Solomon Bililign, Department of Physics

1. Study of Vibrational Overtone Induced Dissociation of Organic Acids From Biomass Burning - Using Cavity Ring Down Spectroscopic Techniques Solomon Bililign, Department of Physics

2. About UNC and NCA&T • Chartered in 1789, UNC was the first public university in the United States and the only one to graduate students in the eighteenth century. Today, UNC is a multi-campus university composed of all 16 of North Carolina's public institutions that grant baccalaureate degrees, as well as the NC School of Science and Mathematics, the nation's first public residential high school for gifted students.

3. NC Agricultural and Technical State University Description North Carolina Agricultural & Technical State University is a public, comprehensive, land-grant and "high research activity" university committed to fulfilling its fundamental purposes through exemplary undergraduate and graduate instruction, scholarly and creative research, and effective public service. The University is accredited by the Commission on Colleges of the Southern Association of Colleges and Schools to award bachelor's, master's and doctoral degrees. Since its inception as a land-grant university in 1891, North Carolina A&T has had a rich tradition of leadership and achievement. Those qualities are still evident today. www.ncat.edu | Explore. Discover. Become.

4. FACTS ABOUT ISETCSC North Carolina A&T State University -Lead North Carolina State University University of Minnesota Fisk University-Tennessee California State University-Fresno University of Alaska Southeast City University of New York Partner Institutions Aligned with the NOAA Office of Atmospheric Research MISSION Train students in NOAA scientific areas and develop technology and analysis techniques of global data sets for improved understanding of climate and environmental change Thirty one scientists and engineers in seven institutions Nine Academic Departments NCA&T Global warming task force report

5. ISETCSC INTERDSICIPLINARY RESEARCH THRUSTS Thrust Area I Sensor science and technology Data assimilation Sensor Data Thrust Area II Global Observing systems: numerical and physical research Thrust Area III Data mining & Fusion Distributed architecture • Climate prediction • Pattern recognition for seasonal hurricane forecasts Sun photometer TA-1 Departments Physics Chemistry Oceanography Electrical Engineering Chemical Engineering TA-III-Departments Mathematics Computer science Physics Electrical engineering CUNY TA -II- Departments Physics Chemistry Meteorology Hydrology Mathematics Computational science Environmental Science Civil Engineering Scientific Environmental Technology Development Fresno Alaska Student field experience ChesapeakeBay FISK

6. Thrust Area I Sensor science, Sensor technology A-7 Sensor technology, Eye safe Lidar, etc. (CUNY) A-1 A-2 A-3 A-4 A-5 a) Cavity ring down spectroscopy b) Negative ion proton transfer mass spectroscopy (Bililign, Fiddler) A-6 A-8 Luminescent Sensors (Assefa, NCA&T). RC(O)O2 + HO2 reaction branching ratios (Hasson, Fresno State).

7. The Research Group Fiddler Begashaw Cochran www.ncat.edu | Explore. Discover. Become.

8. What is in the atmosphere? • 1950s: Atmosphere is 99.999% composed of N2, O2, CO2, He, Ar, Ne. All are inert! (no chemistry). O3 in the stratosphere. Trace CH4, N2O • 1960s: Recognized that reactive compounds in the atmosphere were important even at extremely low levels. • 1970s: Regional air quality becomes a major research topic. • 1980s: Global atmospheric chemistry becomes a major research topic.

9. Emission Sources Natural (Biogenic/Geogenic) • Lightning (NOx) N2 NOx • Volcanoes (SO2, aerosols) • Oceans • Vegetation * Highly variable in space and time, influenced by season, T, pH, nutrients… Anthropogenic • Mobile sources • Industry • Power generation • Agriculture FIRE Source: Lecture notes by Christine Wiedinmyer NCAR

10. Example: Emissions from fires Courtesy of Brian Magi, NOAA GFDL

11. What is emitted from fires? Urbanski et al., Wildland Fires and Air Pollution, 2009

12. Acids emitted in Biomass burning Source: Veres, P., et al. Journal of Mass Spectrometry, 2008. 274(1-3): p. 48-55. www.ncat.edu | Explore. Discover. Become.

13. Global Emission Estimates: Particles Andreae and Rosenfeld, Earth Science Reviews, 2008

14. Atmospheric Abundance CH3OOH 700 H2 H2O2 500 500 Nitrogen 78% Ethane 500 CO2 380 NH3 400 N2O 310 HCHO 300 HNO3 300 Ne SO2 Oxygen 200 CO NOx He (5) 100 100 20% 18 others CH4 (1.8) Ozon H2OArgon 1% 30 ppb ppt ppm Image courtesy of Max-Planck-Institut für Meteorologie, Hamburg

15. Biomass burning- Why do we care? • Biomass burning is a significant source of atmospheric gases and particles • It occurs naturally in wildfires • It occurs when people clear forests for agriculture, cooking fuel. • Most abundant compounds emitted: water vapor, CO2, CO, and thousands of additional compounds and aerosols. • Many of these compounds are poorly characterized due to analytical challenges. • One poorly understood, but significant class of Volatile organic compounds (VOC) present in biomass burning is gas-phase organic acids. Theyare extremely difficult to measure because of their adsorptive nature. • Accurate measurement of optical properties( single scattering albedo) of aerosols is crucial for quantifying the influence of aerosols on climate in climate models and remote sensing applications www.ncat.edu | Explore. Discover. Become.

16. Research activities related to biomass burning in Bililign’s Group • Negative Ion proton transfer mass spectrometry to Measure: a) Acidities of gas-phase acids; b) Rate of H-transfer; c) Water cluster characterization. • Investigate vibrational overtone initiated photodissociation processes that are significant sources of atmospheric radicals using cavity ring down spectroscopy • Measurement of the Henry's law coefficient and first order loss rate of Isocyanic Acid in Water Solutions • NEW work: measurement of optical properties of biomass aerosols using cavity ring down spectroscopy www.ncat.edu | Explore. Discover. Become.

17. Some Important Photolysis Reactions O2 + hn (l < 240 nm)  O + O source of O3 in stratosphere O3 + hn (l < 340 nm)  O2 + O(1D) source of OH in troposphere NO2+ hn (l < 420 nm)  NO + O(3P) source of O3 in troposphere CH2O + hn (l < 330 nm)  H + HCO source of HOx, everywhere H2O2 + hn (l < 360 nm)  OH + OH source of OH in remote atm. HONO + hn (l < 400 nm)  OH + NO source of radicals in urban atm.

18. Interest in OH Radical Formation • Life time 10−9 seconds and a high reactivity • The hydroxyl radical is often referred to as the "detergent" of the troposphere and has an important role in eliminating some Greenhouse gases dominant removal mechanism of for large number of volatile organic compounds (VOCs) • The rate of reaction with the hydroxyl radical often determines lifetime of many pollutants. • Major loss mechanism for methane www.ncat.edu | Explore. Discover. Become.

19. THE DRIVING FORCE O2 R’OO• O3 O2 H2O NO NO2 R R’• R’O• • The radiation from the Sun drives several processes in the atmosphere: • Retention of short and long-wave radiation. • Photo-induced chemistry. OH•

20. Atmospheric Photochemistry • Ozone photolysis (λ<310nm) • OH radical formation

21. The Oxidative Power of the Atmosphere 21

22. λ ≤ 698 nm Relatively strong vibrational overtone Activation energy = 168.0 ± 3.4 kJ/mol + OH• + OH• Other products • Peractic acid: K. A. Sahetchian, et al., Symp. Int. Combust. Proc., 1992, 24, 637-643. www.ncat.edu | Explore. Discover. Become.

23. Direct Overtone Photolysis • In polyatomic molecules the X-H stretching at considerably higher frequency than other vibrational modes. • They dominate the ground electronic vibrational overtone spectrum-uncoupled from other vibrations • These modes can have direct excitation from the ground vibrational level to several excited levels (“overtone transitions”). • If the vibrational level accessed in this way lies above the dissociation limit, dissociation is followed.

24. VIBRATIONAL OVERTONE-INDUCED PHOTODISSOCIATION E = hv ν = 5 ν = 4 ν = 3 ν = 2 ν = 1 Figure adapted from a diagram by Mark M. Somoza

25. Cavity Enhanced Spectroscopy O’Keefe and Deacon 1988 • • Light introduced and detected through a mirror. • • Light intensity inside of the optical cavity depends on a number of factors, and can be much smaller or much larger than the incident intensity • Allows the measurement of absorption coefficient on an absolute scale • • Effective or average path length may be very (very, very) long • Limited partly, but not exclusively, by mirror reflectivity • • So …. offers potential for very high sensitivity absorption (or extinction) spectroscopy

26. Beer’s Law (Lambert-Beer Law) dz I0 I Light Source Detector L  Absorption (Extinction) Coefficient  (cm-1) = NAbs (cm-3)  (cm2) If  is known, N can be determined absolutely

27. Absorber [A] I THEORY z LA d The detector receives a series of pulses separated by the roundtrip time t = 2d/c with decreasing power from pulse to pulse. After one pass through cavity: • a is the absorption coefficient • After each round trip the pulse power decreases by an additional factor T = 1− R− A << 1- Transmission is very small EBAL-08, Cairo, January 2008

28. THEORY • After m rounds the power has decreased to: If the detector time constant is large compared to the pulse width it just detects the envelope of the pulse and records an exponential decay with the decay time With out a gas a = 0; The decay time will be lengthened to EBAL-08, Cairo, January 2008

29. Cavity Ring-Down Absorption (Extinction) Spectroscopy Define: Minimum detectable absorption is limited by the reflectivity R, the unavoidable losses A of the resonator and accuracy of measuring to and t1 Minimum detectable absorption =  R= Reflectivity, L cavity length

30. What is an Optical Cavity ? “A region bounded by two or more mirrors that are aligned to provide multiple reflections of light waves” Triangular Bow-Tie www.ncat.edu | Explore. Discover. Become.

31. Stable Optical Resonators R = mirror radius of curvature d = mirror separation “g parameter” Stability condition

32. Gaussian Beams Most optical beams propagating in free space are almost TEM, field component lie in a plane perpendicular to the direction of propagation The wave is propagating with a velocity = c. The major variation of the field with z is a term of approximate form: exp(-ikz), with k = wn/c= 2pn/lo. Since lo is quite small for optical frequencies, k is a large number. If the beam has a finite diameter D the transverse divergence can be approximated by Et/D ;So that the ratio of Ez/Et is very small. Assuming a solution of the form E(x,y,z) = Eoy(x,y,z) e-ikz and substituting into the wave equation, and after some approximation is the central equation for Gaussian beams www.ncat.edu | Explore. Discover. Become.

33. Transverse Electric Modes (TEM) TEM00 TEM10 TEM20 TEM00 Longitudinal TEMnm Transverse TEM11  (or )

34. CONDITIONS • mode matched Laser mode to the fundamental TEM00q resonator mode.. • Mode of laser in resonance With a mode of the cavity • The relaxation time of the resonator must be longer than that of excited molecules, i.e. R > 0.9999 and careful alignment. • Due to the spectral bandwidth of the laser pulse many fundamental resonator modes within the bandwidth dwR can be excited. Therefore in order to resolve absorption lines the laser bandwidth dwL should be smaller than the absorption width. EBAL-08, Cairo, January 2008

35. Resonancesin Optical Cavities Round trip phase shift = 2kd = n.2p Note: k = 2p/l = w/c Cavity Transmission Free Spectral Range (nq+1-nq ) Resonant Non-resonant Full Width Half Max FSR ∆

36. Limitations on 0 (effective path length) “Empty” Cavity Loss Mirror Transmission Rayleigh Scattering Mie Scattering Interfering Absorptions = + + + • Mirror Reflectivity • Best achievable is T ≈ 5 ppm • Strong function of  • Cavity Length • Rayleigh scattering • Rayliegh-4 ! • Mie Scattering - Aerosol • Also scales steeply with  • Aerosol extinction can be large! • Interfering absorbers 1-2 specific to CRDS 3-5 common to any direct absorption measurement but … particularly acute when min < 10-8 cm-1

37. RING-DOWN TIME Number density of absorber (molec•cm-3) I0 Absorption coefficient (cm-1) Absorption cross section (cm2•molec) Intensity Speed of light in air I0/e Without sample With sample Time

38. Components of a CRD setup Optical cavity with two highly reflective mirrors (~99.995%) Translated light intensity into an electronic signal Positioning mirrors Photomultiplier Tube (PMT) Data Acquisition Telescope Dye Laser Optical Isolator Determines Tau values Matches the lasers pulse and optical cavity modes. Pulsed laser source with variable wavelength light. This protects the laser from back reflection 38

39. EXPERIMENTAL SETUP Pin Hole Wave- plate Turning Mirror Iris Polarizer Lens 1 Lens 2 Nd:YAG Dye Laser Sample Flow HeNe Laser Pressure Transducer Purge Flow Silver Turning Mirrors Collimator Turning Mirror Optical Fiber Ring-down Cavity Mirror Mount and Bellows Fitting Purge Flow PMT Zinc Lamp Copper Tubing Teflon Tubing PC Phototube Detector UV Cell Exhaust Bandpass Filter

40. Flow system N2 Inline Filter Bubbler MFM Sample Ring-down Cell MFM UV Cell

41. CRDS SETUP

42. CONTROL AND DATA ANALYSIS

43. INSTRUMENT SPECIFICATIONS • Ring-down cavity length: 91 cm • Typical ring-down times at 620 nm: ~100 μs • Dye laser wavelength accurate to ±0.02 nm against HITRAN • The minimum detectable extinction coefficient from taking the limit at τ approaches τ0 αmin~3.5*10-9 cm-1•Hz-1/2

44. CALIBRATION AND COMPARISON WITH WATER

45. Quantifying Photolysis Processes Photolysis reaction: AB + hn A + B Photolysis rates: • Photolysis frequency (s-1) J = lF(l) s(l) f(l)dl • (other names: photo-dissociation rate coefficient, J-value)

46. CALCULATION OF PHOTOLYSIS COEFFICIENTS J (s-1) = lF(l) s(l) f(l) dl F(l) = spectral actinic flux, quanta cm-2 s-1 nm-1  probability of photon near molecule. s(l) = absorption cross section, cm2 molec-1  probability that photon is absorbed. f(l) = photodissociation quantum yield, molec quanta-1  probability that absorbed photon causes dissociation.

47. Difficult: must measure absolute change in n (products) and I (photons absorbed) www.ncat.edu | Explore. Discover. Become.

48. Spectral Irradiance, L Typical light ray striking a thin layer of air in the atmosphere adapted from Madronich, 1987..

49. Actinic flux solar zenith angle Watts m-2 www.ncat.edu | Explore. Discover. Become.

50. ABSORPTION CROSS SECTION ∫ J = [σ(λ)•Φ(λ)•I(λ)]dλ • Data was collected by flowing diluted and undiluted acetic acid sample, which varied the concentration. • The number density of monomeric acetic acid in the UV cell (nUV,M) was calculated from the UV absorbance (A) in each experiment, the known equilibrium constant for dimerization (Keq), and the known absorbance cross sections for acetic acid monomer and dimer at 214 nm.