Figure 2. Trend of chemical bext at (a) BLIS1 and (b) SOLA1. The whiskers and boxes indicate 90th, 80th, 20th, and
10th percentile of bext for each year. Small bars indicate the median values. Linear regressions for the 90th and 10th
percentile data are calculated and presented. Red lines indicate the current 90% and 50% standards for (a) regional
and (b) sub-regional visibility. The mean natural background level is also indicated.
Spatial Variation Assessed By Satellite
• AOD- column-total aerosol light extinction:
AOD = ∫bext(dz) If all aerosol is in boundary layer AOD
approximated by bext·L (L=boundary layer depth)
•NASA MODIS AOD (Level II) data from Satellites Terra
and Aqua
•10 km  10 km resolution at nadir view
• Twice daily scans during day time (about 11:00 AM
and 1:00 PM
• Less valid AOD data available in winter due to frequent
cloudy skies over the Lake Tahoe basin
• Spatial distribution of AOD is calculated based on valid
AOD data points
Assumptions for surface/satellite
comparability
• Spatial homogeneity across pixels and within
mixed layers
• Cloudless sky
• Temporal correspondence between surface and
satellite estimates
• Constant aerosol extinction properties (e.g.,
size, composition, and shape)
Domain of Study
120.3  W 120.2  W
120.1  W 120.0  W
119.9  W 119.8  W
39.3  N
39.2  N
39.1  N
39.0  N
BLIS
38.9  N
SOLA
• 11  11 points in the domain of interest
• Surface chemical visibility measurements were made at BLIS and SOLA
Resources
http://modis-atmos.gsfc.nasa.gov/MOD04_L2/acquiring.html
HDF Viewer
Matlab hdfread
AOD Data Interpolation

120.0 W
39.6  N
120.4  W
120.2  W
120.0  W
119.8  W
119.6  W
0.3
0.3
39.4  N
0.25
0.25
0.2
39.2  N
0.2

40.0 N
0.15
0.15
39.0  N
0.1
0.1
38.8  N
0.05
0.05
38.6  N
(a)
(b)
MODIS (Aqua Level 2) AOD data for (a) the western U.S. and (b) Lake Tahoe
region on 6/27/2007. Blanks indicate missing or invalid pixels
Seasonal Variation of Data Availability
Number of Valid Grid Points in 2008
# of Valid Grid Points
8000
6000
4000
2000
0
1
2
3
4
5
6
7
8
Month in 2008
9
10
11
12
AOD versus Surface bext (Mm-1)
140
120
Aerosol b ext
100
80
60
40
20
0
0
0.1
0.2
0.3
0.4
0.5
Stratified AOD
All concurrent data for BLIS from 2007 to 2009
How many % differs from BLIS?
120.3  W
120.2  W
120.1  W
120.0  W
119.9  W
119.8  W
0.4
0.3
39.3  N
0.2

39.2 N
0.1
39.1  N
0
-0.1
39.0  N
BLIS
38.9  N
-0.2
SOLA
-0.3
-0.4
•
So BLIS represents a very large area (for July 2007)
2007
2008
120.3  W 120.2  W 120.1  W 120.0  W 119.9  W 119.8  W
120.3  W 120.2  W 120.1  W 120.0  W 119.9  W 119.8  W
39.3  N
39.2  N
39.2  N
39.1  N
39.1  N
39.0  N
39.0  N
39.3  N
0.25
2009
120.3  W 120.2  W 120.1  W 120.0  W 119.9  W 119.8  W
0.3
39.3  N
120.1  W 120.0  W 119.9  W 119.8  W
IS
SOLA
Average July AOD
0.3
0.25
39.2  N
39.1  N
0.2
0.2
39.0  N
0.15
BLIS
38.9  N
0.15
BLIS
SOLA
BLIS
SOLA
38.9  N
SOLA
38.9  N
0.1
0.1
0.3
0.25
0.2
0.15
0.1
120.1  W 120.0  W 119.9  W 119.8  W
IS
SOLA
Average October AOD
2007
2008
120.3  W 120.2  W 120.1  W 120.0  W 119.9  W 119.8  W
120.3  W 120.2  W 120.1  W 120.0  W 119.9  W 119.8  W
120.3  W 120.2  W 120.1  W 120.0  W 119.9  W 119.8  W
0.3
39.3  N
39.3  N
39.2  N
39.2  N
39.1  N
39.1  N
39.0  N
39.0  N
39.3  N
0.25
2009
0.3
0.25
39.2  N
39.1  N
0.2
0.2
39.0  N
0.15
BLIS
38.9  N
0.15
BLIS
SOLA
BLIS
SOLA
38.9  N
SOLA
38.9  N
0.1
0.1
0.3
0.25
0.2
0.15
0.1
W19.9 W119.8 W
Episodic Aerosol Optical Depth
0.7
120.1  W 120.0  W
0.6
120.3  W 120.2  W
0.5
0.4
119.9  W 119.8  W
120.1  W 120.0  W
0.3
0.2
0.1
120.3  W 120.2  W
120.3  W 120.2  W
119.9  W 119.8  W
39.3  N
39.3  N
39.3  N
39.2  N
39.2  N
39.2  N
39.1  N
39.1  N
39.1  N
39.0  N
39.0  N
39.0  N
BLIS
38.9  N
11:00, 6/24/2007
120.3  W 120.2  W
SOLA
38.9  N
120.1  W 120.0  W
120.3  W 120.2  W
120.1  W 120.0  W
38.9 N
13:00, 6/25/2007
120.3  W 120.2  W
119.9  W 119.8  W
39.3  N
39.3  N
39.3  N
39.2  N
39.2  N
39.2  N
39.1  N
39.1  N
39.1  N
39.0  N
39.0  N
38.9  N
SOLA

38.9 N
11:00, 6/26/2007
120.1  W 120.0  W
119.9  W 119.8  W
39.0  N
BLIS
BLIS
SOLA

11:00, 6/25/2007
119.9  W 119.8  W
119.9  W 119.8  W
BLIS
BLIS
SOLA
120.1  W 120.0  W
BLIS
SOLA
11:00, 6/27/2007
SOLA
38.9  N
13:00, 6/27/2007
AOD map during the Angora Fire event
Spatial Analysis suggests that:
1) background PM and bext levels are pretty uniform
across the Tahoe basin, except during severe wildfire
events,
2) influences such as traffic and residential wood
combustion are confined in urban “neighborhoods”
(sub-pixel), and
3) visibility within the basin would be well bounded
by BLIS1 (background) and SOLA1 (maximum)
measurements