Making sense of air quality. Daniel J. Jacob

Transcription

1 Making sense of air quality Daniel J. Jacob

2 The industrial revolution and air pollution Pittsburgh in the 1940s Make great efforts to build China into a strong and prosperous industrialized country under the leadership of the Party and chairman Mao!

3 London fog Aerosols a.k.a. particulate matter (PM) from domestic+industrial coal combustion Killer fog of December 1952 resulted in 10,000 excess deaths Altitude inversion < 1km Sulfur dioxide (SO 2 ) particles sulfate organic carbon black carbon Temperature Coal combustion

4 Los Angeles smog Respiratory problems, vegetation damage due to high surface ozone altitude produced by photolysis of oxygen (O 2 ) ~ 10 km stratosphere troposphere ~ 1 km ozone temperature inversion Nitrogen oxides (NO x NO + NO 2 ) Volatile organic compounds (VOCs) Sunlight Ozone (O 3 ) PM vehicles, industry, vegetation

5 AIR POLLUTION TODAY: Ozone and fine particulate matter (PM 2.5 ) are the major pollutants US population exposed to air pollutants in excess of national ambient air quality standards (NAAQS), 2015 Ozone 108M 70 ppb (8-h average) PM M 12 µg m -3 (annual), 35 µg m -3 (24-h) PM 10 SO 2 5M 11M Million environmental deaths per year worldwide Lead 1M 1.5 CO 0 NO 2 0 US EPA [2017], OECD [2012] data PM2.5 ozone water supply indoor air malaria

6 Ozone air quality standards in the US and in the world Europe AQS (8-h avg.) Europe AQS (seasonal) Canadian AQS (8-h avg.) U.S. AQS (8-h avg.) U.S. AQS (1-h avg.) ppb Preindustrial ozone background Present-day ozone background at northern mid-latitudes China AQS (8-h avg.)

7 Days per year exceeding 70 ppb ozone standard, TOAR [2017]

8 How to control ozone pollution? Decrease emissions of nitrogen oxides (NO x NO + NO 2 ) and/or volatile organic compounds (VOCs) NO x : efficient combustion (power plants, vehicles) VOCs: inefficient combustion (vehicles, fires), industry, vegetation but complicated by non-linear dependence Ozone production mechanism: Ozone (ppb) vs. NO x and VOC emissions O2 RH OH RO H O RO + NO RO + NO 2 2 O2 NO h NO O + ν (1) (2) (3) Ozone production can be limited by reaction (1) (VOC-limited regime) or reaction (2) (NO x -limited regime) VOC Sillman et al. [1988]

9 Zhu et al. [2017] US is NO x -limited due to high emissions of biogenic isoprene isoprene oxidation hν, OH HCHO ~ 1 hour ~ 1 hour Other VOCs OMI satellite observations of formaldehyde (HCHO) columns, May-Aug

10 China is more complicated: high anthropogenic VOCs OMI annual mean tropospheric column data, Glyoxal (CHOCHO) Formaldehyde (HCHO) NO 2 ( nm) ( nm) ( nm) VOC emissions collocated with NO x emissions (combustion) though there is also a large biogenic component Glyoxal hotspot over Pearl River Delta from large aromatic emissions Chan Miller et al. [2016]

11 NO x controls are needed to meet current ozone standards even if production is locally VOC-limited NO x -limited regime VOC Natural NO x background start point Biogenic VOC background VOC-limited regime VOC controls will only get you so far until you are limited by biogenic background NO x controls are only way to get to current ozone standards and have side benefits (NO 2 air quality, nitrogen deposition)

12 NO x emissions observed from space OMI NO molecules cm -2

13 NO x emission trends observed from space OMI NO 2 OMI 2013NO 2 trend Chinese NO x emissions molecules cm -2 s -1 fractional change Verstraeten et al. [2014]

14 Trend in #days/year with ozone > 70 ppb, summer Megacity trends Ozone at Chinese sites, Parrish [2014], TOAR [2017], Wang et al. [2017]

15 Models still overestimate surface ozone in Southeast US models 24-h ozone observed A major reason is that the EPA NO x emission inventory is a factor of 2 too high: NASA SEAC 4 RS aircraft campaign, Aug Sep 2013 Observed GEOS-Chem with EPA NO x emissions with EPA NO x emissions/2 Fiore et al. [2009]; Travis et al. [2016]

16 NO x emissions in US are sufficiently low that VOC oxidation by low-no x pathways (poorly understood!) becomes important Segregation between isoprene and NO x emissions increases importance of low-no x pathways Isoprene NO emission x emission SEAC 4 RS aircraft campaign, Aug-Sep 2013: oxidation products from the 3 pathways isoprene nitrate isoprene peroxide observed GEOS-Chem model isoprene hydroxy aldehyde NO 60% ozone, carbonyls, nitrates + OH ISOPO 2 HO 2 30% organic peroxides, epoxides PM Isoprene (ISOP) isomerization hydroxyaldehydes, HOx, glyoxal? 10% Travis et al. [2016], Yu et al. [2016]

17 As ozone standard tightens, the nature of the problem changes Seasonal dose in excess of 60 ppb [EPA, 2014] 60 ppb exceedances are largest in Intermountain West

18 Ozone in Intermountain West originates out of N America Background = ozone present in absence of anthropogenic sources in North America MDA8 ozone, ppb observed GEOS-Chem model GEOS-Chem background GEOS-Chem natural Pinedale, Wyoming 2.4 km altitude 2006 Altitude, km Mean profile over NE Pacific, Apr-May Ozone over 2006 NE (INTEX-B) Pacific (INTEX-B, Apr-May 2006) observed GEOS-Chem downwelling Ozone, ppb o Domestic emissions have little influence on intermountain west o Anthropogenic background contributes ~15 ppb with little day-to-day variability Zhang et al. [2014]

19 Intercontinental transport of ozone pollution 2012 OMI NO 2 column, molecules cm -2 Strong westerlies transport ozone around mid-latitudes Ozone pollution transport (GEOS-Chem model) N America Europe Asia

20 Global tropospheric ozone is rising and we don t know why Mean 500 hpa ozone in JJA 2013 OMI satellite data GEOS-Chem model Partly natural: stratospheric influence, lightning, wildfires Partly anthropogenic: methane, intercontinental pollution, fires, ships, aircraft OMI tropospheric ozone column trend, : increasing almost everywhere Models can reproduce present-day levels but not long-term trends Hu et al. [2017], TOAR [2017]

21 Annual mean concentrations of fine particulate matter (PM 2.5 ) inferred from satellite data US air quality standard China air quality standard van Donkelaar et al. [2010]; Rhode and Muller [2015]

22 PM 2.5 composition Two components dominate PM 2.5 mass under almost all conditions: Sulfate-nitrate-ammonium (SNA) Organic carbon (OC) Wang et al. [2014]

23 Formation of sulfate-nitrate-ammonium aerosol H 2 SO 4 (g) SO oxidation 2 NH 3 HNO 3 High RH (aqueous aerosol) H 2 O H 2 SO 4 (aq) HSO - 4 SO 2-4 NH 3 (aq) NH + 4 HNO 3 (aq) NO - 3 Low RH (dry aerosol) NH 4 HSO 4 (NH 4 ) 2 SO 4 NH 4 NO 3 NO x oxidation EMISSION SO 2 : coal combustion NH 3 : agriculture NO x : fuel combustion Sulfuric acid produced from SO 2 oxidation is ~100% incorporated into the aerosol Ammonium and nitrate are incorporated as determined by acid-base titration

24 Three different regimes for SNA aerosol formation Total sulfate [S(VI] = [H 2 SO 4 (aq)] + [HSO 4- ] + [SO 4 2- ] Total ammonia [N(-III)] = [NH 3 (g)] + [NH 3 (aq)] + [NH 4+ ] Total nitrate [N(V)] = [HNO 3 (g)] + [HNO 3 (aq)] + [NO 3- ] Equivalents per m 3 of air N(V) S(VI) N(-III) gas aerosol N(V) S(VI) N(-III) gas aerosol N(V) S(VI) N(-III) gas aerosol [S(VI)] > [N(-III)] all ammonia in aerosol no nitrate in aerosol [S(VI)]+[N(V)] > [N(-III)] > [S(VI)] [S(VI)]+[N(V)] < [N(-III)] almost all ammonia in aerosol ammonia partly in aerosol nitrate partly in aerosol almost all nitrate in aerosol REGIME 1 REGIME 2 REGIME 3

25 Long-term trends in SO 2 emissions

26 New SO 2 pollution frontier: India OMI satellite instrument shows rapid growth in SO 2 emissions from coal use Lu et al. [2013]

27 Sulfate in US has shown linear response to SO 2 emissions OH, H 2 O 2 (in cloud), ozone (in cloud) SO 2 sulfate atmospheric oxidation Coal-fired power plant observed (circles), model (background) 1990 SO 2 emissions decreased by 3% per year from 1990 to Sulfate decreased by 3% per year 2010 Leibensperger et al. [2012] µg m -3

28 Problem with explaining winter sulfate haze in Beijing Observations Model Concentration (μg m -3 ) Observations GEOS-Chem model PM 2.5 percentage Date (January 2013) Sulfate production in models is limited by supply of photochemical oxidants Possible solutions: transition-metal-catalyzed oxidation by O 2, formation of SO 2 -formaldehyde complexes; strongly dependent on aerosol/cloud ph Wang et al. [2014]; Jonathan Moch, Harvard

29 Sulfate aerosol should now be titrated by ammonia in eastern US but it isn t Emission Ratio NH 3 / SO 2 Wet Deposition Ammonium-sulfate Ratio [NH 4+ ] / [S(VI)] Aerosol Ammonium-sulfate Ratio [NH 4+ ] / [S(VI)] Data: EPA NEI & MASAGE Data: NADP CSN SEARCH SOAS Excess ammonia Major departure from standard sulfate-ammonium aerosol thermodynamics Silvern et al. [2017]

30 Long-term trends in Southeast US also depart from thermodynamics Averages for Southeast US -6.1% a % a -1 no trend -8.5% a % a % a -1 Ammonium-sulfate aerosol ratio decreases as sulfate decreases! Silvern et al. [2017]

31 Possible retardation of thermodynamic equilibrium by organic aerosol US aerosol has changed over past decade from sulfate-rich to organic-rich Organic/sulfate ratio (g g -1 ) +9.8% a -1 inorganic core (NH 4 +, SO 4 2- ) Evidence of phase separation, kinetic limitation to NH 3 uptake Organic aerosol coating Retardation/modification of equilibrium by organics could affect uptake of other gases, including water NH 3 (g) Liggio et al., 2011; You et al., 2012

32 Primary and secondary organic aerosol VOC atmospheric oxidation low-volatility products condensation secondary organic aerosol VOCs VOCs VOCs primary organic aerosol (direct emission) open fires fuel combustion and industry vegetation

33 Two models for formation of secondary organic aerosol Classical model for reversible uptake by pre-existing organic aerosol GAS ORGANIC PHASE VOC oxidation semi-volatile gas semi-volatile aerosol Alternate model for irreversible uptake by aqueous aerosol GAS AQUEOUS PHASE VOC oxidation water-soluble gas dissolved gas nonvolatile oxidation species complexation oligomerization

34 Sources of organic aerosol in the Southeast US in summer Observed (circles), GEOS-Chem (background) Aug-Sep 2013 GEOS-Chem source attribution (29%) (12%) Most organic aerosol in summer is biogenic; isoprene accounts for ~50%, most by low-no x channel Kim et al. [2015]

35 Aqueous-phase mechanism for secondary organic aerosol from isoprene: the short version isoprene OH glyoxal OH OH epoxide (IEPOX) Gas-phase aerosol precursors HO HO oligomerization. oxidation acid catalysis Aqueous aerosol Marais et al. [2016]

36 Association of IEPOX secondary organic aerosol with sulfate Correlation with sulfate in SEAC 4 RS and at Centerville, Alabama research site GEOS-Chem Centerville, AL SEAC 4 RS J. Jimenez (CU) Sulfate Marais et al. [2016] Aerosol volume Aerosol acidity IEPOX SOA

37 Equatorial Asia: new frontier for air pollution Massive agricultural fires in one of the most densely populated regions of Earth MODIS fire observations, June Smoke in Singapore, June agricultural fires caused 100,000 excess deaths MODIS AERONET Mean aerosol optical depths, Sept-Oct Koplitz et al. [2017]

38 Large planned increases of coal emissions in Equatorial Asia GEOS-Chem simulations with vs. without 2030 coal emissions Planned 2030 coal power plants Transboundary transport of Southeast Asia pollution to cause excess deaths in China by 2030 excess deaths per year from coal emissions Koplitz et al. [2017]

39 Geostationary satellite constellation for air quality ( launch) TEMPO (hourly) Sentinel-4 (hourly) GEMS (hourly) GEMS to be launched by Korean Aerospace Research Institute (KARI) UV/Vis solar backscatter: aerosol, NO 2, SO 2, HCHO, O 3 (free troposphere) 7x8 km 2 pixels, hourly data TEMPO will also detect O 3 in boundary layer (weak nm Chappuis bands)