# Presentation: 693401

## Validation of MODIS aerosol observations over the Netherlands with GLOBE student measurements

- Folkert Boersma*1,2* and Joris de Vroom*1*
*1. KNMI, De Bilt, the Netherlands*
*2. Now at Harvard University, Cambridge, USA*
- _[boersma@fas.harvard.edu](mailto:boersma@fas.harvard.edu)_
_*www.knmi.nl/globe*_

## Why a student-based aerosol validation project?

- = **G**lobal **L**earning and **O**bservations to **B**enefit the **E**nvironment – since 1995

- Outreach
-  get satellite research (OMI) and
-  atmospheric research to schools
-  generate publicity for OMI
- 2. Science validation of OMI aerosols (and first MODIS as a demonstration)

**Notes:**
De negatieve charge at the cloud base repels the negative charge initially at the surface –positive charge is induced by the cloud
The charge separation is thought to occur from COLLISION of precipitation particles. The HEAVY particles fall to the cloud base and these are negatively charged. The LIGHTER ones are carried upward to the cloud top, and these are positively charged.
As soon as the gas cools faster than NO equilibrium mixing ratio’s are settled we gave a freeze-out of NO.

## School measurements provide potential for dense network that cannot be reached with professional instruments! (D. Brooks – Drexel University)

**Notes:**
De negatieve charge at the cloud base repels the negative charge initially at the surface –positive charge is induced by the cloud
The charge separation is thought to occur from COLLISION of precipitation particles. The HEAVY particles fall to the cloud base and these are negatively charged. The LIGHTER ones are carried upward to the cloud top, and these are positively charged.
As soon as the gas cools faster than NO equilibrium mixing ratio’s are settled we gave a freeze-out of NO.

## How do students measure AOT?

**How do students measure AOT?**

-  Use a simple hand-held Sun photometer developed by D. Brooks
-  Point to the Sun in a cloud-free sky
-  Two LEDs detect light at 508 nm and 625 nm
-  Record light intensity, local time, surface pressure
-  Apply Lambert-Beer’s law

- Students do
- measurement

**Notes:**
De negatieve charge at the cloud base repels the negative charge initially at the surface –positive charge is induced by the cloud
The charge separation is thought to occur from COLLISION of precipitation particles. The HEAVY particles fall to the cloud base and these are negatively charged. The LIGHTER ones are carried upward to the cloud top, and these are positively charged.
As soon as the gas cools faster than NO equilibrium mixing ratio’s are settled we gave a freeze-out of NO.

## How good is AOT measured by a simple Sun photometer?

**How good is AOT measured by a simple Sun photometer?**

**Notes:**
De negatieve charge at the cloud base repels the negative charge initially at the surface –positive charge is induced by the cloud
The charge separation is thought to occur from COLLISION of precipitation particles. The HEAVY particles fall to the cloud base and these are negatively charged. The LIGHTER ones are carried upward to the cloud top, and these are positively charged.
As soon as the gas cools faster than NO equilibrium mixing ratio’s are settled we gave a freeze-out of NO.

## How good is AOT measured by students with a Sun photometer?

**How good is AOT measured **_**by students**_** with a Sun photometer?**

- In the 2002-2003 period we found
-  22 collocated measurements of
-  students by De Populier (The Hague) and the
-  AERONET CIMEL Sun Photometer (TNO-FEL)
-  x = 4 km, t = 30 min

**Notes:**
De negatieve charge at the cloud base repels the negative charge initially at the surface –positive charge is induced by the cloud
The charge separation is thought to occur from COLLISION of precipitation particles. The HEAVY particles fall to the cloud base and these are negatively charged. The LIGHTER ones are carried upward to the cloud top, and these are positively charged.
As soon as the gas cools faster than NO equilibrium mixing ratio’s are settled we gave a freeze-out of NO.

## How good is AOT measured by students with a Sun photometer?

**How good is AOT measured **_**by students**_** with a Sun photometer?**

-  Theoretical error analysis:  precision better than 0.02 AOT
-  KNMI testcase: bias <0.005, precision ~0.01 AOT
-  De Populier testcase: bias < 0.04, precision ~0.03 AOT
- 	- time differences[30m]
- 	- larger distance [4km]
- 	- students vs. professionals
- 	- calibration issues

- ...Good enough to try and validate MODIS AOT!

**Notes:**
De negatieve charge at the cloud base repels the negative charge initially at the surface –positive charge is induced by the cloud
The charge separation is thought to occur from COLLISION of precipitation particles. The HEAVY particles fall to the cloud base and these are negatively charged. The LIGHTER ones are carried upward to the cloud top, and these are positively charged.
As soon as the gas cools faster than NO equilibrium mixing ratio’s are settled we gave a freeze-out of NO.

## The Hague school reveal land-water boundary problem with MODIS

**The Hague school reveal land-water boundary problem with MODIS**

**Notes:**
De negatieve charge at the cloud base repels the negative charge initially at the surface –positive charge is induced by the cloud
The charge separation is thought to occur from COLLISION of precipitation particles. The HEAVY particles fall to the cloud base and these are negatively charged. The LIGHTER ones are carried upward to the cloud top, and these are positively charged.
As soon as the gas cools faster than NO equilibrium mixing ratio’s are settled we gave a freeze-out of NO.

## Slide 9

**Notes:**
De negatieve charge at the cloud base repels the negative charge initially at the surface –positive charge is induced by the cloud
The charge separation is thought to occur from COLLISION of precipitation particles. The HEAVY particles fall to the cloud base and these are negatively charged. The LIGHTER ones are carried upward to the cloud top, and these are positively charged.
As soon as the gas cools faster than NO equilibrium mixing ratio’s are settled we gave a freeze-out of NO.

## MODIS vs. GLOBE AOT over the NL

**MODIS vs. GLOBE AOT over the NL**

- September 2002 – October 2005

*r**2 *= 0.66
*n *= 61

*r**2 *= 0.50
*n *= 61

- bias 	= +0.03 AOT
- RMS 	= 0.11 AOT
- y	= 0.10 + 0.78x

- bias 	= -0.01 AOT
- RMS 	= 0.11 AOT
- y	= 0.05 + 0.69x

**Notes:**
De negatieve charge at the cloud base repels the negative charge initially at the surface –positive charge is induced by the cloud
The charge separation is thought to occur from COLLISION of precipitation particles. The HEAVY particles fall to the cloud base and these are negatively charged. The LIGHTER ones are carried upward to the cloud top, and these are positively charged.
As soon as the gas cools faster than NO equilibrium mixing ratio’s are settled we gave a freeze-out of NO.

## Instrument

- Broad sensitivity

## Instrument

- Broad sensitivity
-  Define an effective wavelength for aerosol retrieval?
-  If yes, what is the stability (error) associated?

## Effective wavelength

-  Instrument measures atmospheric transmission:

- eff is the wavelength for which it holds that:

## Effective wavelength

- Effective wavelength thus depends on:
- I0()
- R()
- T() and thus on M, R, O , a

-  = 2.0

## Effective wavelength

- Aerosol type:

- M = 2.0
- AOT = 0.5
- O3 = 300 DU
- p = 1013 hPa

## Effective wavelength

- Neglect sensitivity to O3, pressure, and Angstrom coefficient
- In the GLOBE project, AOT reported at fixed wavelengths:
- 	Green: 	508 nm
- 	Red: 	625 nm
- Use Lookup Table to find ***eff**(M, AOT)* to correct for wavelength errors

## Effective wavelength

- Use Lookup Table to find ***eff**(M, AOT)* to correct for wavelength errors

## Typical corrections over the Netherlands

- Range AOT 508 nm: 0.0 - 0.8
- Range AOT 625 nm: 0.0 – 0.6

- AOT 508 < 0.002
- AOT 625 < 0.001

## Calibration: Langley method

- Conditions

-  Observations of irradiance for several zenith angles
*	**T = *e-***	**	**	**	*(1)
*	**I = I**0**·*e-*m**** *			(2)
- 	ln *I* = ln *I**0* + *m* ln *T**	**	*(3)
-  Clear day
-  Atmospheric properties (*T*) constant
-  Correction for Earth-Sun distance
-  Extrapolate to *m*=0: *I=I**0*

## Langley method

- 7 april 2003

- V0 = 2.209 (±0.011)
- r2 = 0.999
- V0 = 1.907
- (±0.009)
- r2 = 0.998

## Langley method

- 8 april 2003

- V0 = 2.167 (±0.019)
- r2 = 0.998
- V0 = 1.849
- (±0.011)
- r2 = 0.999

## Langley method

- 17 april 2003

- V0 = 2.292 (±0.021)
- r2 = 0.997
- V0 = 2.051
- (±0.016)
- r2 = 0.997

## Langley method

- 9 september 2004

- V0 = 2.181 (±0.013)
- r2 = 0.999
- V0 = 1.845
- (±0.014)
- r2 = 0.996

## Langley method

- Four Langley analyses RG2-047:

| Date | V0 Green | V0 Red | Visibility |
| --- | --- | --- | --- |
| 07-04-2003 | 2.209 (0.011) | 1.907 (0.009) | 40 km |
| 08-04-2003 | 2.167 (0.019) | 1.849 (0.011) | 30 km |
| 17-04-2003 | 2.292 (0.021) | 2.051 (0.016) | 15 km |
| 09-09-2004 | 2.181 (0.013) | 1.845 (0.014) | 25 km |
| Average | 2.186 (0.021) | 1.867 (0.035) |  |

## GLOBE calibration concept

- No calibration relative to SPUV
- Demonstration project
- Comparison with SPUV gives impression of quality of retrieval
- Differences due to
- calibration differences
- algorithm differences

## Comparing GLOBE and SPUV voltages

- r2: 0.995 (n=149)

- r2: 0.985 (n=149)

- r2: 0.987 (n=149)

- Simultaneous collocated observations
- September 2002 - April 2003

## Comparing GLOBE and SPUV AOTs

- Use Angstrom’s relationship:
***1**: *GLOBE wavelength
***2**: *SPUV wavelength
**: Angstrom coefficient (from SPUV 501 nm and 670 nm)

## Comparing GLOBE and SPUV AOTs

*y* = -0.02+1.10*x *(0.06)
- Av. bias:	-0.005
- RMS:	0.009

- r2 = 0.992 (n=50)

## Comparing GLOBE and SPUV AOTs

*y* = -0.01+1.13*x *(0.08)
- Av. bias:	+0.002
- RMS:	0.011

- r2 = 0.98 (n=50)

## GLOBE calibration concept

- Calibration constants for all schools relative to RG2-047:
- R: instrument ratios from
- 	simultaneous measurements
- Advantage:
- - Independent retrievals
- - Demonstrate attainable accuracy with independent approach

## Comparing GLOBE and AERONET AOTs in The Hague

- Use Angstrom’s relationship:
***1**: *GLOBE wavelength
***2**: *AERONET wavelength
**: Angstrom coefficient (from AERONET 440 nm and 670 nm)

## Comparing GLOBE and AERONET AOTs in The Hague

- Comparing GLOBE and AERONET AOTs in The Hague

*y* = 0.05+0.94*x *(0.05)
- Av. bias:	+0.04
- RMS:	0.03

- r2 = 0.96

## Comparing GLOBE and AERONET AOTs in The Hague

- Comparing GLOBE and AERONET AOTs in The Hague

*y* = 0.06+0.80*x *(0.05)
- Av. bias:	+0.03
- RMS:	0.03

- r2 = 0.93

## Comparing GLOBE and AERONET AOTs in The Hague

- Comparing GLOBE and AERONET AOTs in The Hague

- Calibration constants derived from RG2-047
- Demonstration project
- Small bias, excellent correlations: students can do it!
- Differences due to (note: n = 22)
- calibration differences
- algorithm differences

## Validation of MODIS AOT with GLOBE schools in the NL

- How?
- MODIS AOT at 470 nm and 660 nm
- Extrapolate GLOBE AOT to MODIS with GLOBE Angstrom coeff.
- Criteria
- School location within MODIS pixel
- |t| < 10’
- n = 11:

## Validation of MODIS AOT with GLOBE schools in the NL

- All data

*y* = 0.08+0.81*x *(0.28)
- Av. bias:	+0.03
- RMS:	0.06

- r2 = 0.84 (n=11)

*y* = 0.01+0.89*x *(0.41)
- Av. bias:	+0.003
- RMS:	0.043

- r2 = 0.76 (n=11)

## Validation of MODIS AOT with GLOBE schools in the NL

- How?
- MODIS AOT at 470 nm and 660 nm
- Extrapolate GLOBE AOT to MODIS with GLOBE Angstrom coeff.
- Criteria
- School location within MODIS pixel
- |t| < 3 hours to generate statistics

## Validation of MODIS AOT with GLOBE schools in the NL

- All data

*y* = 0.07+0.75*x *(0.11)
- Av. bias:	+0.03
- RMS:	0.12

- r2 = 0.71 (n=62)

*y* = 0.02+0.77*x *(0.14)
- Av. bias:	+0.01
- RMS:	0.09

- r2 = 0.72 (n=62)

## Validation of MODIS AOT with GLOBE schools in the NL

- Increasing time criterion does not (significantly) affect
- Slope
- Av.bias stays the same
- RMS doubles

## Validation of MODIS AOT with GLOBE schools in the NL

- Bernard Nieuwentijt - Amsterdam

*y* = 0.01+0.94*x *(0.22)
- Av. bias:	-0.003
- RMS:	0.091

- r2 = 0.73 (n=22)

*y* = 0.04+0.91*x *(0.27)
- Av. bias:	+0.043
- RMS:	0.096

- r2 = 0.40 (n=22)

## Validation of MODIS AOT with GLOBE schools in the NL

- De Populier – The Hague

*y* = 0.05+0.78*x *(0.51)
- Av. bias:	+0.01
- RMS:	0.12

- r2 = 0.45 (n=7)

*y* = -0.002+0.70*x *(0.80)
- Av. bias:	-0.04
- RMS:	0.09

- r2 = 0.15 (n=7)

## Validation of MODIS AOT with GLOBE schools in the NL

- Zwin  – Oostburg

*y* = 0.13+0.81*x *(0.41)
- Av. bias:	+0.10
- RMS:	0.05

- r2 = 0.98 (n=5)

*y* = 0.01+0.79*x *(0.43)
- Av. bias:	-0.04
- RMS:	0.06

- r2 = 0.98 (n=5)

## Validation of MODIS AOT with GLOBE schools in the NL

- KNMI  – De Bilt

*y* = 0.10+0.53*x *(0.20)
- Av. bias:	+0.02
- RMS:	0.07

- r2 = 0.53 (n=28)

*y* = 0.05+0.31*x *(0.23)
- Av. bias:	-0.02
- RMS:	0.05

- r2 = 0.50 (n=28)

## Validation of MODIS AOT in coastal regions

- Marken and The Hague locations, 3 hour time difference

- Coastal av. bias:	+0.10 (0.16)
- Inland av. bias:	-0.01 (0.09)

- Coastal:	r2 = 0.50 (n=15)
- Inland:	r2 = 0.58 (n=12)

- Coastal: 	r2 = 0.35 (n=15)
- Inland:	r2 = 0.47 (n=12)

- Coastal av. bias:	+0.09 (0.15)
- Inland av. bias:	-0.01 (0.06)

## Extra slides

- Comparing Angstrom coefficients in De Bilt
- SPUV 501-670 nm: 		1.15  0.34 (n = 50)
- GLOBE 508-625 nm: 	0.66  0.73
- AOT 508 nm > 0.15
- SPUV 501-670 nm: 		1.11  0.39 (n=13)
- GLOBE 508-625 nm: 	0.73  0.36

## Extra slides

- Comparing Angstrom coefficients in The Hague
- AERONET 440-670 nm: 	1.46  0.62 (n = 22)
- GLOBE 508-625 nm: 	1.35  0.65
- AOT 508 nm > 0.15
- AERONET 440-670 nm: 	1.44  0.58 (n=18)
- GLOBE 508-625 nm: 	1.43  0.66