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Horizontal variability of trace gases over Houston, TX derived from airborne remote sensing, in-situ aircraft measuremen

Horizontal variability of trace gases over Houston, TX derived from airborne remote sensing, in-situ aircraft measurements and regional chemical models. Melanie Follette-Cook 1 , Ken Pickering 2 , Zhining Tao 1 , Scott Janz 2 , Jim Crawford 3 , Matt Kowalewski 1

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Horizontal variability of trace gases over Houston, TX derived from airborne remote sensing, in-situ aircraft measuremen

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  1. Horizontal variability of trace gases over Houston, TX derived from airborne remote sensing, in-situ aircraft measurements and regional chemical models Melanie Follette-Cook1, Ken Pickering2, Zhining Tao1, Scott Janz2, Jim Crawford3, Matt Kowalewski1 1 University of Maryland, Baltimore County 2 NASA Goddard Space Flight Center 3 NASA Langley Research Center

  2. Introduction • Airborne measurements of column trace gas molecular density were taken by the Airborne Compact Atmospheric Mapper (ACAM) instrument during the September, 2008 Newly-Operating and Validated Instruments Comparison Experiment (NOVICE) field campaign over Houston, TX. • Tropospheric column NO2 observations from ACAM, flown on the WB-57 aircraft, are used to quantify the horizontal variability of this gas over the Houston region in support of planning for the future NASA Geostationary Coastal and Air Pollution Events (GEO-CAPE) satellite. • The horizontal variability measured by ACAM and that from in-situ instruments during past field campaigns will be compared with results from the WRF-Chem regional chemical model

  3. Airborne Compact Atmospheric Mapper (ACAM) • Two spectrographs + HD video camera • Air Quality (AQ) 304-520 nm, 0.8 nm resolution for NO2, O3, UV absorbing aerosols, SO2, and HCHO Data from WB-57 are at 2-km resolution along flight track and averaged over the pixels in 12-km cross-track swath • Ocean Color (OC) 460-900 nm, 1.5 nm resolution • Video camera (2592x1936 pixels) – 3 pixel FWHM

  4. Regional Model Output • WRF-Chem (Weather Research and Forecasting model with on-line chemistry) • 4km domain over eastern Texas and western Louisiana, nested within a 12 km domain over the Central US, nested in a 36 km domain covering the entire US • 31 levels in the vertical up to 110 hPa, one hour output interval

  5. Analysis of Horizontal Variability • Structure functions are a useful way to quantify variability in either space and time. The equation for a spatial structure function is as follows: • (Z,y)  <Z(x +y) - Z(x)q> • Where < > denotes the average of data pairs separated by distance y, Z is the variable of interest at a given location x, and q is a scaling exponent. In this case, q=1. • The ACAM NO2 data are retrieved using the DOAS technique and converted geometrically to vertical columns • Only ACAM data where the aircraft was above 14 km were included • Data are assumed to be isotropic (i.e. vector direction between data pairs is not important) • In-situ NO2 aircraft observations from CARB-ARCTAS, ICARTT, TEXAQS2000, and TEXAQS2006 • WRF-Chem structure functions computed using tropospheric NO2 columns (sfc-200hPa) from the Houston region in the 4-km resolution model domain at 1 PM LT. • WRF-Chem structure functions also computed for a small domain (4-km res.) over the Maryland/Washington, DC area and are an average of the structure functions for 7/9/2007 at 7am, 9am, 11am, 1pm, 3pm, and 5pm LT

  6. ACAM OMI

  7. GEO-CAPE Baseline Precision ACAM NO2 Structure Functions at Varying Resolutions 1 km 2 km 4 km 8 km 10 km Average Difference (1015molec/cm2) Distance (km) ACAM data show decreasing NO2 gradients (decreasing variability) with coarser resolution

  8. Structure Function Normalization • The gradient in NO2 from polluted to background conditions is larger than for other species • The magnitudes of the structure functions of field campaign data are therefore highly sensitive to the sampling of urban versus rural regions  • To account for this sensitivity, normalized structure functions are evaluated for polluted conditions only, and the differences are divided by the larger of the two measurements (i.e. one of the measurements has to be > 1 ppbv in the boundary layer or > 3.8 x1015molec/cm2 in the column) • This eliminates measurement pairs at low concentrations where the differences are dominated by measurement uncertainty • The resulting structure function displays the variability in pollution plumes for NO2 exceeding 1 ppbv in the PBL and 3.8 x1015molec/cm2 in the trop column

  9. Normalized Structure Functions for Houston Region 1900 UT 4-km resolution data NEI-05 NOx emissions used in WRF-Chem likely too large

  10. WRF-ChemTropospheric Column NO2 - July 9, 2007 1 PM LT Baltimore-Washington Region 1015molec/cm2

  11. ACAM 4km Houston NO2 Normalized Structure Function and WRF-Chem 4km Baltimore-Washington Normalized NO2 Structure Function After normalization the structure functions represent the rates at which plumes dilute in the two regions ACAM Fractional Average NO2 WRF-Chem Distance (km)

  12. ICARTT NOAA P-3 (7/5/2004 – 8/15/2004) TEXAQS 2000 NOAA P-3 (8/16/2000 – 9/13/2000) TEXAQS 2006 NOAA P-3 (8/31/2006 – 10/13/2006) ARCTAS CARB NASA DC-8 (6/18/2008 – 6/24/2008)

  13. Variability in WRF-Chem vs. In-situ Aircraft Data Flight data: • All data pairs are below 2 km, and span less than 30 min • Measurements at 1 hz (~100 m resolution for NOAA P-3, and 150 m resolution for NASA DC-8) • Data are assumed to be isotropic (i.e. vector direction between data pairs is not important) • Data are assumed to represent a well-mixed boundary layer WRF-Chem: • Structure functions were calculated for the surface model layer over a small domain (4 km resolution) covering the Maryland/Washington, DC area and are an average of the structure functions for 7/9/2007 at 7am, 9am, 11am, 1pm, 3pm, and 5pm LT

  14. NO2 Structure Functions for Near Surface Without normalization With normalization Average Difference (ppbv) Fractional Average NO2 Distance (km) WRF-ChemCARBICARTTTEXAQS2000TEXAQS2006 Balt-Wash WRF-Chem Houston results will be added to these sets of structure functions after improving accuracy of NOx emissions

  15. Tropospheric Column NO2 Structure Functions – Baltimore-Washington Average Difference (1015molec/cm2) GEO-CAPE threshold precision GEO-CAPE baseline precision Distance (km) 7am9am11am1pm3pm 5pm (EST)

  16. Summary • ACAM observations over Houston yield valuable column information for assessing NO2 variability • Structure functions for horizontal gradients computed from ACAM data suggest that GEO-CAPE at 4-km resolution would be capable of obtaining observations exceeding the STM baseline precision over Houston • WRF-Chem simulation over Houston region produced gradients in NO2 tropospheric columns larger than seen in ACAM. NOx emissions likely need updating. • Normalized tropospheric column NO2 structure function from WRF-Chem simulation for Baltimore-Washington region similar in shape to that from ACAM Houston observations. • Normalized structure functions for Baltimore-Washington WRF-Chem near-surface NO2 are similar to those from aircraft campaigns in various regions of the US • WRF-Chem Baltimore-Washington column NO2 at 4-km resolution exceeds both GEO-CAPE STM threshold and baseline precisions during all daytime hours

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