Lecture 17 Atmospheric Aerosols

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Presentation transcript:

Lecture 17 Atmospheric Aerosols ATS 621 Fall 2012 Lecture 17 Atmospheric Aerosols

All PM2.5, 2011 average ammonium sulfate organic carbon ammonium nitrate soil (“dust”)

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 NH3 is available after sulfate titration HNO3 and excess NH3 can also form a solid aerosol  Ammonium nitrate formation is favored at LOW TEMPERATURE, HIGH RH Thermodynamic rules: Highest concentrations in industrial Midwest (coal-fired power plants) Observed aerosol acidity in US

For current monitor readings: Front Range “average” PM2.5 aerosol ammonium nitrate = ammonium sulfate For current monitor readings: http://www.colorado.gov/airquality/aqi_map_ags.aspx

DUST: MOST IMPORTANT(?) NATURALLY EMITTED AEROSOL Sources: arid / semi-arid regions Emission in both fine and coarse mode, depends on surface properties and wind speed. Resulting lifetime ~weeks Dust Emissions (2001) g m-2 y-1 [Fairlie et al. 2007] [Husar et al., 2002]

DESERT DUST CAN BE TRANSPORTED ON INTERCONTINENTAL SCALES April 16, 2001: Asian dust! clear day Glen Canyon, Arizona Annual mean PM2.5 dust (mg m-3), 2001 Asia Sahara Most fine dust in the U.S. (except in southwest) is of intercontinental origin

March May July September PM2.5 soil concentrations, 2011 Big year for wildfires in New Mexico….

MEAN SEA SALT AEROSOL CONCENTRATIONS Lower marine boundary layer (0-100 m) [Alexander et al. 2005]

CARBONACEOUS AEROSOL SOURCES ORGANIC CARBON (OC) ELEMENTAL CARBON (EC) GLOBAL 22 Tg yr-1 130 Tg yr-1 = BSOA UNITED STATES 0.66 Tg yr-1 2.7 Tg yr-1

BLACK CARBON EMISSIONS DIESEL DOMESTIC COAL BURNING BIOMASS BURNING

SECONDARY ORGANIC AEROSOL PRODUCTION FROM BIOGENIC VOC EMISSIONS Nucleation (oxidation products) Oxidation Reactions (OH, O3,NO3) Growth Condensation on pre-existing aerosol Over 500 reactions to describe the formation of SOA precursors, ozone, and other photochemical pollutants [Griffin et al., 2002; Griffin et al., 2005; Chen and Griffin, 2005]

BIOGENIC HYDROCARBONS Isoprene (C5H8) Monoterpenes(C10H16) Sesquiterpenes (C15H24) Anthropogenic SOA-precursors = aromatics (emissions are 10x smaller) "Trees cause more pollution than automobiles do.“ (when talking about ozone in 1981)

WILDFIRES: A GROWING AEROSOL SOURCE S. California fire plumes, Oct. 25 2004 Total carbonaceous (TC) aerosol averaged over U.S. IMPROVE sites Interannual variability is driven by wildfires

Radiocarbon (14C) is present at a small but approximately constant level in living (or contemporary) materials but is absent in fossil fuels, which are much older than the radiocarbon half-life of 5730 years. Schichtel, B. A., W. C. Malm, G. Bench, S. Fallon, C. E. McDade, J. C. Chow, and J. G. Watson (2008), Fossil and contemporary fine particulate carbon fractions at 12 rural and urban sites in the United States, J. Geophys. Res., 113, D02311, doi:10.1029/2007JD008605.

PRIMARY BIOLOGICAL AEROSOL PARTICLES (PBAP) BACTERIA VIRUSES POLLEN FUNGUS PLANT DEBRIS ALGAE Very large and likely short-lived These particles have not traditionally been considered part of the OA budget, but this has been revised in recent years. Not much is known about emissions, processing, climate effects.

GLOBAL SULFUR BUDGET [Chin et al., 1996] (flux terms in Tg S yr-1) SO42- t = 3.9d SO2 t = 1.3d cloud 42 OH H2SO4(g) 4 18 8 NO3 OH (CH3)2S 64 DMS t = 1.0d 10 dep 27 dry 20 wet dep 6 dry 44 wet 22 Phytoplankton Volcanoes Combustion Smelters

GLOBAL SULFUR EMISSION TO THE ATMOSPHERE 2001 estimates (Tg S yr-1): Industrial 57 Volcanoes 5 Ocean 15 Biomass burning 1 [Chin et al. 2000]

GLOBAL EMISSIONS OF AMMONIA [Bouwman et al., 1997] GLOBAL UNITED STATES 55 2.8 Ammonia, Tg N yr-1

STRATOSPHERIC AEROSOL PSCs (nitric acid / water vapor) Injection of volcanic ash (SiO2, Al2O3, Fe2O3) as well as gases (H2S, SO2, HCl) TROPOPAUSE Transport of long-lived S gases (eg. COS) Aerosols in the stratosphere are long-lived due to absence of precipitation and “layered” transport (due to stability)

SURFACE AEROSOL NUMBER CONCENTRATION GLOMAP: 2 moment sectional model simulating sulfuric acid / sea salt Dec July Continental: > 250 cm-3 Urban/polluted: > 2000 cm-3 Marine BL: ~ 200 cm-3 [Spracklen et al., 2006]

Particle size distributions We use frequency functions to describe the particle size distribution Number of particles per unit volume, per interval of the x variable (usually x=diameter): The AREA under the curve has meaning (total # concentration of particles between the two diameters) The value of the function itself at a particular point has little physical meaning we often choose x=log(diameter) We can convert the number distribution readily into surface area or volume distributions by multiplying by area or volume per particle

Normalize N in each bin by width of bin Example “data” Normalize N in each bin by width of bin Can use dDp or dlnDp as the bin width variable

TYPICAL AEROSOL SIZE DISTRIBUTION ultrafine fine coarse accumulation PM2.5 PM10 N=number concentration (particles/cm3)

Mean diameters Mean diameter of the NUMBER distribution Mean diameter of the SURFACE AREA distribution Mean diameter of the VOLUME (~MASS) distribution

The lognormal distribution N is the total number concentration of particles. Typical values of sg for atmospheric aerosols: 1.8 -- 2

Volume distribution is most closely related to light scattering

Aerosol water contents (Kreidenweis et al., ERL, 2008) Relative Humidity

UPTAKE OF WATER BY AEROSOLS

PM Welfare Effects: Visibility and Haze Visual Range: 250 km Visual Range: 25 km

PM10 Levels and Visibility in Beijing WHO Guideline: 50 ug/m3 averaged over 24 hrs 8 ug/m3 12 July 26 ug/m3 15 July 32 ug/m3 20 July 104 ug/m3 5 August 191 ug/m3 7 August 278 ug/m3 10 August http://news.bbc.co.uk/2/hi/in_pictures/7506925.stm (slide courtesy J. Volckens)

Visibility Human ability to see is a function of Characteristics of object observed Amount and distribution of light Concentration of suspended particles and gases Particles & gases scatter & absorb light causing Appearance of haze Decrease in contrasts Change in perceived color (absorption / scatter of certain λ’s) Scattering usually has a greater effect on visibility than absorption does 2 & 3 affect contrast between 1 & 4

Rayleigh scattering (isentropic) applies to gases, very small particles http://www.philiplaven.com

Haze particle http://www.atm.damtp.cam.ac.uk/people/mgb/aniso.html

Contrast that can be discerned by typical viewer Koschmieder equation I I0 Assumes: Homogeneous atmosphere Uniform sky brightness Viewing horizontally Neglected curvature of Earth Contrast that can be discerned by typical viewer This equation relates visual range to the total extinction coefficient

Components of the extinction coefficient, σext σext = σRayleigh + σabs,gas + σabs,part + σscat,part Visible light: 0.4 μm < λ < 0.7 μm Consider components: σRayleigh Due to gases (small particles also scatter this way); scattering α λ-4 -> shorter wavelengths scatter more (blue sky in clean conditions; red sky at sunset) At sea level and 0.45 μm , σRayleigh ~ 0.03 km-1 -> Lv = 130 km (81 miles) σabs,gas NO2 is the only important absorber – absorbs most strongly at blue λs -> colors plumes yellow, red or brown σabs,part Absorption = f(composition, size distribution) -> soot is most important (5-40% of total visibility reduction) -> overall soot is ~3x as efficient as sulfate, nitrate or organic aerosol species in terms of visibility reduction per unit mass of airborne particles -> new data suggest organic species contribute to light-absorbing carbon (“brown carbon”) – prevalent in wood smoke

Scattering by particles σscat,part Particle scattering is ~60 – 95% of visibility reduction Scattering importance (generally): sulfate > organic carbon > nitrate - Particle WATER CONTENT is very important! Most complicated to compute Very small particles act like gases Very large particles (Dp >> λ) extinguish light according to 2x their cross-sectional area (the “extinction paradox” – light scattered at very small angles to the beam is still seen when close to the object, but not when at a distance) Intermediate-sized particles (“Mie scattering”) have the most effective light scattering mechanism Extinction cross section can be up to ~4 – 5 times the cross sectional-area 0.1 – 1 μm are of similar λ to visible light: the “accumulation mode”

Scattering efficiency of a spherical particle m=1.5 m=1.5 + 0.1 i Diameter in microns

Scattering by particles The scattering coefficient due to N particles, each of radius r, is Can compute this for each ‘section’ of a size distribution to get the overall sscat Similar for absorption by particles and total extinction due to particles In practice, we often use “mass scattering efficiencies” and “mass absorption efficiencies” to compute overall extinction from measured mass concentrations

Effect of refractive index Mass scattering efficiency

Effects of relative humidity on visibility More aerosol mass is present (in the form of condensed water in the aerosol phase)  visibility is reduced The SIZES of the particles shift  generally to a more efficient scattering diameter, but can also grow OUT of the most effective scattering range if already very large The refractive index DECREASES  this decreases the extinction coefficient, but effect is much smaller than the other effects

http://views.cira.colostate.edu/web/

Estimated Annual Visual Range http://vista. cira. colostate ATS 555 F10 Lecture 3