Those who oppose smoking in movies aimed at young people planned to step up their assault on the science-fiction epic “Avatar” on Tuesday with advertisements in Hollywood trade papers that accuse the film of providing the equivalent of $50 million in free tobacco advertising. The full-page ads were scheduled to run in both The Hollywood Reporter and Daily Variety. For every $100 million it earns at the box office, “Avatar” will “deliver an estimated 40 million tobacco impressions to theater audiences,” said the ads, referring to scenes involving a cigarette-smoking character played by Sigourney Weaver. That estimate of the film’s value to tobacco companies included anticipated home video sales. To date, “Avatar” has taken in more than $1.3 billion at the worldwide box office. In a statement last week, James Cameron, its director, took sharp issue with the notion that the movie advocates smoking simply because it portrays cigarette use. The new advertisements were paid for by Smoke Free Movies, a campaign supervised by Stanton A. Glantz, director of the Center for Tobacco Control Research and Education at the University of California, San Francisco, with backing from the American Legacy Foundation. The foundation was established with money from a 1998 settlement of legal claims between tobacco companies and 46 states.