Using Spectral Averaging and Signal Processing to Leverage Exis7ng Short- term Fence Line UV Spectroscopic Data to Retrieve Accurate, Long- term Gas Concentra7ons to Meet New Monitoring Goals Robert Crampton Ph.D Brent Olive PH.D, Don Gamelis PH.D. Argos Scien7fic
What is UV DOAS Spectrographic technique for measuring atmospheric components in situ.
Ac7ve vs. Passive
Principles of DOAS The Beer-lambert law OD i = σ i c i l OD i = optical density, σ i = absorption cross-section, c i = concentration, l = pathlength (for species i)
Advantages of DOAS Overlapping absorp7on structures due to different species can be separated Species not an7cipated can be measured later. Spectral fingerprint Presence of Gases verified Warning against unexpected absorbers (residual) Mostly Immune against con7nuous (broad band) ex7nc7on due to e.g. aerosol or molecules High sensi7vity, since many trace gas lines (bands) are used. No analy7c cost associated with the sample (no lab) And Low maintenance costs
Disadvantages Atmospheric turbulence induce intensity varia7ons in the spectra. Upfront costs considerable 40 200k Only a limited number of molecules have suitable absorp7ons in UV and visible region. Rain, snow, fog and clouds make measurements impossible due to the strong ayenua7on in the UV- Visible region. Path Average concentra7on vs. Peak concentra7on
Some Gases/features
Can detect a wide range of UV absorbing chemical compounds Acrolein Acetaldehyde Ammonia Benzene 1,3-Butadiene Carbon disulfide Formaldehyde Hydrogen sulfide Nitric oxide Nitrogen dioxide Ozone Phenol m-, o-, p-xylene Styrene Sulfur dioxide Toluene..and more
Some Example Spectra
Hardware
Data Trail Raw Spectra - Background - Sample Absorb7on Concentra7on vs Time
Deuterium Source Raw Spectrum 4000 Raw Data - Deuterium Source 3500 O3 Benzene Toluene Xylenes 3000 2500 2000 NO NO2 SO2 Formaldehyde CS2 CLO2 Counts 1500 1000 Acrolein 1-3 But NH3 SO2 single beam H2S 500 0-500 175.9716339 180.3062418 184.6236179 188.9237305 193.2065478 197.4720381 201.7201696 205.9509104 210.164229 214.3600935 218.5384721 222.6993332 226.8426449 230.9683754 235.0764931 239.1669662 243.2397628 247.2948513 251.3321999 255.3517768 Nanometers 259.3535503 263.3374886 267.3035599 271.2517325 275.1819747 279.0942546 282.9885406 286.8648008 290.7230035 294.5631169 298.3851093 302.1889489 305.974604 309.7420428 313.4912336 317.2221445
Xenon Source Raw Spectrum Raw Data - Xenon Source 1400 1200 Formaldehyde CS2 Chlorine Dioxide SO2 1000 Toluene Xylenes 800 Counts 600 Benzene Single Beam 400 O3 200 0-200 176.1561279 180.4756965 184.7785364 189.0645923 193.3338088 197.5861305 201.8215022 206.0398685 210.2411739 214.4253631 218.5923808 222.7421716 226.8746801 230.989851 235.0876288 239.1679584 243.2307842 247.2760509 251.3037032 255.3136857 nanometers 259.305943 263.2804198 267.2370607 271.1758104 275.0966134 278.9994145 282.8841582 286.7507892 290.5992522 294.4294917 298.2414525 302.035079 305.8103161 309.5671083 313.3054003 317.0251367
Background Spectrum Collect a data spectrum in the atmosphere when the target gas is not present. Define this as the background spectrum. Single Beam Raw Spectrum - Background 3500 3000 2500 2000 1500 1000 500 0 180 185 190 195 200 205 210 215 220 225 230 235 240 245 250 255 260 265 270 275 280 285 290 295 300 305 310 315 320 Signal Strength Wavelength (nm)
Sample Spectrum Collect a data spectrum in the atmosphere when the target gas is present. Define this as the Sample spectrum. Single Beam Raw Spectrum Sample Spectrum 3500 3000 2500 Signal Strength 2000 1500 1000 500 0 180 185 190 195 200 205 210 215 220 225 230 235 240 245 250 255 260 265 270 275 280 285 290 295 300 305 310 315 320 Wavelength (nm)
Absorbance Spectrum Subtract the logarithms of the two spectra. This resulting spectrum is defined as an absorbance spectrum. Absorbance Spectrum 1.4 1.2 1 Absorption 0.8 0.6 0.4 0.2 0 180 200 220 240 260 280 300 320 Wavelength (nm)
Concentra7on 7me series Chalmette Air Monitoring (SO2) PPB 250 200 150 100 50 0-50 9:31 12:15 3:05 6:09 8:50 11:32 Time 14:54 6:45 9:58 12:41 3:22 Ambient SO2 10 Min Limit 24 Hour limit Annual Limit
Limits of Detec7on How do you know you have a detec7on? Spectral Feature of Gas larger than the noise 2x to 6x depending on false posi7ve vs. false nega7ves - level of cau7on Absorbance spectrum correla7on to ref spec.
Current Use UV DOAS Fence- line 5 min 7me resolu7on 5 bbp Benzene LOD Detect Peaks and Leaks Use with Wind data to pinpoint sources
Absorbance Spectra Various concentra3ons Benzene, 5 min 0.003 0.002 0.001 Absorbance - 0.001 0 252 253 254 255 256 257 258 259 260 5 min 0 ppb 5 min.6ppb 5 min 1ppb 5 min 1.8 ppb 5 min 6ppb - 0.002-0.003-0.004 Nanometers
New Monitoring Goals Longer term exposures Lower Levels of detec7on Required Fine Time resolu7on not needed EPA Refinery Rule - 1 ppb Benzene - 2 week sample 7me
Absorbance Spectra Various concentra3ons Benzene, 40 min 0.002 0.001 0 252 253 254 255 256 257 258 259 260 Absorbanc e - 0.001-0.002-0.003 40 min 0 ppb 40 min.6 ppb 40 min 1ppb 40 min 1.8 ppb 40 min 6ppb - 0.004-0.005-0.006 Nanometers
Requirements UV system has to save spectra for each small 7me frame Signal Strength changes need to be accounted for Changes in Pressure cause differen7al Oxygen absorp7on which need to be accounted for
Theory vs. Reality Limit of detec7on improvement 1/sqrt N 100 7mes sample 7me implies 1/10 L.O.D
Theory vs Data 0.00035 0.0003 0.00025 RMS Noise Abs 0.0002 0.00015 Noise Data Theory 0.0001 0.00005 0 0 5 10 15 20 25 30 35 40 45 Sample Time Min
Why not as good as Theory Signal strength fluctua7on O2 Absorp7on Atmos Pressure change Missing /bad data Advanced algorithms can get closer to Theory
Next Genera7on Argos UV - More thermally stable spectrometer/beyer detector array - 5 min Detec7on limits for benzene in the parts- per- trillion (Based on EPA's ETV methodology for determining detec7on limits) - Detec7on limits generated using field data - Final independent valida7on study completed in Summer 2015
New Spectrometer 0.0025 New Spectrometer 2015 0.002 0.0015 ABS 0.001 1 ppb Benzene 5 min Noise 0.0005 0 250.0 252.0 254.0 256.0 258.0 260.0 262.0 264.0-0.0005 Nanometers
Conclusions Exis7ng UV fence- line monitors can meet or exceed new requirements BeYer Time resolu7on 1 ppb LOD Benzene much less than 14 day 7me resolu7on. Several Compounds can be measured Can average spectra from a subset of wind direc7ons much beyer source appor7onment. New Spectrometer promises to deliver 1 ppb L.O.D with 5 min scans