Figure 2. Trend of chemical bext at (a) BLIS1 and (b) SOLA1. The whiskers and boxes indicate 90 th, 80th, 20th, and

10th percentile of bext for each year. Small bars indicate the median values. Linear regressions for the 90 th 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

BLIS

SOLA

120.3 W 120.2 W 120.1 W 120.0 W 119.9 W 119.8 W

38.9 N

39.0 N

39.1 N

39.2 N

39.3 N

• 11 11 points in the domain of interest

• Surface chemical visibility measurements were made at BLIS and SOLA

Resourceshttp://modis-atmos.gsfc.nasa.gov/MOD04_L2/

acquiring.html

HDF Viewer

Matlab hdfread

120.0 W

40.0 N

0.05

0.1

0.15

0.2

0.25

0.3

(a)

120.4 W 120.2

W 120.0

W 119.8

W 119.6

W

38.6 N

38.8 N

39.0 N

39.2 N

39.4 N

39.6 N

0.05

0.1

0.15

0.2

0.25

0.3

(b)

AOD Data Interpolation

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 AvailabilityNumber of Valid Grid Points in 2008

0

2000

4000

6000

8000

1 2 3 4 5 6 7 8 9 10 11 12

Month in 2008

# o

f V

alid

Gri

d P

oin

ts

AOD versus Surface bext (Mm-1)

All concurrent data for BLIS from 2007 to 2009

0

20

40

60

80

100

120

140

0 0.1 0.2 0.3 0.4 0.5

Stratified AOD

Aer

oso

l bex

t

BLIS

SOLA

120.3 W 120.2 W 120.1 W 120.0 W 119.9 W 119.8 W

38.9 N

39.0 N

39.1 N

39.2 N

39.3 N

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

How many % differs from BLIS?

• So BLIS represents a very large area (for July 2007)

BLIS

SOLA

120.3 W 120.2 W 120.1 W 120.0 W 119.9 W 119.8 W

38.9 N

39.0 N

39.1 N

39.2 N

39.3 N

0.1

0.15

0.2

0.25

0.3

Average July AOD

BLIS

SOLA

120.3 W 120.2 W 120.1 W 120.0 W 119.9 W 119.8 W

38.9 N

39.0 N

39.1 N

39.2 N

39.3 N

0.1

0.15

0.2

0.25

0.3

BLIS

SOLA

120.3 W 120.2 W 120.1 W 120.0 W 119.9 W 119.8 W

38.9 N

39.0 N

39.1 N

39.2 N

39.3 N

0.1

0.15

0.2

0.25

0.3

BL

IS

SO

LA

120.3 W

120.2

W 120.1

W

120.0 W

119.9 W

119.8

W

38.9 N

39.0 N

39.1 N

39.2 N

39.3 N

0.1

0.15

0.2

0.25

0.3

2007

2008

2009

BLIS

SOLA

120.3 W 120.2 W 120.1 W 120.0 W 119.9 W 119.8 W

38.9 N

39.0 N

39.1 N

39.2 N

39.3 N

0.1

0.15

0.2

0.25

0.3

Average October AOD

BL

IS

SO

LA

120.3 W

120.2

W 120.1

W

120.0 W

119.9 W

119.8

W

38.9 N

39.0 N

39.1 N

39.2 N

39.3 N

0.1

0.15

0.2

0.25

0.3

2007

2008

2009

BLIS

SOLA

120.3 W 120.2 W 120.1 W 120.0 W 119.9 W 119.8 W

38.9 N

39.0 N

39.1 N

39.2 N

39.3 N

0.1

0.15

0.2

0.25

0.3

BLIS

SOLA

120.3 W 120.2 W 120.1 W 120.0 W 119.9 W 119.8 W

38.9 N

39.0 N

39.1 N

39.2 N

39.3 N

0.1

0.15

0.2

0.25

0.3

BLISSOLA

120.3 W 120.2

W 120.1 W 120.0

W 119.9 W 119.8

W 38.9

N 39.0

N 39.1

N 39.2

N 39.3

N 0.1 0.2 0.3 0.4 0.5 0.6 0.7

BLIS

SOLA

120.3 W 120.2

W 120.1 W 120.0 W 119.9

W 119.8

W

38.9 N

39.0 N

39.1 N

39.2 N

39.3 N

11:00, 6/24/2007

BLIS

SOLA

120.3 W 120.2

W 120.1

W 120.0

W 119.9

W 119.8

W

38.9 N

39.0 N

39.1 N

39.2 N

39.3 N

11:00, 6/25/2007

BLIS

SOLA

120.3 W 120.2 W 120.1 W 120.0 W 119.9 W 119.8 W

38.9 N

39.0 N

39.1 N

39.2 N

39.3 N

13:00, 6/25/2007

BLIS

SOLA

120.3 W 120.2 W 120.1 W 120.0 W 119.9 W 119.8 W

38.9 N

39.0 N

39.1 N

39.2 N

39.3 N

11:00, 6/26/2007

BLIS

SOLA

120.3 W 120.2

W 120.1

W 120.0

W 119.9

W 119.8

W

38.9 N

39.0 N

39.1 N

39.2 N

39.3 N

11:00, 6/27/2007

BLIS

SOLA

120.3 W 120.2 W 120.1 W 120.0 W 119.9 W 119.8 W

38.9 N

39.0 N

39.1 N

39.2 N

39.3 N

13:00, 6/27/2007

Episodic Aerosol Optical Depth

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