WordPress – customize posts per page for tag, category and date archive pages

add_filter('pre_option_posts_per_page', 'posts_per_page_filter');
function posts_per_page_filter() {
  if ( is_archive() ) {
    // date,tag
    if (is_date() || is_tag() ) {
      return 30;
    }
    if (is_category() ) {
      return 20;
    }
  }
  return 10; // default
}