Techniques for cooling a data center include operating an air handling system in a first mode of operation to provide an outside airflow cooled through a direct evaporative cooling module to a data center without substantial mixing of the cooled outside airflow with another airflow; determining that a measured outside air contaminant level exceeds a setpoint contaminant level; and based on the determination, operating the air handling system in a second mode of operation to provide a datacenter return airflow cooled through an indirect evaporative cooling module to the data center without substantial mixing of the cooled return airflow with another airflow.