# Presentation: 506339

## New MultiFunction Differential Optical Absorption Spectroscopy Instrument for Aura/OMI Validation –  Preliminary Results for NO2 from INTEX B

**	****	****Elena Spinei****1****, George Mount****1****	****	****	****	****	****	****	**** P. K. Bhartia****2****,  Ernest Hilsenrath****2**** ****	****	****	****	****	****	****	****Anne Thompson****3****, Brett Taubman****3****	****1**** Washington State University, Pullman, WA ****	****	****	****	****	****2**** Laboratory for Atmospheres, NASA/GSFC, Greenbelt, MD****	****	****	****3**** Pennsylvania State University, University Park, PA****	**

**1. Introduction:**
-  The Aura satellite was launched on July 15, 2004 with four scientific instruments on board, including the Ozone Monitoring Instrument (OMI).  OMI is a nadir viewing UV/VIS imaging spectrometer that measures ultraviolet and visible backscattered solar radiation for retrieval of tropospheric and stratospheric vertical columns of such gases as NO2, O3, SO2, BrO, HCHO and OClO. Measurement uncertainties from satellite instruments require them to be properly validated. A MultiFunction Differential Optical Absorption Spectrometer instrument (MFDOAS), built for ground based validation of OMI, was first fielded in a prototype form during the INTEX B campaign at Pacific Northwest National Laboratory in Richland, WA, April 17 – May 17, 2006.

**2. Objectives:**
- The objectives of this presentation are:
-   Description of the MFDOAS instrument
-   Description  of our INTEX B campaign participation
-   Presentation of data analysis and NO2 slant column results from INTEX B measurements
-   Comparison of tropospheric NO2 ΔSCD with in-situ CO measurements
-   Preliminary comparison of NO2 slant column results with OMI tropospheric
-     NO2 products
-   Future work

**6. NO****2**** slant column results**
- Figure 3 presents the spatial and temporal variation of NO2 differential slant column for May 9, 2006, a polluted day. Higher column densities were observed to the south and east towards the urban center. Measurements taken at 5-degree elevation showed higher NO2 tropospheric column compared to 15 and 45-degree angles, as expected. These elevated NO2 slant column densities were particularly pronounced during the morning rush hour.

- To evaluate the ability of the MFDOAS instrument to measure local pollution we compared CO mixing ratio taken at the PNNL site with the MFDOAS derived differential NO2 slant column densities. CO was chosen to represent local vehicular pollution. The NATIVE trailer produced in-situ measurements of O3, NO, NOy, CO, SO2, and meteorological conditions. Figures 4, 5, 6 show NO2 differential SCD and CO mixing ratios for one unpolluted  and two polluted days. Good correlation is observed between the temporal variations of both species from the NATIVE trailer and MFDOAS. Differences can be generally attributed to the fact that CO measurements are point measurements north of the city center versus long path measurements from the MFDOAS.

**8. Preliminary comparison of NO****2****  differential slant column results with the OMI tropospheric NO****2**** **
- The Aura satellite flies over Tri-Cities area around 13:00hr with a spatial resolution of 13km x 24km. Figure 7 shows a contour plot of tropospheric NO2 VCD for May 9, 2006. OMI tropospheric NO2 vertical column densities “integrated” over several pixels in the MFDOAS observation direction were compared to MFDOAS differential NO2 SCDs  derived from 5-degree elevation south observations. Figure 8 shows preliminary results for April 30 through May 13, 2006. Reasonable correlation between MFDOAS measurements and OMI results is observed, however not for all days. Some of the discrepancies can be attributed to “integration” of OMI data along the MFDOAS observation path. The comparison will become more quantitative when our slant column numbers can be replaced by vertical column abundances after application of a spherical radiation code.

**9. Conclusions**
- The MFDOAS instrument is unique in its ability to take both scattered sky measurements and direct sun measurements. This combination allows measurement of air pollution with simple AMFs from the direct sun and also makes use of the MAXDOAS azimuth/elevation technique coupled with a spherical radiative transfer code to derive spatial and full day temporal dependence of urban pollutants. The tropospheric NO2 pollution trends are captured by the MFDOAS instrument as confirmed by in-situ measurements. Preliminary intercomparison with OMI NO2 tropospheric column densities show reasonable correlation, but require further investigation.

**10. Future work**
-    Reduction of the differential slant column abundances to vertical tropospheric columns for direct comparison with OMI for both NO2 and ozone, and use of the new instrument in future field campaigns
-    Direct sun measurements will be conducted during fall 2006 in combination with the scattered sky measurements in Washington state
-    Intercomparisons at Goddard Space Flight Center and at Table Mountain Observatory (JPL) will be conducted in the winter/spring time frame.

**Acknowledgment **
- We thank the DOMINO team at KNMI (Ruud Dirksen, Henk Eskes, and Folkert Boersma) for providing us with the OMI tropospheric NO2 data, NASA Headquarters for sponsoring the development and deployment of the MFDOAS instrument, and Mr. Kurt Hutchinson and the WSU Engineering Shop for building the instrument.

**4. INTEX B campaign**
- The Intercontinental Chemical Transport Experiment – Phase B (INTEX-B) took place from 17 April to 17 May 2006. The primary goal of the (mainly aircraft) campaign was to quantify the transpacific  transport and evolution of Asian pollution to North America with the emphasis on validation of satellite observations of tropospheric composition. The ground-based MFDOAS instrument was first fielded in a prototype form during this campaign for Aura/OMI validation. This presentation will focus on tropospheric NO2 slant column results derived from the scattered skylight observations.

**4b. Measurements**
- The MFDOAS instrument operated mainly in scattered skylight mode (MAXDOAS). The observation sequence included elevation angles of 5o, 15o, 45o, and zenith at each of azimuths north, south, east, and west. Direct sun observations were unfortunately not made reliably. The observation time at each azimuth/elevation position was about 1 min, with the spatial time resolution of about 25min per full scan.

- Richland is located in a semi-rural area known as Tri-Cities (merged cities of Kennewick, Pasco and Richland, WA) with total population of approximately 150,000 in an area of 250 km2. PNNL is situated north from the center of Richland (approximately 15 km) and north-west of the population center.

**4a. Location**
- The MFDOAS instrument was positioned on the roof of a building at Pacific North  West National Laboratory, Richland, WA (latitude: 46.3409, longitude:
- -119.2787)

- The major source of local NO2 pollution is vehicular exhaust.

**5. Data Analysis **
- NO2 slant columns were derived using the differential optical absorption spectroscopy (DOAS) technique. DOAS is based on the Beer – Lambert law, where σ**’** is a differential absorption cross section for molecule “i”, and I**’**o is a modified I o.
- Vertical column retrieval from the multi-axis measurements requires application of spherical radiative transfer theory and is not addressed at this point.   We will concentrate on derivation of slant columns.  A nonlinear least squares algorithm was utilized to fit cross sections of NO2, O3, Ring, and instrument polarization in a small spectral region 400nm-419nm. A polynomial was also added to remove slowly varying Rayleigh and Mie Scattering. The reference spectrum used for data analysis (Io in Beer – Lambert Law) was taken at local noon zenith on April 30, 2006, an unpolluted day. Raw spectra were corrected for detector darks and flat field. Figure 2 shows residual optical densities after fitting procedure for observations taken on May 9, 2006 at 5-degree elevation and all directions.

**3. MFDOAS instrument description:**
- The MFDOAS instrument observes scattered skylight with a 1° FOV at varying viewing azimuth and elevation angles as well as direct sun light in the uv-visible spectral region for determination of NO2, O3, SO2, and CH2O slant columns. The MFDOAS spectrograph is a single pass Czerny-Turner (Acton SP-2300i) spectrograph of focal length 300mm. The instrument covers a wavelength range from 305nm to 480nm, and has a spectral resolution of 0.82nm (6 pixels FWHM). Scattered sky light is collected by a 12cm telescope and passes into the spectrograph through two filter wheels which contain depolarizers, spectral flattening filters, and uv cutoff filters. Direct sun light is fed into a spectralon integrating sphere (diameter 8cm) before passing through the filter wheels.

**Figure 1. **
**Schematic of the MFDOAS**

**7. Comparison of tropospheric NO****2**** SCD with CO in-situ measurements**

- A two-dimensional CCD detector (400 x 1340 pixels2) is used in the focal plane. Spectrograph stray light is reduced by a spectral flattening filter which reduces the long wavelength throughput of the  instrument. Tracking/positioning utilizes a Kipp & Zonen tracker which moves the entire instrument. An instrument schematic is presented in Figure 1.