+ H 2 O R + O 2 +M RO 2 + M RO 2 + NO RO + NO 2 RO + O 2 HO 2 +R CHO HO 2 + NO OH + NO 2 2 NO 2 + hν NO +O 2 O+ O 2 + M O 3 + M Net: RH + 4O 2 R CHO + H 2 O + 2 O 3 VOC: Volatile Organic Compounds Produced by trees and fossil fuel vapor Strong source of HO x (OH & HO 2 ) & O 3 (depending on NO x levels) Examples of RH and R CHO : CH 4 (methane) CH 2 O (formaldehyde) : C 2 H 6 (ethane) CH 3 CHO (acetaledhyde) : C 3 H 8 (propane) CH 3 COCH 3 (acetone) Ozone Production limited by k[ho 2 ][NO] + k i [RO 2 ] i [NO] 9 Tropospheric Ozone Production CO + OH CO 2 + H H + O 2 +M HO 2 + M HO 2 + NO OH + NO 2 NO 2 + hν NO +O O+ O 2 + M O 3 + M Net: CO + 2 O 2 CO 2 + O 3 RH + OH R+ H 2 O R + O 2 +M RO 2 + M RO 2 + NO RO + NO 2 RO + O 2 HO 2 +R CHO HO 2 + NO OH + NO 2 2 NO 2 + hν NO +O 2 O+ O 2 + M O 3 + M Net: RH + 4O 2 R CHO + H 2 O + 2 O 3 Chain Mechanism for production of ozone Chemical Initiation: Human emission of NO, CO and either human (RO 2 ) or natural (HO 2 ) hydrogen radicals Ozone production: k[ho 2 ][NO] Termination: can occur via either: HO 2 + HO 2 H 2 O 2 +O 2 or OH + NO 2 + M HNO 3 + M 10

6 Tropospheric Ozone Production versus NO As NO x rises: [HO 2 ] falls faster than [NO] rises, leading to a decrease in the value of the product of k [HO 2 ] [NO], and hence the production rate of O 3. Illustrative calculation of the dependence of O 3 production on [NO] This curve has key policy implications! 11 Tropospheric Ozone Production versus NO x and VOCs NO x Emission Rate (10 12 molec cm -2 s -1 ) VOC limited regime NO x limited regime Jacob, Chapter 12, Introduction to Atmospheric Chemistry, 1999 VOC Emission Rate (10 12 molec cm -2 s -1 ) Figure: 12

7 Temperature Inversions and Air Quality Temperature inversion: increase in temperature with height Inversions are important for Air Quality because they inhibit vertical mixing of air Altitude Slide 21, Lecture 3 Pollution trapped below inversion Temperature Inversion Temperature 13 Day-to-day meteorology (weather!) affects severity and duration of pollution episodes Probability of ozone exceedance vs. daily max. temperature Lin et al Why does probability of high ozone rise with increasing temperature? Faster chemical reactions, increased biogenic emissions, and stagnation of air. 14

8 Day-to-day meteorology (weather!) affects severity and duration of pollution episodes Maryland has worst air quality during summer, when Bermuda High sets up over the Carolinas H 15 Day-to-day meteorology (weather!) affects severity and duration of pollution episodes Produced by: Daniel Silversmith, UMCP Directed by: Tim Canty & Ross Salawitch 16

9 Significant Improvements in U.S. Air Quality, Past 3 Decades Chapter 1 Chemistry in Context 17 Significant Improvements in U.S. Air Quality, Past 3 Decades 1980 to 2010: 82% decrease 1980 to 2010: 28% decrease to 2010: 52% decrease 2000 to 2010: 27% decrease

10 Significant Improvements in U.S. Air Quality, Past 3 Decades 19 Removal of NO x from Power Plants Brandon Shores, Md Coal Power Plant NOx Control: SCR Selective Catalytic Reduction 4NO + 4NH 3 + O 2 4N 2 + 6H 2 O 6NO 2 + 8NH 3 7N H 2 O Catalyst Maximize NOx N 2 Minimize SO 2 SO 3 Slide courtesy John Sherwell, Md Dept of Natural Resources 20

11 Removal of NO x from Power Plants Kim et al., GRL, Significant Improvements in Local Air Quality since early 1980s earchcenter/publications/general/emde/vol4no8/article4_photo1.aspx 22

12 Significant Improvements in Local Air Quality since early 1980s Pages/HistoricalData.aspx 23 Probability of Surface O 3 Exceedance: DC, MD, and Northern VA Catalytic Converters Mandatory Low NOx burners 1990 CAAA Phase 1 SCR Scrubbing of NOx from Power Plants (SIP) 24

13 Probability of Surface O 3 Exceedance: DC, MD, and Northern VA Catalytic Converters Mandatory Reformulated Gasoline 1990 CAAA Phase 1 SCR Scrubbing of NOx from Power Plants (SIP) 25 Probability of Surface O 3 Exceedance (DC, MD, No. VA) vs Daytime NO 2 Hot Summer Days (T BWI > 90 F) BWI T > 90 F Hosley, Canty, and Salawitch, in preparation, 2013 Analysis in this framework motivated by Pusede and Cohen, ACP,

14 Subtropical Jet Ferrel cell Hadley cell Subtropical Jet: area where poleward descending branch of the Hadley Circulation meets the equatorward descending of the Ferrel Cell (see Lecture 3) Semi-permanent area of high pressure, fair weather, low rainfall: conditions conductive to high ozone 27 Climate Change and Air Pollution Poleward expansion of the sub-tropical jet: Surface ozone highs occur along Subtropical Jet Number of days Subtropical Jet within 150 miles of Baltimore has increased by ~50% between 1979 and 2003, due to frontal movement Driving force: weakening of the equator to pole temperature gradient, caused by more rapid warming at high latitudes compared to tropics Seidel et al., Nature Geoscience, 2008 Computer models predict increase in severity and duration of pollution episodes over Midwest, Mid-Atlantic, and Northeast U.S. in 2050, even for constant emissions 28