Google Inc.’s Android may be king of the mobile operating systems, but Apple Inc.’s iOS, which runs the iPhone, iPod Touch and iPad, rules the mobile advertising roost, boosted by the March debut of the third generation iPad.

In Q1 2012, 36.8% of North American mobile ad impressions—when a display advertisement is served on a mobile web or app page—occurred on iOS devices, up 3.7 percentage points from 33.1% in Q4 2011, reports mobile ad network InMobi. 34.1% occurred on Android devices, up 1.6 percentage points  from 32.5%; and 7.3% on BlackBerry devices, down 4.6 percentage points  from 11.9%. The remaining impressions occurred on devices running a variety of mobile operating systems with very small market shares.

From January 1 to March 31, the number of mobile ad impressions InMobi served grew by 18% to 93.4 billion, the company reports.

Mobile ad impressions show how smartphone owners use the mobile web and apps. Smartphone market share tells a story different from mobile ad impressions share. For the three-month period ending February 2012, Android topped smartphone market share at 50.1%, according to web and mobile measurement firm comScore. Apple iOS came in second with 30.2%, BlackBerry 13.4%, Microsoft Windows Phone 3.9%, Symbian 1.5% and other 0.9%.

Yet Apple wins again in mobile ads when InMobi breaks down impressions served to specific devices, highlighting the strength of the Apple brand, InMobi says. Following are the percentages of mobile ad impressions for Q1 2012 for the leading devices: iPhone, 19.7%; iPod Touch, 12.2%; iPad, 4.9%; BlackBerry 8520, 1.9%; and HTC PC36100, which runs Android, 1.8%.

“The mobile ad market is more mature on the Apple iOS platform, so publishers and developers are further along in delivering ads to consumers in that space,” says Anne Frisbie, vice president and managing director for North America at InMobi. “The ad data reports also include impressions for all types of mobile-connected devices, including tablets, iPod Touch devices and others. While Android has a larger smartphone market share than iOS, the iPad dominates tablet market share. All of these factors explain why we served more ad impressions on iOS than Android in spite of the fact that Android smartphone market share is higher.”

More ads are served on smartphones than tablets, InMobi adds.

But new devices could mean a shakeup in the results when InMobi studies mobile ad impressions again in the future.

“The new iPad certainly helped Apple’s overall position,” Frisbie says. “However, we know that fierce competition is created across the operating systems when new devices enter the market, and this time last year Android surpassed iOS globally. We will see further disruption if devices such as the much-anticipated Amazon smartphone and Google tablet hit the U.S. market this year. Momentum surrounding Windows 8 could also change the market significantly.”

Windows Phone 8 is expected later this year. It is the latest update to Microsoft Corp.’s mobile operating system. Some industry observers say if the new operating system is used on enough new smartphones, and if it’s more powerful than its previous incarnations, it could help Windows gain smartphone market share. Windows owns only 3.9% of smartphone market share by operating system, according to comScore.

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