The Atmospheric Chemistry and Physics of Ammonia

Transcription

1 The Atmospheric Chemistry and Physics of Ammonia Russell Dickerson Dept. Meteorology, The University of Maryland Presented at the National Atmospheric Deposition Program Ammonia Workshop October 23, 2003 Photo from UMD Aztec,

2 I. Fundamental Properties Importance Reactions Aerosol formation Thermodynamics Role as ccn Talk Outline II. Local Observations Observed concentrations Impact on visibility Box Model results New Detection Technique III. Fun Stuff if there s time. 2

3 Atmospheric Ammonia, NH 3 I. Fundamental Properties Importance Only gaseous base in the atmosphere. Major role in biogeochemical cycles of N. Produces particles & cloud condensation nuclei. Haze/Visibility Radiative balance; direct & indirect cooling Stability wrt vertical mixing. Precipitation and hydrological cycle. Potential source of NO and N 2 O. 3

4 Fundamental Properties, continued Thermodynamically unstable wrt oxidation. H rxn = kcal mole -1 G rxn = kcal mole -1 NH O 2 NO + 1.5H 2 O But the kinetics are slow: NH 3 + OH NH 2 + H 2 O k = 1.6 x cm 3 s -1 (units: (molec cm -3 ) -1 s -1 ) Atmospheric lifetime for [OH] = 10 6 cm -3 τ NH3 = (k[oh]) -1 6x10 6 s = 72 d. Compare to τ H2O 10 d. 4

5 Gas-phase reactions: Fundamental Properties, continued NH 3 + OH NH 2 + H 2 O NH 2 + O 3 NH, NHO, NO NH 2 + NO 2 N 2 or N 2 O (+ H 2 O) Potential source of atmospheric NO and N 2 O in low-so 2 environments. Last reaction involved in combustion denox operations. 5

6 Fundamental Properties, continued Aqueous phase chemistry: NH 3(g) + H 2 O NH 3 H 2 O (aq) NH OH Henry s Law Coef. = 62 M atm -1 Would not be rained out without atmospheric acids. Weak base: K b = 1.8x10-5 6

7 Aqueous ammonium concentration as a function of ph for 1 ppb gas-phase NH 3. From Seinfeld and Pandis (1998). 7

8 Formation of Aerosols Nucleation the transformation from the gaseous to condensed phase; the generation of new particles. H 2 SO 4 /H 2 O system does not nucleate easily. NH 3 /H 2 SO 4 /H 2 O system does (e.g., Coffman & Hegg, 1995). 8

9 Formation of aerosols, continued: NH 3(g) + H 2 SO 4(l) NH 4 HSO 4(s, l) (ammonium bisulfate) NH 3(g) +NH 4 HSO 4(l) (NH 4 ) 2 SO 4(s, l) (ammonium sulfate) Ammonium sulfates are stable solids, or, at most atmospheric RH, liquids. Deliquescence to become liquid through the uptake of water at a specific RH ( 40% RH for NH 4 HSO 4 ). Efflorescence the become crystalline through loss of water; literally to flower. We can calculate the partitioning in the NH 4 /SO 4 /NO 3 /H 2 O system with a thermodynamic model; see below. 9

10 Cloud 10

11 Formation of aerosols, continued G rxn = kcal mole -1 NH 3(g) + HNO 3(g) NH 4 NO 3(s) [NH 4 NO 3 ] K eq = = exp ( G/RT) [NH 3 ][HNO 3 ] K eq = 1.4x10 16 at 25 C; = 1.2x10 19 at 0 C Solid ammonium nitrate (NH 4 NO 3 ) is unstable except at high [NH 3 ] and [HNO 3 ] or at low temperatures. We see more NH 4 NO 3 in the winter in East. 11

12 Ammonium Nitrate Equilibrium in Air = f(t) NH 3(g) + HNO 3(g) NH 4 NO 3(s) ln(k) = ln(T) (ppb) 2 1/K eq 298K = [NH 3 ][HNO 3 ] (ppb) 2 = 41.7 ppb 2 ( ppb each) 1/K eq 273K = 4.3x10-2 ppb 2 Water in the system shifts equilibrium to the right. 12

13 Radiative impact on stability: Aerosols reduce heating of the Earth s surface, and can increase heating aloft. The atmosphere becomes more stable wrt vertical motions and mixing inversions are intensified, convection (and rain) inhibited (e.g., Park et al., JGR., 2001). 13

14 Additional Fundamental Properties Radiative effects of aerosols can accelerate photochemical smog formation. Condensed phase chemistry tends to inhibit smog production. Too many ccn may decrease the average cloud droplet size and inhibit precipitation. Dry deposition of NH 3 and HNO 3 are fast; deposition of particles is slow. 14

15 II. Local Observations 15

16 Fort Meade, MD Annual mean visibility across the United states (Data acquired from the IMPROVE network) 16

17 Fort Meade, MD 17

18 Inorganic compounds ~50% (by mass) Carbonaceous material ~40% (by mass) Summer: Sulfate dominates. Winter: Nitrate/carbonaceous particles play bigger roles. 18

19 Seasonal variation of 24-hr average concentration of NO 19 y, NO 3-, and NH + 4 at FME.

20 ISORROPIA Thermodynamic Model (Nenes, 1998; Chen 2002) Inputs: Temperature, RH, T-SO 4 2-, T-NO 3 -, and T-NH 4 + Output: HNO 3, NO 3 -, NH 3, NH 4 +, HSO 4 -, H 2 O, etc. 20

21 ISORROPIA Thermodynamic Model (Nenes, 1998; Chen, 2002) Inputs: Temperature, RH, T-SO 4 2-, T-NO 3 -, and T-NH 4 + Output: HNO 3, NO 3 -, NH 3, NH 4 +, HSO 4 -, H 2 O, etc. 21

22 (Data acquired in July 1999) 22

23 (Water amount estimated by ISORROPIA) 23

24 Interferometer for NH 3 Detection Schematic diagram detector based on heating of NH 3 with a CO 2 laser tuned to 9.22 µm and a HeNe laser interferometer (Owens et al., 1999). 24

25 Linearity over five orders of magnitude. 25

26 Response time (base e) of laser interferometer 1 s. 26

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28 *Emissions from vehicles can be important in urban areas. 28

29 Summary: Ammonia plays a major role in the chemistry of the atmosphere. Major sources agricultural. Major sinks wet and dry deposition. Positive feedback with pollution thermal inversions & radiative scattering. Multiphase chemistry Inhibits photochemial smog formation. Major role in new particle formation. Major component of aerosol mass. Thermodynamic models can work. Rapid, reliable measurements will put us over the top. 29

30 Acknowledgements Contributing Colleagues: Antony Chen (DRI) Rob Levy (NASA) Charles Piety Lackson Marufu Bruce Doddridge Jeff Stehr Bill Ryan (PSU) Melody Avery (NASA) Funding From: Maryland Department of the Environment NC Division of Air Quality VA Department of Environmental Quality NASA-GSFC EPRI 30

31 The End. 31

32 MODIS: August 9, 2001 Highest Ozone of the Summer Visible Composite Aerosol Optical Depth at 550 nm Balt GSFC Phila Balt GSFC Phila AOT Robert Levy, NASA

33 Donora, PA Oct. 29,

34 Madonna Harten Castle Germany: Ruhr area Portal figure Sandstone Sculptured 1702 Photographed

35 Madonna Harten Castle Germany: Ruhr area Portal figure Sandstone Sculptured 1702 Photographed

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