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add_action( 'pre_get_posts', 'archive_sort_order'); 
    function archive_sort_order($query){
        if(is_archive()):
         //If you wanted it for the archive of a custom post type use: is_post_type_archive( $post_type )
           //Set the order ASC or DESC
           $query->set( 'order', 'ASC' );
           //Set the orderby
           $query->set( 'orderby', 'title' );
        endif;    
    };