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Revisiting HCHO Pandora direct sun measurements: 2016 - 2019

Revisiting HCHO Pandora direct sun measurements: 2016 - 2019. Elena ( Spinei ) Lind, Virginia Polytechnic Institute and State University Nash Kocur , Virginia Polytechnic Institute and State University Nabil Nowak , Virginia Polytechnic Institute and State University

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Revisiting HCHO Pandora direct sun measurements: 2016 - 2019

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  1. Revisiting HCHO Pandora direct sun measurements: 2016 - 2019 Elena (Spinei) Lind, Virginia Polytechnic Institute and State University Nash Kocur, Virginia Polytechnic Institute and State University Nabil Nowak, Virginia Polytechnic Institute and State University GSFC/NASA Pandora team Luftblick Pandora Team EPA (in situ HCHO data from Westport, CT) KORUS-AQ team

  2. DOAS principle 2

  3. Pandora observations: DS and MAX-DOAS

  4. Multi-axis observation geometry • Absorption depends on: • Wavelength • Aerosol profile and aerosol properties • Clouds • Viewing zenith and relative azimuth angles • Solar zenith angle • Surface albedo • Gas profiles

  5. Pandora “standard” MAX-DOAS MAX-DOAS Pandora standard short scan -Each measurement is analyzed relative to a reference spectrum within 2-6 min; - Instrument typically does not change within this time frame; - Retrievals are mostly insensitive to instrumental changes 4-12 min 5

  6. Pandora observations • Direct sun: • most preferred mode due to simplicity in analysis (total column only) and high temporal resolution; • Sensitive to instrument changes: e.g. HCHO in the Pandora head sensor (temperature driven); • Requires SCDreference estimation • MAX-DOAS: • Researchers have typically less confidence in the profile products due to Air Mass Factor uncertainties; • Has lower time resolution • Less sensitive to instrument changes

  7. Pandora head sensor Lens Baffle holding tube Electronics board Dimensions are in mm Delrin components: - 2 filter wheels - baffle holding tube

  8. What is Delrin and why use it? • Polyoxymethylene(POM), also known as polyformaldehydeis an engineering thermoplastic used in precision parts requiring high stiffness, low friction, and excellent dimensional stability. • is produced by different chemical firms with slightly different formulas and sold under such names as Delrin, Celcon, Ramtal, Duracon, Kepital, and Hostaform. • Typical applications for injection-molded POM include high-performance engineering components such as small gear wheels, eyeglass frames, ball bearings, ski bindings, fasteners, knife handles, and lock systems. The material is widely used in the automotive and consumer electronics industry.

  9. Does Pandora HS make HCHO? Enclosure set T = 45C Internal T = 58C Enclosure set T = 35C Internal T = 45C 1 ramp up 2 Cool down 2 Ramp up 1 cool down Enclosure set T = 15C Internal T = 19C Enclosure set T = 15C Internal T = 19C Window and collimator baffle removed 1 ramp up – from 35 C to 45 C (manual) 1 cool down – 70 min for 5 C decrease (automated) from 45C to 15C set box temperature 2 ramp up – 30 min per 5C from 15C to 45C set box temperature (30 min at 15C before) 2 cool down – 40 min per 5C from 45C to 15C set box temperature (30 min at 15C after)

  10. Pandora HS makes its own HCHO Internal T = 58oC HCHO column INSIDE P148 head sensor [DU] No accumulation at internal T = 58 C for 13 hrs (lamp off) Internal T = 44oC Internal T = 19oC Internal T = 19oC No accumulation at internal T = 44 C for 5 hrs (lamp off) Window and collimator baffle removed Complete “deposition” of HCHO Light Source: FEL at 7A; Integration time 300 sec; reference spectrum taken by Pandora 148 at internal T = 19oC, box T = 15oC; U340 filter

  11. Pandora HS makes its own HCHO Delrin parts (filter wheels and “baffle” tube) off gas HCHO as a function of temperature HCHO column density INSIDE Pandora 148 head sensor [DU] RELATIVE to 10°C Exponential fit Lab measurements (HCHO cross section Meller et al at 297K) Temperature INSIDE Pandora 148 head sensor [oC] Temperature decrease and increase rates varied from 0.07 to 0.4 °C/min

  12. Pandora makes its own HCHO Delrin parts (filter wheels and “baffle” tube) off gas HCHO as a function of temperature • HCHO production/deposition within head sensor has strong temperature dependence (small T-rate dependence) • Measurement spectra temperature vs. reference spectrum temperature determines the “severity” of the internally produced HCHO • Choice of the reference spectrum and analysis can minimize the differential column retrieved within the Pandora head sensor

  13. Approaches to mitigate HCHO: Direct Sun Delrin parts (filter wheels and “baffle” tube) off gas HCHO as a function of temperature • For short-term campaigns: creating reference spectra by averaging spectra from multiple days and SZA => reduces the derived HCHO within the head sensor • For long-term measurements: subsetting Pandora spectra based on expected temperature within the head sensor and processing these subsets independently => almost eliminates the problem (assuming Pandora is stable otherwise) • Estimating HCHO from head sensor temperature,…but there are no temperature sensors inside (prior to April 2019) => thermodynamics studies Tmeasurements≅ Treferenceand similar wind speeds => Minimum interference TmeasurementsandTreference (internal) < 30°C => Minimum interference Tmeasurements>> Treferenceor/and different wind speeds (e.g. summer vs. winter) => MAXIMUM interference 13

  14. HCHO tropospheric columns Direct sun columns derived using modified Langley plot (50th percentile ) Similar trends between DS and MAX-DOAS trop columns

  15. HCHO tropospheric columns Point-by-point differences between DS and MAX-DOAS trop columns. Some of it is due to Delrin off gassing.

  16. LISTOS: Westport, CT (2018) High linear correlation betweenin situ and MAX-DOAS surface concentrations: slope 0.86, R2 = 0.81 for all conditions slope 0.94, R2 = 0.92 for cloud free conditions Near Surface HCHO volume mixing ratio [ppb] EPA in situ surface concentration MAX-DOAS near surface concentration

  17. LISTOS: Westport, CT (2018) Direct sun Max-DOAS Tropospheric HCHO columns [DU] Maximum 0.3 DU differences between DS and MAX-DOAS trop columns. Some of it is due to Delrin off gassing.

  18. Heat transfer to/from Pandora head sensor • Heat produced by electronics board and filter wheel motors; • Convective heat transfer as a function of ambient temperature, wind speed and sensor orientation relative to wind direction; • Short-wave solar flux absorption (direct and diffuse components); • Long-wave radiation emission. 18

  19. Heat transfer to/from Pandora head sensor • Heat produced by electronics board and filter wheel motors: heat flux measurements under typical DS operation => 18 locations (2 front and back and 16 along the cylinder) 19

  20. Heat transfer to/from Pandora head sensor VirginiTech 0.7m subsonic open jet wind tunnel 20

  21. Heat transfer to/from Pandora head sensor • Convective heat transfer coefficients as a function of wind speed (0.5 – 12 m/s) and head sensor orientation 21

  22. Heat transfer to/from Pandora head sensor • Short-wave solar flux: total and diffuse only (correlation with integrated Pandora corrected counts and H2O column) still in progress. Effect of clouds? 295 to 2685 nm Apogee Thermopile Pyranometers (SP-610 model): sun tracking 22

  23. Temperature inside Pandora 148 HS Blacksburg, VA (May, June 2019) Temperature [oC] Measured inside head sensor Estimated inside head sensor Ambient

  24. KORUS AQ (Olympic Park): no T-sensor Estimated inside head sensor: wind data from Taehwa Temperature [oC] Estimated inside head sensor Ambient temperature

  25. KORUS AQ (Olympic Park): no T-sensor Estimated inside head sensor: wind data from Taehwa Inside head sensor HCHO column [DU]

  26. KORUS AQ: Olympic Park - OLD ~ 0.45 DU ~ 0.5 DU ~ 0.8 DU ~ 0.8 DU ~ 0.8 DU ~ 0.4 DU • Most likely not all of the differences cannot be attributed to HCHO formed inside the sensor: • New fitting window to represent median of 36 DOAS fitting cases (328.5 – 360 nm) • Still the question of SCDref determination remains Atmosphere + head sensor HCHO column [DU] • Aircraft in situ integrated measurements • Pandora vertical column densities from direct sun data • ---Surface in situ “ground up” columns using different profile assumptions

  27. Conclusions and future work • HCHO production/deposition within head sensor has strong temperature dependence (small T-rate dependence) and can be estimated based on ambient temperature, head sensor orientation and wind speed and direction and solar flux measurements; • Old systems should be tested by using temperature surface sensors and LED source in the field to ensure the same HCHO production and heat transfer; • Choice of the reference spectrum and heat transfer analysis can significantly minimize the differential column retrieved within the Pandora head sensor

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