New York—U.S. digital ad spending will reach $42.26 billion this year, up 14.9% over last year's spending, according to a new forecast from eMarketer Inc. The projection is up slightly from a June forecast of $41.94 billion. eMarketer made the adjustment largely due to a revision in its mobile ad spending forecast, which is now set at $8.51 billion for the year, up from a June projection of $7.65 billion. This year, 22.1% of all U.S. digital search spending will be on mobile devices, according to eMarketer. By 2017, U.S. advertisers will spend 59.6% of all digital search dollars on mobile, it projected. Next year, digital ad spending is expected to hit $47.58 billion, up 12.6% over this year, and by 2017 will reach $61.35 billion, according to eMarketer. This year, digital ad spending will account for 24.7% of all media spending, and by 2017 it will account for 31.1%, eMarketer predicted.