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Particulate Matter. Atmospheric Particles (aerosol) Armospheric particles come in many sizes and shapes and can be made up of hundreds of different chemicals.

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Presentation on theme: "Particulate Matter. Atmospheric Particles (aerosol) Armospheric particles come in many sizes and shapes and can be made up of hundreds of different chemicals."— Presentation transcript:

1 Particulate Matter

2 Atmospheric Particles (aerosol) Armospheric particles come in many sizes and shapes and can be made up of hundreds of different chemicals. Some particles, known as primary particles are emitted directly from a source, such as construction sites, unpaved roads, fields, smokestacks or fires. Others form in complicated reactions in the atmosphere of chemicals such as sulfur dioxides and nitrogen oxides that are emitted from power plants, industries and automobiles. These particles, known as secondary particles, make up most of the fine particle pollution in the country.

3 Particulate Matter (Aerosol) TSP : Total suspended particulate PM10 PM2.5 Ultrafine particle

4 Chemical composition –chemical reactions –refractive index (i.e., optics) Shape –light scattering –efficiency for condensational growth Concentration –efficiency of transformations (e.g., coagulation goes as the number concentration 2 ) dN/dt = -kN 2 Size –lifetime –optical properties –mobility Aerosol properties

5 Aitken mode smallest particles (d < 0.1  m) formed by gas to particle conversion (homogeneous nucleation) or condensation Accumulation mode 0.1 < d < 2.5  m Direct emissions (e.g., biomass burning), condensation on existing particles, or growth from Aitken mode by coagulation Coarse mode d > 2.5  m mechanically generated (e.g., dust blown up by winds) Sizes and sources of particles

6 TYPICAL AEROSOL SIZE DISTRIBUTION finecoarse ultrafine accumulation PM 2.5 PM 10

7 OZONE AND PARTICULATE MATTER (PM): THE TOP TWO AIR POLLUTANTS IN THE U.S. # millions of people living in areas exceeding national ambient air quality standards (NAAQS) in  g m -3 (day), 65 (annual) 75 ppb (8-h average) 65  g m -3 (24-h), 15 (annual)

8 8 PM 2.5 versus Ozone In some parts of the country, PM 2.5 and ozone concentrations can be high at the same time. In the Pacific Northwest, for example, PM 2.5 concentrations are highest in the winter where ozone concentrations are the lowest. PollutantOzoneParticles PropertiesInvisible gas, three oxygen atoms Visible solids and liquids, many different compounds FormationReaction of precursor gases in sunlight Directly emitted and formed by multiple processes SeasonSummerAll seasons Health effectsAggravates lung and respiratory diseases Aggravates lung and heart diseases

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10 10 About PM 2.5 A complex mixture of solid and liquid particles Both a primary and secondary pollutant Significant particle size variation Seasonal and regional differences Forms in many ways Clean-air levels are <5 µg/m 3 * U.S. concentrations range from 0 to 200+ µg/m 3 * Health concerns Ultra-fine fly- ash or carbon soot "Night at Noon." London's Piccadilly Circus at midday during deadly smog episode in the winter of Source: When Smoke Ran Like Water, Devra Davis, Perseus Books * 24-hr average

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14 FINE AEROSOL COMPOSITION IN NORTH AMERICA Annual mean PM 2.5 concentrations (NARSTO, 2004) Current air quality standard is 15  g m -3

15 WORLDWIDE MEASUREMENTS OF FINE AEROSOL COMPOSITION

16 ORIGIN OF THE ATMOSPHERIC AEROSOL Soil dust Sea salt Size range:  m (molecular cluster) to 100  m (small raindrop)

17 17 PM 2.5 Formation and Growth Coagulation: Particles collide with each other and grow. Cloud/Fog Processes: Gases dissolve in a water droplet and chemically react. A solid particle exists when the water evaporates. SO 2 NH 4 Ammonium Sulfate Chemical Reaction: Gases react to form particles. NO x  HNO 3 + NH 3 NH 4 NO 3 (particle) Ammonium Nitrate Nitric Acid Ammonia Fn (Temp, RH) Formation Condensation: Gases condense onto a small solid particle to form a bigger particle. Growth

18 U.S. SO 2 EMISSIONS Sulfur emissions, Tg a GLOBAL UNITED STATES Main source is coal combustion

19 燃料中的硫份燃燒後產生 SO 2 ,排放進入大氣後,會產生包括氣 相和液相的一系列的反應,空氣中的硫化物除了 SO 2 以外還包括 H 2 SO 3 (aq) 、 HSO 3 - 、 SO 3 2- 、 H 2 SO 4 (g,aq) 、 HSO 4 - 、 SO 4 2- 等,前面三 種屬於 +4 價的硫,後面三種則為 +6 價的硫,所以在大氣中的反 應為將 +4 價的硫氧化成為 +6 價的硫。 SO 2 氧化成 H 2 SO 4 的過程可以分為兩種方式,其中一種方式是先 在氣相中被氧化成硫酸,然後被水滴吸收成為酸雨,另外一種 方式則是 SO 2 先溶解在水滴中,然後再氧化成為硫酸。 SO 2 在大氣中的轉換

20 在氣相中將 SO 2 氧化成 H 2 SO 4 包含下面步驟: (1) 二氧化硫被氧化爲三氧化硫。 (2) SO 3 與水反應產生 H 2 SO 4 (g) 。 (3) 因為 H 2 SO 4 的飽和蒸氣壓很低,所以上面 方式所產生的硫酸會很快地凝結在固體或液滴的表面,成為 H 2 SO 4 (aq) ,並 且解離產生 SO 4 2- 和 H + 。此種方式在溼度較低的情況下容易發生。 有幾種不同方式可將 SO 2 氧化成 SO 3 ,第一種為利用光化學反應。二氧化 硫在大氣中有兩個大於 2.9μm 的吸收光譜,一個在 2.9μm 處,一個在 3.8μm 處, 當 SO 2 在大氣中吸收不同光波時,可形成不同激發態的 SO 2 ,然後與氧分子 化合形成 SO 3 ,後者再與水結合形成硫酸。即 SO 2 + hν -> S0 2 * SO 2 * + O 2 -> SO 3 + [O] SO 3 + H 2 O -> H 2 SO 4 當被污染了的大氣中有碳氫化合物、氮氧化物和自由基 ( 如 : OH - 、 HO 2 等 ) 存在,則 SO 2 會和 OH 自由基反應,先產生 SO 3 (g) ,然後再形成 H 2 SO 4 SO 2 (g)+ OH + M --> HOSO 2+ M HOSO 2+ O 2 --> SO 3 (g) + HO 2 (fast) SO 3 (g) + H 2 O(g) + M --> H 2 SO 4 (g) + M (fast) 在 SO 2 之氧化反應中 OH 自由基影響很大。大氣中如果有碳氫化合物和自由 基存在, SO 2 的氧化速率會大大超過在潔淨空氣中的反應速率,在清潔大氣 中,二氧化硫的氧化速率爲每小時 0.023% - 1.0% ,當有碳氫化合物和氮氧 化物時,二氧化硫氧化速率可高達每小時 48% - 294% 。

21 另外一種方式則是 SO 2 先溶解在水滴中,然後再氧化成為硫酸,其反應可以分為三個 步驟,包括: (1) SO2 氣體溶解在水中 SO 2 (g) --> SO 2 (aq) (2) 在水中 SO 2 (aq) 轉換為 H 2 SO 3 ,並且解離產生 HSO 3 - 和 H + SO 2 (aq) + H 2 O --> HSO H + (3) 在水滴中反應將 HSO 3 - 氧化成為 H 2 SO 4 。 液相中 HSO 3 - 可被水滴內的 H 2 O 2 、 O 3 、 O 2 等氧化成為 H 2 SO 4 ,其反應如下: HSO H + + H 2 O 2 (aq) --> SO H + + H 2 O 2HSO H + + O 2 (aq) --> 2SO H + HSO H + + O 3 (aq) --> SO H + + O2 上述反應可在有催化劑與沒有催化劑反應,前者反應速率遠超過後者,而最有效之 催化劑為 Mn 2+ 、 Fe 3+ 、 Cu 2+ 等微量金屬。 H 2 O 2 很容易溶解在水中,在大氣中 H 2 O 2 溶解在水中的濃度比 O 3 高出 6 個數量級 (order) ,在一般情況下,當 pH 小於 4 到 5 時, H 2 O 2 為主要的路徑,當 pH>5 以後 O 3 的 反應開始主宰氧化反應,當 pH 值較高時,而且有 Fe 和 Mn 催化的情況下, O 2 所進行的 氧化反應可能相當重要。

22 GLOBAL SULFUR BUDGET [Chin et al., 1996] (flux terms in Tg S yr -1 ) Phytoplankton (CH 3 ) 2 S SO 2  1.3d (DMS)  1.0d OHNO 3 Volcanoes Combustion Smelters SO4 2-  3.9d OH cloud dep 27 dry 20 wet dep 6 dry 44 wet H 2 SO 4 (g)

23 NO 可與空氣中之 O 3 發生化學反應,形成 NO 2 NO + O 3 → NO 2 + O 2 NO 2 與 OH 自由基反應會產生 HNO 3 (g) NO 2 + OH - + M → HNO 3 + M HNO 3 (g) 被微粒表面吸收,轉變為無機性硝酸鹽或硝酸,硝酸再與氨 (NH3) 反應生成硝酸銨 (NH 4 NO 3 ); HNO 3 + HN 3 → NH 4 NO 3 或經由水滴之直接吸收,將溶解之 NO 2 轉變為 NO 3- ,其反應式,可以表 示如下: NO 2 + H 2 O + M → H + + NO M NO X 在大氣中的轉換

24 FORMATION OF SULFATE-NITRATE-AMMONIUM AEROSOLS Sulfate always forms an aqueous aerosol Ammonia dissolves in the sulfate aerosol totally or until titration of acidity, whichever happens first Nitrate is taken up by aerosol if (and only if) excess NH 3 is available after sulfate titration HNO 3 and excess NH 3 can also form a solid aerosol if RH is low Thermodynamic rules: Highest concentrations in industrial Midwest (coal-fired power plants) Conditionaerosol pHLow RHHigh RH [S(VI)] > 2[N(-III)]acidH 2 SO 4 nH 2 O, NH 4 HSO 4, (NH 4 ) 2 SO 4 (NH 4 +, H +, SO 4 2- ) solution [S(VI)] ≤ 2[N(-III)]neutral(NH 4 ) 2 SO 4, NH 4 NO 3 (NH 4 +,NO 3 - ) solution

25 AMMONIA EMISSIONS Ammonia, Tg N a -1 GLOBAL UNITED STATES

26 SULFATE-NITRATE-AMMONIUM AEROSOLS IN U.S. (2001) Highest concentrations in industrial Midwest (coal-fired power plants) SulfateNitrate Ammonium Acidity

27 CARBONACEOUS AEROSOL SOURCES IN THE U.S. ORGANIC CARBON 2.7 Tg yr -1 ELEMENTAL CARBON 0.66 Tg yr -1 Annual mean concentrations (2001) elemental organic

28 BC is emitted by incomplete combustion “BC” or “soot” is optically defined and includes both graphitic elemental carbon (EC) and light-absorbing heavy organic matter Diesel engines are large BC sources Freshly emitted BC particle

29 Atmospheric aging and scavenging of BC Emission Hydrophobic BC resistant to scavenging coagulation gas condensation Hydrophilic BC coated with sulfate, nitrate Scavenging Aging time scale τ ~ 1 d Implications for BC export from source continents: OCEAN aging scavenging Hydrophobic BC aging long-range transport FREE TROPOSPHERE BOUNDARY LAYER

30 LONG-RANGE TRANSPORT OF BC TO THE ARCTIC (ARCTAS aircraft campaign, April 2008) Altitude, km BC, ng m -3 STP mg m -2 month -1 Arctic vertical profiles BC sources Qiaoqiao Wang, Harvard

31 ORGANIC AEROSOL IN STANDARD GEOS-Chem MODEL fuel/industry open fires OH, O 3,NO 3 SOGSOA POA K vegetation fuel/industry open fires 700 isoprene terpenes oxygenates… 30 alkenes aromatics oxygenates… alkanes alkenes aromatics… VOC EMISSIONPRIMARY EMISSION VOC Global sources in Tg C y -1 secondary formation

32 TERPENES Terpenes are biogenic hydrocarbons produced in plants by combination of isoprene units (C 5 H 8 ) Monoterpenes: C 10 H 16 β-pinene Sesquiterpenes: C 15 H 24 δ-cadinene

33 FORMATION OF ORGANIC AEROSOL FROM VEGETATIVE EMISSIONS (C 5 H 8 ) (C 10 H 16 ) OH, O 3 Aldehydes RC(O)H Ketones RC(O)R Dicarbonyls RC(O)-C(O)R absorption into aerosol oxidation Carboxylic acids RC(O)OH polymerization

34 GENERAL SCHEMATIC FOR HETEROGENEOUS CHEMISTRY A(g)A(g) s A(aq) s A(aq) B(aq) BsBs B(g) diffusion surface reaction aqueous reaction GAS AEROSOL Aerosols enable surface and ionic reactions that would not happen in the gas phase; also concentrate low-volatility species in condensed phase interfacial equilibrium

35 FLUX AT THE GAS-PARTICLE INTERFACE A(g)A(g) s A(aq) s diffusion interfacial equilibrium l = mean free path of air (0.18  m at STP) a = particle radius Knudsen number Kn = a/l Kn >>1:  continuum (diffusion-limited) regime Kn<<1: free molecular (collision-limited) regime n bulk GAS PARTICLE n(aq) s a r 0 distance from center of particle

36 WHY CARE ABOUT ATMOSPHERIC AEROSOLS? Public health Visibility Ocean fertilization Chemistry Climate forcing Cloud formation

37 Effects of secondary aerosol Health Effects : Particle pollution - especially fine particles - contains microscopic solids or liquid droplets that are so small that they can get deep into the lungs and cause serious health problems. Numerous scientific studies have linked particle pollution exposure to a variety of problems, including: –increased respiratory symptoms, such as irritation of the airways, coughing, or difficulty breathing, for example; –decreased lung function; –aggravated asthma; –development of chronic bronchitis; –irregular heartbeat; –nonfatal heart attacks; and –premature death in people with heart or lung disease.. Environmental Effects Visibility reduction. Environmental damage Particles can be carried over long distances by wind and then settle on ground or water. The effects of this settling include: making lakes and streams acidic; changing the nutrient balance in coastal waters and large river basins; depleting the nutrients in soil; damaging sensitive forests and farm crops; and affecting the diversity of ecosystems. Aesthetic damage Particle pollution can stain and damage stone and other materials, including culturally important objects such as statues and monuments. Change weather: Global warming, increase rainfall

38 aerosols scatter and absorb solar radiation (direct effect) –scattering aerosol can reduce radiation going to the ground (cooling) –absorbing aerosols heat the air, changing dynamics aerosols modify cloud properties –cloud droplets form on small aerosol particles called cloud condensation nuclei (CCN) –more aerosols = possibly more CCN –for a given amount of water vapor, if there are more CCN the cloud droplets formed are smaller and the clouds look “brighter” from space (Twomey effect, 1 st indirect effect) –smaller cloud droplets don’t coalesce efficiently to form large raindrops, so the clouds last longer (2 nd indirect effect). Climatic effects

39 大氣能見度由大氣對太陽光的散射和吸收的消光效應決 定,能見度降低一是由於物體和背景兩者之間的對比度 減少,二是由於細粒子和氣態污染物對光的吸收和散射, 使來自物體的光信號減弱。通常光衰減的強弱可用消光 係數 (b ext ) 表示。 ( 用 530nm 或 550nm 作為可見光區的基準 波長 ) 通常規定能見度是當來自物體輻射只剩 0.02 時的距離, 因此能見度 V d 可以由下式計算: 能見度 (Visibility)

40 Copyright © 2010 R.R. Dickerson40 Optical Properties & Visibility Change in intensity of light reflecting off an object  I / I = exp( ‑ b ext  X) where: I = incident intensity of light  I = change in intensity of light b ext = extinction coefficient (m ‑ 1 )  X = distance (m)

41 Copyright © 2010 R.R. Dickerson41 Extinction Coefficient, b ext Sum of scattering and absorption coefficients: b ext = b scat + b abs Decomposed further from gases and particles: b abs = b ag + b ap b scat = b sg + b sp Where: b ag = absorption coefficient due to gases (Beer's law) b ap = absorption coefficient due to particles b sg = scattering coefficient due to gases (Rayleigh scattering) b sp = scattering coefficient due to particles (Mie scattering)

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48 cloud cover (PATMO S-x 1982–2008)

49 EPA REGIONAL HAZE RULE: WILDERNESS AREAS MUST ACHIEVE NATURAL VISIBILITY CONDITIONS BY 2064 Glacier National Park 7.6 µgm µgm µgm µgm -3 U.S. air quality standard Visibility degradation by aerosols at Glacier National Park, Montana Natural aerosol concentrations are typically less than 2  g m -3

50 VISIBILITY IN U.S. WILDERNESS AREAS Statistics for 20% worst visibility days Deciviews 2001 observationsNatural Background; includes transboundary pollution Visual range (km) Park et al. [2006]

51 INTERCONTINENTAL TRANSPORT OF DESERT DUST Glen Canyon, Arizona clear day April 16, 2001: Asian dust! Annual mean PM 2.5 dust (  g m -3 ), 2001 Asia Sahara Most fine dust in the U.S. (except in southwest) is of intercontinental origin

52 WILDFIRES: A GROWING AEROSOL SOURCE S. California fire plumes, Oct Total carbonaceous (TC) aerosol averaged over U.S. IMPROVE sites Interannual variability is driven by wildfires

53 ANNUAL MEAN PM 2.5 CONCENTRATIONS (2002) derived from MODIS satellite instrument data


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