If dataDir is defined, data will be loaded from this local
dirctory. Otherwise, data will be loaded from the monitoring data repository
maintained by PWFSL.
The daily files loaded by this function are updated once a day, shortly
after midnight and contain data for the previous 45 days.
For the most recent data, use airnow_loadLatest().
For data extended more than 45 days into the past, use airnow_loadAnnual().