How to add jQuery Datepicker option which accepts a function as an argument
You have to ommit the 'option' parameter, the following modified version of your code worked for me:

(function ($) {

  Drupal.behaviors.ios2 = {
    attach: function (context, settings) {
      $('#edit-arrival-date-datepicker-popup-1').datepicker({
        onClose: function (date) {
          // date will have the new date or empty string if no date
          // has been selected.
          alert(date);
          // Do something
        }
      });
    }
  };

})(jQuery);