Thanks…
STO- is the only link I saw… It was the first item that came up on my search.
If I can say a quick word or 2 .
From what I have noticed having water proofed and repaired stucco walls before. Also seeing this on brick veneer.
The staining in this photo looks like it is from the tin-work overhead. Possible aluminum gutters or flashing or both.
Gutters not doing there job because of several reasons. I can not see them up close. Gutters to small, dirty, pulling away from the wall and leaking, ETC.
Tin or aluminum and wall cladding, be it brick or stucco in this case, will be stained by the dirt in the gutters to the metals in tin-work that cling to the wall surface.
WHY
Many times brickwork gets stained from aluminum, and certain metals flashing and gutters. Not so much today because they are using cheaper furious metals to produce gutters.
Large wooden eliminates on exterior walls are being flashed instead of scraped ,repaired and painted, are cover with aluminum and the oxides in the product cling to the sand in the stucco or the sand in the masonry and cling to brick. 
The sand has traces of iron ( the amount varies by quarry ), and it attracts the molecules in the dirty gutter water and particles in the aluminum flashing’s or sidings that are in the aluminum causing the staining.
Its like ghosting on interior walls. Not exactly because its water not air that is the carrier.
Also the atmospheric grim that is in the rain and on the roof is carry downwards and spills onto walls. It to is attracted to the sands in the stucco.
Why is the sand showing here.
The painted surface is coming to its life expectancy and the sands are showing through. The sand has iron in it.That is what is causing the staining. There ability from the iron in the sand to attract charged particles, negatively or positively charged particles.:o
Also the surface is not as smooth and allowing the wall to absorb contaminants.
When I point ( re-dress the masonry bedding and butt joints between the bricks ) a home and it has aluminum flashing anywhere on the wall, I tell them to paint the surface because staining will happen on the new jointed brick.
I have washed brick walls to remove this staining.
Seen this 25 plus years now.
I hope I made myself clear. I will look for the scientific evaluation.
This is just my assessment.
Sorry it is so long.
Robert this thread is 5 years old.
Looks to me like the picture was removed.