Your trees symptoms are a little strange. Looking at your pictures for some time I don't think it is a nitrogen problem,. The symptoms for a nitrogen deficiency are:....Totally yellow leaves with no variation of color, or yellow/orange veins with some green out on the far sides. A nitrogen deficiency is a mobile deficiency, meaning that the deficiency shows up on the older leaves of the tree with the newer leaves still retaining some green. This is because the tree removes the nitrogen from the older leaves to supply the nitrogen to the new growth. Most of your tree's leaves show a yellowing on the new growth. As you are in the Bay area, and it now January, I really would not think that the weather has been all that hot to cause sun damage, it not like you lived inland in the desert. The leaf symptoms don't seem to be correct for a magnesium deficiency either. I know there has been more and more Citrus Greening in California, especially in the southern part of the state, Have you heard of it showing up in your area? You can always call the county Ag Agent and ask for a person familiar with citrus to come out an look at the tree. Normally this is done at no cost, or a very slight cost. Foliar nutrient spray are 8 to 20 times faster clearing up a deficiency (if it is indeed a deficiency). You could spray the tree with a fertilizer solution containing all the micro nutrients. If you decide to do this, the best time to spray is in the morning on a cloudy day with relatively higher humidity. Add a teaspoon of a surfactant per gallon of the nutrient solution. The best to you and this tree. I would be very interested in hearing further about this tree, and what steps you take.