# Presentation: 425545

## NO2 absolute columns in the stratosphere and troposphere from ground-based UV-visible measurements with the FTUVS: Retrieval method, results, and comparison to OMIThomas J. Pongetti,Shuhui Wang, and Stanley P. SanderJet Propulsion Laboratory California Institute of Technology

**Notes:**
I will first be explaining how the FTUVS total column technique shown by Shuhui is taken a step further to separate the stratospheric and tropospheric partial columns.  Then I will show how the partial column FTUVS data spanning the past three years compares to the stratospheric and tropospheric OMI overpass data provided on the AVDC.

## Table Mountain FTUVS Background

- Rural Los Angeles County
- Altitude : 2.3km (34.4°N, 117.8°W)
- Spectral window : 488 – 498 nm
- Resolution : 0.053 cm-1 ~ 0.001 nm
- Temporal resolution : 17 minutes
- Signal to Noise : ~1000
- NO2 measured 2005 - present

- Atmospheric Spectrum

- Solar Spectrum

- Atmospheric Spectrum Ratio

- High Resolution Spectroscopy
- Absolute NO2 retrieval from direct sun measurements
- Solar Doppler shift method eliminates need for a solar reference
- Straightforward AMF calculation
- Resolves NO2 rotational lines
- Gives vertical profile information

**Notes:**
Direct Sun absolute retrieval basics:
A field stop allows only 2.5% of the solar disk to enter the FTUVS.  Light from the east and west solar limbs are Doppler shifted due to the Sun’s rotation.
-Spectrum from the west solar limb is then shifted to match that of the east.
-When the east spectrum is divided by the west only the ratio of the atmospheric spectrum remains.
-This atmospheric ratio is then fit with shifted and ratioed cross section reference spectra.

## Retrieval Method

- Pressure broadening seen in the NO2 rovibronic features is used to
- resolve stratospheric and tropospheric vertical columns.

- Total column
- Convolution to smear resolution
- NO2 reference and atmospheric data are convolved to eliminate altitude information
- Total column a priori vertical profile assumptions have a limited impact

- Two layer partial column
- Tropospheric and stratospheric references are formed using standard profile assumptions. Then simultaneously fit to the data while being constrained by the total column.

**Profiles**

**Reference**

**Strat.**

**Trop.**

**Notes:**
The two layer retrieval method simply uses the total column method described by Shuhui to constrain a simultaneous fit of both stratospheric and tropospheric references.  First the resolution is smeared in the total column method to reduce the pressure broadening seen in NO2.  This resolution smearing limits the impact of the initial vertical profile assumption which allows for an accurate total column estimate.  Once the total column is known then there exist only one combination of high resolution stratospheric and tropospheric references that fit the measured spectrum.  This process initially uses a clean vertical profile assumption, 80% stratospheric, then iteratively allows the tropospheric value to change until the output stratospheric/tropospheric partition is the same as the input partition.

## Comparison to OMI April, 2005 to Present

- Comparison to OMI April, 2005 to Present

- Terrain image of Table Mountain’s surrounding area looking southwest  including OMI pixel center points that coincide with FTUVS days.  The San Gabriel Mtns. separate the L.A. pollution from the clean desert air.  Note: surface elevation has been exaggerated.

- North

- Los Angeles

- High Desert

- Tejon Pass

- San Gabriel Mountains

- Cajon Pass

**Notes:**
Since April 2005 we have acquired over 80 days of suitable NO2 data.  This image is looking south west from the Mojave desert to los angels.  The green points are the OMI pixel centers for which we have FTUVS NO2 data.  With the highly variable terrain surrounding table mountain combined with the OMI pixel size we expect the OMI tropospheric values to be higher than that of the FTUVS.  Later I will discuss a crude correction for these tropospheric differences. 

## Diurnal Variations at Table MountainUnpolluted                 Polluted

- -Table Mountain

- Diurnal Variations at Table MountainUnpolluted                 Polluted

**Los Angeles**

**Los Angeles**

**Pacific Ocean**

**Pacific Ocean**

**Notes:**
At an elevation of 2.3 km, Table Mountain generally escapes the pollution coming from the Los Angeles basin.  To the left we see an example of this, the prevailing wind is coming from the rural high desert to the north keeping the tropospheric NO2 levels low.  However there are days like the example to the right when the air from LA finds its way up to Table Mountain causing afternoon levels of tropospheric NO2 to rise.  The pink data points are the OMI total and stratospheric values which show very good agreement in the stratosphere but poor agreement in the total column.  This poor agreement is due to the topography of the surrounding area, the OMI pixel covering TMF always includes areas of lower elevation with higher concentrations of tropospheric NO2.  

## Comparison of Stratospheric Vertical Column : FTUVS vs. OMI

**Notes:**
This stratospheric comparison shows a good correlation value of 0.92.  The slope is very close to 1 with a y-intercept 3 orders of magnitude smaller then the average stratospheric value.  Since the stratospheric NO2 is not effected by the terrain we see good agreement between OMI and FTUVS.

## Comparison of Tropospheric Vertical Column : FTUVS vs. OMIShows dependence upon terrain surrounding TMF

- Generally the OMI and FTUVS show better agreement  when the terrain pressure is closer to the pressure at TMF

**Notes:**
In the case of tropospheric NO2 we see significant disagreement between FTUVS and OMI.  This tropospheric comparison plot is colored according to the OMI terrain pressure value reported in the AVDC overpass data set.  Essentially all of the tropospheric NO2 values are higher then that of the FTUVS.  This makes sense because the FTUVS only measures the NO2 above the TMF altitude and the OMI pixel covers an area with a significantly lower average terrain altitude.  The atmospheric pressure at the FTUVS is about 760 hPa, as the OMI average terrain pressure gets further away from this value the tropospheric agreement between the two worsens. 

## Tropospheric Terrain Correction

- TMF pressure

- OMI terrain pressure

- The correction is applied to the FTUVS tropospheric column

**Notes:**
A crude method for adjusting for this discrepancy is to use an assumed tropospheric profile and add the estimated column that lies in between TMF and the OMI terrain average pressure.  The measured FTUVS tropospheric value is used to shift the profile right or left then the calculated amount below TMF is simply added to the FTUVS value.  Currently all tropospheric terrain adjustments have been made using this polluted profile  For a more accurate adjustment, different profiles should be used for different OMI center points and measured NO2 values.  For example if the NO2 value is relatively low and the OMI pixel is centered over the mountain to the south then a cleaner profile should be used.

## Comparison of tropospheric Vertical Column : FTUVS vs. OMIFTUVS column corrected to OMI terrain pressure

**Notes:**
The tropospheric comparison after the terrain pressure adjustment shows a significant improvement over the unadjusted values.  Again this plot is colored to represent the OMI terrain pressure.  The adjustment seems to over compensate at higher terrain pressures and under compensate at low pressures.  This is most likely due to the assumed profile used in the terrain adjustment.  I hope to improve upon this by using different profiles depending upon the where the OMI pixel lands and the relative pollution level measured by the FTUVS.

## Comparison of total Vertical Column : FTUVS vs. OMIFTUVS stratospheric + adjusted tropospheric

- Comparison of total Vertical Column : FTUVS vs. OMIFTUVS stratospheric + adjusted tropospheric

**Notes:**
This total column comparison uses the terrain adjusted FTUVS tropospheric measurement added to the stratospheric value.  While the scatter is moderate, the slope of 0.82 and relatively low y-intercept shows good agreement between the NO2 columns measured from OMI and FTUVS.

## NO2 Seasonal Variation

- Seasons are evident in stratospheric columns, not in tropospheric or Total

-   April 2005               2006                   2007                  2008

**Notes:**
The NO2 seasonal variation in the stratosphere is quite pronounced.  There is no obvious seasonal variation in the tropospheric NO2.  The black line is the FTUVS tropospheric column before the terrain adjustment is made.  Sporadic wind events throughout the year can cause NO2 to be pushed up into the region surrounding TMF but generally speaking TMF is a clean site.

## Summary

|  | FTUVS Relative
to OMI | Uncertainty
+/- | Slope | Correlation |
| --- | --- | --- | --- | --- |
| Stratospheric NO2 | -0.95% | 4.32% | 0.98 | 0.82 |
| Trop. NO2, uncorrected | -34.4% | 28.5% | 0.26 | 0.51 |
| Trop. NO2, corrected | 3.95% | 20.8% | 0.95 | 0.80 |
| Total NO2 | 0.03% | 13.1% | 0.97 | 0.82 |

- FTUVS provides an absolute two-layer retrieval which is directly comparable to the OMI two-layer product.
- OMI and FTUVS agree well in the stratosphere.
- Good agreement is observed in the troposphere after correcting for the mean terrain difference (TMF and OMI footprint).
- Future work:
- improved vertical profile retrieval and terrain adjustment
- continuation of the long-term NO2 data record
- modeling of the observed stratospheric NO2 seasonal variability

**Notes:**
The FTUVS at the Table mountain Facility, CA provides absolute NO2 column measurements in the stratosphere and troposphere which are directly comparable to the OMI data product.  OMI values in the stratosphere show very good agreement to FTUVS showing a 1.43% bias with a 4.32% uncertainty.  Highlighted are the tropospheric values before the terrain adjustment has been made, these values show poor agreement due to the mountainous terrain surrounding TMF.  A terrain adjustment has be shown to estimate the NO2 which lies below TMF and above the OMI terrain height.  This terrain adjustment greatly improves the agreement between OMI and FTUVS tropospheric NO2, reducing the bias form 30.6% to 4.2%.  Future work will include rigorous testing of the retrieval technique, the assumed profiles, and the terrain adjustment method.  Unfortunately the FTUVS is a stationary instrument so we will continue acquiring data at TMF for the long term record.  Other mobile instruments are encouraged to come to TMF for future inter-comparison campaigns.

Thank you and enjoy your lunch.