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Assessment of causes of dust at Class I areas in western U.S .

This study assesses the causes of dust at Class I areas in the western U.S. and identifies the types of events resulting in dust on worst dust days. The study utilizes an empirical/heuristic approach, including multivariate linear regression analysis, air mass backward trajectories, land use/land cover data, and wind erodibility metrics. The study also analyzes the seasonal variation and spatial distribution of the identified events.

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Assessment of causes of dust at Class I areas in western U.S .

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  1. Assessment of causes of dust at Class I areas in western U.S. Ilias Kavouras, Vicken Etyemezian, Dave DuBois, Jin Xu, Mark Green DAS/DRI Las Vegas

  2. Worst dust days = 20% worst-case visibility days when dust was the largest contributor (2001-2003) Worst dust days occur more frequently in southern states with dust concentrations as high as 65 μg/m3

  3. Seasonal variation of worst dust days Most of worst dust days are observed during spring in TX, NM, CO, UT, NV For sites in AZ, and CA, dust was the major contributor to reduced visibility during summer

  4. Scope of the study - methodology • Identify type of events resulting in dust on worst dust days in WRAP • Developed an empirical/heuristic approach (EHA) using • Asian Dust Score, a combination of elemental concentration ratios • Multivariate linear regression analysis • Air mass backward trajectories; • Land use/land cover data for US; • USGS Wind erodibility group (soil erosion metric) for US • Use of the EHA to assign worst dust days into a source/event • Integrate all tools into a GIS system and generate maps for each worst dust day • Develop a set of guideline criteria • Long-range transport (Asia) • Windblown events • Upwind transport • Undetermined sources

  5. Trajectories Land use and wind erosion Forests & wetlands Low erodibility based on soil texture Trajectory endpoint at 8:00 a.m. (CST) 0.00 < speed < 14.00 mph " High erodibility based on soil texture 14.00 < speed < 20.00 mph " speed > 20.00 mph " Shrubland and grassland areas Trajectory endpoint at 2:00 p.m. (CST) Low erodibility based on soil texture ! 0.00 < speed < 14.00 mph 14.00 < speed < 20.00 mph ! High erodibility based on soil texture speed > 20.00 mph ! Trajectory endpoint at 8:00 p.m. (CST) Representation of multiple linear regression of wind conditions vs. total measured dust available for this site day Human-induced areas B # 0.00 < speed < 14.00 mph Precipitation occurred at the site Low erodibility based on soil texture # 14.00 < speed < 20.00 mph IMPROVE site with a valid sample ! # speed > 20.00 mph Ï IMPROVE site IMPROVE site without a valid sample High erodibility based on soil texture Local windblown dust (only shown for worst dust days § No Met data Asian Dust Score (only shown for worst dust days) ADS < 750 ! LWD/TMD = 0.00 3 3 750 < ADS < 1500 LWD/TMD < 0.25 ! 3 0.25 < LWD/TMD < 0.50 ! ADS > 1500 ! 0.50 < LWD/TMD < 1.00 ! LWD/TMD > 1.00 Map legends

  6. Low/moderate erodible forest areas Low/moderate erodible human-influenced areas IMPROVE site without a valid sample Precipitation occurred at this IMPROVE site Moderate speed 8:00 trajectory over low/moderate erodible shrubland areas IMPROVE sites with a valid sample but not a worst dust day High speed 14:00 trajectory for WICA over moderate/high erodible shrubland areas that are more than 24 hours away from the site The ADS is higher than 1500 and no LWD was calculated because of no meteorological data Moderate/high speed trajectories upwind of the site The ADS is higher than 1500 and the LWD/TMD is lower than 0.25 Low speed trajectories at and near the site Moderate/high WEG shrubland areas No ADS and LWD/TWD were calculated because meteorological and chemical data were not available The ADS is not calculated due to absence of reliable chemistry data and the LWD/TMD is between 0.50 and 1.00 Moderate speed 20:00 trajectory over Mexico The ADS is higher than 1500 and the LWD/TMD is 0.00

  7. IMPROVE Sites in western U.S 126 IMPROVE sites (all bullets) 72 sites with complete met data (blue and green bullets) 41 sites showed associations between meteorology and dust concentrations (green bullets)

  8. Worst dust days attributed to events 80% of 644 “worst” dust days were assigned to a specific type of event 55 cases were attributed to long range transport from Asia 201 cases were associated with local windblown dust and 252 cases were assigned to regional transport from upwind sources

  9. Spatial variation of events types • NM,CO,UT,WY,ID,TX, : >75% explained; Mostly local windblown dust • AZ, NV, OR, MT: Mostly regional windblown dust (inputs from local dust) • CA, AZ: ~ 30% of cases, the causing event was not determined • ND,SD (local and regional windblown dust), AK (Exclusively Asian dust), WA (mostly regional windblown dust),

  10. Seasonal variation – 1 for western US Almost all cases in spring/fall and most of the summertime cases were attributed to one of the events Undetermined events were mostly observed in summer: MAYBE an indicator of the source

  11. Seasonal variation - 2 • Long range transport from Asia was observed only in spring (in accordance with Asian dust outbreaks) • Local windblown dust occurs more frequently in spring • Regional windblown dust from upwind transport was identified during spring, summer and fall

  12. Seasonal variation – for AZ and CA Arizona • Most of the worst dust days during summer were not explained • Considering: • Agricultural activities (as compared to NM, TX, etc) • Susceptible to wind erosion soil (as compared to MT, ND, SD) • Agricultural emissions are not related to wind speed (usually ag activities occur under calm winds) • Agricultural dust may be a significant source of dust in CA, AZ California

  13. Conclusions-I • Reduced visibility due to high dust concentrations are more frequently observed in southern states (approximately 30% of 20% worst-case visibility days) (644 days during 2001-2003) • A GIS methodology to determine the causes of high dust concentrations at Class I areas in western U.S. was developed • 508 (~80%) worst dust days were attributed to long-range transport from Asia (55 cases), local windblown dust (201 cases) and regional windblown dust from upwind sources (252 cases). • Asian dust occurred during spring, local windblown dust was more pronounced during spring and transport from upwind sources is more important in summer • Undetermined events were mostly observed in CA, AZ; Agricultural dust may be the source.

  14. Case study: Salt Creek and White Mt, NM Based on the previous study: Worst dust days were entirely attributed to local windblown and/or regional transported dust

  15. Regression analysis of wind conditions vs. dust • High-speed (>26 miles/hour) SSW winds are strongly associated with high dust concentrations for both sites • (Blue triangle at 225o) • Moderate associations of • High-speed N-NE winds • Low-speed (14-20miles/hour) E and SE winds • Low-speed E, SSE winds • Indicators of possible source areas Salt Creek White Mt Regression coefficients: wind vs. dust

  16. Methodology Identify the source areas that contributed to elevated dust concentrations during the worst dust days over the period 2001 – 2003 at Salt Creek and White Mountain Wildernesses areas in NM Develop a metric of windblown dust for each area utilizing • Windblown Dust Index (WDI • Range: 0-1 • Highlights the source areas that are likely to exert the greatest influence. • WDI is specific for the site, time period and threshold trajectory speed. • WDI do not provide a direct source attribution for windblown dust. • WDI/distance ratio can provide qualitative information about the relative contribution of source areas. 1. Dust emissions potential (DEP) • Wind Erodibility Group (WEG): • - Indicator of susceptibility to wind erosion • - Classifies soils with similar surface properties affecting their resistance to windblowing in cultivated areas. • The range of valid entries for wind erodibility group data is 1, 2, 3, 4, 4L, 5, 6, 7, and 8 2. Trajectory analysis Integration of points with trajectory speed was higher than 20 or 26 miles/hour during worst dust days using the Kernel spatial probability density normalized by the total number of points

  17. WDI for SACR 2000-2003 20 mph • Moderate-to-strong influence (WDI>0.30): • Local sources (up to 40 miles) from the site including the urban area of Roswell; • Regional sources (up to 150 miles from the site) from the southwest, southeast and northeast; • Upwind transport (up to 450 miles) from the northeast; • Poor-to-moderate influence (WDI<0.30): • Regions located in southwest and central NM, southeast and northeast AZ, north TX and Mexico

  18. WDI for WHIT 2000-2003 20 mph • Moderate-to-strong influence (WDI>0.30): • Local sources (up to 40 miles) southwest of the site including the White Sands Missile Range area • Regional sources (up to 150 miles from the site) from the southwest, and northeast sectors in NM • Poor-to-moderate influence (WDI<0.30): • Regions located up to 450 miles from Mexico and, the northeast,northwest and southwest

  19. Winter Spring Summer Fall

  20. WDI/dist. for SACR 2000-2003 20 mph • Moderate-to-high WDI/distance (and WDI) values are mostly associated with specific types of land use: • Open and closed shrublands, Urban development and agricultural areas near the site (> 50 miles) • Bare ground and open shrubland in White Sands Missile Range area located southwest of the site • Closed shrublands, grasslands and croplands in Texas (not shown here) • Open shrubland and bare grounds in northern Mexico (not shown here)

  21. WDI/dist. for WHIT 2000-2003 20 mph • Moderate-to-high WDI/distance (and WDI) values are mostly associated with specific types of land use: • Bare gound and open shrubland in White Sands Missile Range area located southwest of the site (> 50 miles) • Closed shrublands, grasslands and croplands in east NM and TX areas • Open shrublands in AZ (not shown here) • Open shrubland and bare grounds in northern Mexico. (not shown here)

  22. Conclusions • Two methods to identify the primary causes of dust at Class I areas in western United States were developed and applied • Asian dust, local windblown dust and transport of windblown dust from upwind sources appeared to be the most important sources; The impact of agricultural dust may be significant in CA and AZ • The impacts of dust sources showed significant spatial and temporal variations • The area sources of windblown dust for Salt Creek and White Mountain sites were determined and include, bare ground, shrub lands and ag fields in NM, TX and to a lesser extend in Mexico.

  23. Conclusions-II • A tool to identify source areas that are likely to exert the greatest influence ondust concentration is developed • For Salt Creek and White Mountain areas, the possible source area include: • The urban area of Roswell and agricultural fields and shrub lands within 40 miles of the site; • Bare grounds and shrub lands at the White Sands region including the White Sands Missile range; • Shrub lands, grasslands and agricultural fields in northern Texas especially during summer and fall • Shrub lands and grasslands in SE NM • Bare grounds and shrub lands in northern Mexico

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