posts_per_pages and AJAX problem

Hi everybody!

I have a Ajax-based products catalog and i want to make a select box to choose posts per page count. After choosing a count page is refreshing with a new $_GET parameter ‘posts_per_page’. I doing it this way:

$query_param['posts_per_page'] = isset($_REQUEST['posts_per_page']) ? $_REQUEST['posts_per_page'] : get_option('posts_per_page');
global $query;
$query = new WP_Query($query_param);

but my posts per page count still getting from options default.
My full code of AJAX-refresh catalog is http://pastebin.com/wXm64VmJ.

How could i fix it? Thanks!