Android devices are landing in the hands of more Americans, according to a new survey by Nielsen. The fast-growing mobile operating system now accounts for 39 percent of the smartphone market — up from 36 percent when Nielsen ran its survey in May.

Apple’s iOS, meanwhile, is in second place with 28 percent of the market and RIM Blackberry fell to 20 percent of the market. Windows Mobile/Windows Phone 7 came in fourth place with nine percent market share.

The study follows a separate report released yesterday by IMS Research which found that sales of smartphones will exceed 420 million devices in 2011. That will make up roughly 28 percent of the global handset market. The research firm predicts that annual sales of smartphones will surpass one billion devices by the end of 2016.

Here’s a look at the IMS research.

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