1. Vertical profiles of aerosol optical properties from EXSCALABAR
Cathryn Fox, Steve Abel, Justin Langridge,
Jonny Taylor, Huihui Wu, Hugh Coe,
Michael Cotterell, Nick Davies, Kate Szpek
CLARIFY meeting, Exeter
3 October 2018
Cathryn’s work; she recently chose to leave OBR to work elsewhere in Science.
I’ll be picking it up, once I’ve cleared a couple of other things from my desk.

2. Vertical profiles of aerosol optical properties from EXSCALABAR
EXtinction, SCattering and Absorption of Light for AirBorne Aerosol Research
Cathryn Fox, Steve Abel, Justin Langridge,
Jonny Taylor, Huihui Wu, Hugh Coe,
Michael Cotterell, Nick Davies, Kate Szpek
CLARIFY meeting, Exeter
3 October 2018
Cathryn’s work; she recently chose to leave OBR to work elsewhere in Science.
I’ll be picking it up, once I’ve cleared a couple of other things from my desk.

3. EXSCALABAR measurements
Technique
Sample conditioning
Wavelengths (nm)
Extinction
CRDS
Cavity Ring-Down Spectroscopy
Dry
405
662
Humidified: 70%, 90%
Gas reference
Absorption
PAS
Photo-acoustic Spectroscopy
515
Thermally denuded
1 μm impactor
Dry measurements have RH < 30%
Activated Carbon scrubber removes Ozone, NO2
Aerosol filter for background check
A brief overview of what EXSCALABAR is
Several instruments in one
Optical spectroscopy techniques providing greater sensitivity and temporal resolution, lower uncertainties than previously possible on FAAM aircraft, all within a relatively small footprint.
CRDS – dry extinction at 2 wavelengths (red and blue), humidified extinction at 2 humidities, both blue.
PAS – dry absorption at 3 wavelengths (red, green and blue), thermally denuded absorption at 2 wavelengths (blue and red (usually))
Thermally denuded channels -> attribution
Matched PAS and CRDS wavelengths -> SSA at two wavelengths
Common inlet conditioning
Dries (nafion)
Scrubs for ozone, NO2 which absorb at the wavelengths we’re interested in
Impactor (cuts at 1µm or 2µm diameter (geometric), interchangeable)
Filter for background check (~every 10 minutes)
First major campaign for EXSCALABAR.

4. CLARIFY EXSCALABAR data availability
Dry extinction
405, 660 nm
Dry absorption
405, 515, 660 nm
SSA
Ext Ang Exp
Abs Ang Exp
Absorption enhancement
Humidified extinction
70% and 90% (405)
NetCDF available
No QC yet; should be available in future
Not operated
Unexpected results; Lab work required
As NetCDFs (comments welcome!), not yet on CEDA. Will be once finalised. Until then, contact me.
EXSCALABAR time NOT core time.
Not operated on return transit from Liberia.
May need correcting for PAS calibration bath gas consideration. Talk to Michael about what he’s going to present. Read up on it.
Transmission from Rosemount -> rack applied? Nope.
Transmission from rack inlet -> cells applied? Yes.
Extinction data in the files is not the true 1Hz data; has been transformed to match PAS’ slower response for e.g. SSA calc.
Note 1µm impactor (geometric for the flow rate, something bigger aerodynamic).
CRDS time-aligned to each other, PAS currently isn’t. Time is EXSCALABAR time only, NOT aircraft.
Red TD laser bust throughout -> only one wavelength abs enhancement.
Humidified extinction at ~70 and ~90% may be available in the future, but channels not yet characterised/QCd.
A brief look to give an idea of the measurements available.

5. Aerosol extinction vertical distribution
405 nm
660 nm
Ambient RH
Extinction at two wavelengths, RH adjusted (more on that in a minute)
Every data point, including SLRs, from all over the place.
Same horizontal scale (Mm-1) for all plots.
Variability due e.g. location, humidity.
Three regimes highlighted.
Integrated take-off profiles yields AOD -> compare with AERONET

6. AOD: AERONET vs EXSCALABAR
Take-off profiles out of ASI, only where AERONET data is available
Instrument started before take-off -> AOD from GND
DRY extinction from EXSCALABAR CRDS (grey in both plots)
AERONET in blue
Mostly AERONET data from airport site, ARM one often cloud covered.
V1.5 data, Nearest time.
EXSCALABAR AOD low, particularly where smoke in BL (up to C032, then from C042).
Similar story for red wavelength (not shown)
Note 1µm impactor on EXSCALABAR.
Sig aerosol (smoke) in BL -> RH enhancement becomes more important -> larger differences.
Initial “crude” estimate of RH enhancement;
assumed composition (AMS), same for all flights (thank you Huihui!)
assumed PSD (SAMBBA BBA), same for all flights
RH from WVSS2 (GE)
Now comparing yellow with blue ->mostly EXSCALABAR AOD > AERONET AOD.
Very crude first bash; much more that can be done, including looking at EXSCALABAR’s humidified channels sometime in the future.
C030, C031, C048, C049 use ARM site AOD. Others from airport AERONET.
C040; different air mass with smaller particles than assumed PSD?
(Only flights where AERONET data available)

7. Extinction enhancement factor, fRH
Calculated using Kappa Koehler theory with:
Assumed PSD
Composition (AMS)
Assumed refractive indices
Same for all flights and aerosol types (i.e. very crude)
Extinction measured at elevated RH by CRDS; not yet QC’d/available.
Until then:
Assumed SAMBBA BBA PSD
Assumed composition (from AMS), same for all flights/aerosol.
Kappa-Koehler theory
Used WVSS2 data where available, GE otherwise (Steve A)
Even by RH=60%, extinction enhanced by 25% (0.83 -> 0.86)
by RH=80%, fRH=1.6 (0.83 -> 0.88)
Ignoring RH enhancement biases SSA low.

8. AOD: AERONET vs EXSCALABAR
Take-off profiles out of ASI, only where AERONET data is available
Instrument started before take-off -> AOD from GND
DRY extinction from EXSCALABAR CRDS (grey in both plots)
AERONET in blue
Mostly AERONET data from airport site, ARM one often cloud covered.
V1.5 data, Nearest time.
EXSCALABAR AOD low, particularly where smoke in BL (up to C032, then from C042).
Similar story for red wavelength (not shown)
Note 1µm impactor on EXSCALABAR.
Sig aerosol (smoke) in BL -> RH enhancement becomes more important -> larger differences.
Initial “crude” estimate of RH enhancement;
assumed composition (AMS), same for all flights (thank you Huihui!)
assumed PSD (SAMBBA BBA), same for all flights
RH from WVSS2 (GE)
Now comparing yellow with blue ->mostly EXSCALABAR AOD > AERONET AOD.
Very crude first bash; much more that can be done, including looking at EXSCALABAR’s humidified channels sometime in the future.
C030, C031, C048, C049 use ARM site AOD. Others from airport AERONET.
C040; different air mass with smaller particles than assumed PSD?
(Only flights where AERONET data available)

9. Scattering enhancement by RH is important!
Dry
Ambient RH
Ambient RH
Scattering enhancement by RH is important!
C042
May not be most recent EXSCALABAR data, but the take home message is the same.
Small enhancement in free trop. Sig enhancement in BL.
Thanks to Steve Abel

10. SSA Scattering/extinction fRH applied Boundary layer 0.9-0.95
Free troposphere
~0.85
Ignore that hard to read detail.
Hopefully you should get the impression that SSA in BL (blue) is larger than SSA in free trop (red).
Gridlines on this plot?
When > 5Mm-1 extinction:
SSA ~0.85 in free trop
SSA ~ in BL
Extinction humidity enhancement applied (crude).
No absorption humidity enhancement.

11. ORACLES HSRL vs EXSCALABAR
Aug 17th
Elevated dust layer
Ascension Is.
Slide provided by Richard Ferrare (NASA P-3)
Slide provided by Richard Ferrare (NASA P-3)
Note time offset between FAAM and P3 data
Top left -> HSRL shows elevated dust layer
Top right, profiles from HSRL-2 near Asi and EXSCALABAR
-> EXSCALABAR misses much of dust layer, 1µm impactor
Bottom right, AERONET AOD time-series
-> EXSCALABAR data in circles
-> P3 data stars
AERONET measures column
HSRL column up to aircraft (3300 m), true ambient extinction
EXSCALABAR layer up to profile top (5200 m), impactor, crude assumptions used to calculate ambient extinction.
Elevated dust layer in HSRL data (top left)

12. C031 inter-comparison FAAM neph and EXSCALABAR CRDS; P3 neph
More evidence that impactor is significant:
A bit of a profile and a SLR
Rough comparison; scattering and extinction being compared. No wavelength adjustment? Nothing done about different operating RHs?
Black – P3 neph scattering
Grey – EXSCALABAR extinction
Red – FAAM neph scattering
Once settled in SLR ~13:25:
CRDS extinction ~20-30% less than P3 neph scattering
Section highlighted in orange: 1µm impactor on P3 neph inlet -> agree well!
FAAM neph scattering ~ 50% of P3 neph scattering
-> Lab work comparing FAAM neph and EXSCALABAR CRDS
Instruments compare well
Nafion dryer reduces FAAM neph by ~10-20% at most
Rosemount inlet?
Courtesy of Paul Barrett

13. Absorption enhancement
𝐸𝑎𝑏𝑠= 𝑎𝑏𝑠 𝑑𝑟𝑦 𝑎𝑏𝑠 𝑑𝑒𝑛𝑢𝑑𝑒𝑑
Denuder
Catalytic stripper, 350°C
Removes coatings from BC core
Mostly ~1
little enhancement from coatings/organics
However…
SP2 suggests particles are thickly coated.
Only one wavelength.
Compare dry: denuded
Operates at 350oC (Paul Williams)
Some layers in free troposphere show small signal for enhancement.
Arrows highlight != 1
HOWEVER SP2 suggests otherwise…

14. MAC from EXSC / SP2 MAC = Absorption Mass Conc. of BC Blue Absorption
Slope (MAC) =
15.51 ± 0.037
Blue Absorption
BC mass conc (SP2)
“Uncoated” BC MAC
(Bond&Bergstrom)
7.5 m2/g at 550 nm
Extrapolate (using AAE=1)
Blue “uncoated” = m2/g
Eabs (coated/uncoated):
Blue = 15.51/10.19 = 1.52
Via thermodenuder
Via MAC = absorption/BC mass
C037 Eabs profile
MAC from 2016
Zuidema et al., GRL 2018 1 2 3 4 5
Eabs
MAC = Mass Absorption X-section
Can combine SP2 BC mass conc and EXSCALABAR (PAS) absorption to calculated MAC.
As shown Top left:
Data from all flights
EXSCALA Abs vs SP2 BC mass con -> slope indicates MAC
Bottom left:
Zuidema et al :
2016 BB season (blue data points)
PSAP
Reasonable agreement for MAC (also at other wavelengths, not shown)
To convert to Eabs:
Need uncoated BC MAC, such as from Bond and Bergstrom.
Extrapolate to the wavelength of interest (550 nm -> 405 nm)
MAC/uncoated -> Eabs.
(<1 in BL since little absorption measured)
Poor agreement in Eabs. Why?
TD is not removing all coatings
very thick (Knox et al – MAC of BC in relation to chemical age)
TD is not characterised properly
Shell/core model or literature MAC value?
Cappa et al – observed low Eabs values, in contrast to theory
BC particles as inclusions – filters?
Thanks Jonny!

15. What next? EXSCALABAR instrument technical paper (in prep).
Finalise primary EXSCALABAR dataset -->CEDA.
Understand denuder performance.
Lab testing needed; waiting for access to an SMPS.
Generate ambient extinction profiles.
Humidified extinction channels, but first need to demonstrate their performance 
Or in the interim: apply less crude PSD and composition assumptions.  
Support FAAM as they investigate nephelometer limitations. 
Things we need from others:
PSD
Composition assumptions
Please be aware there may well be an issue with FAAM core nephelometer data; EXSCALABAR may well be the only source of extinction (scattering) data.