Within the next month, ftp://ftp.airnowapi.org/ will be discontinued, replaced by the amazon s3 server files.airnowtech.org. Many users will need to update their scripts to grab data from the new location.
Note that there are different paths to the files on the new server. For example, for today's files, they can be viewed at https://files.airnowtech.org/?prefix=airnow/today/ This URL is for viewing only. If you are pulling the file automatically/using a script, the path to a file in the today folder is https://files.airnowtech.org/airnow/today/filename
If you used the command line program "Copy" to download & copy files on the old FTP server, it will no longer work on files.airnowtech.org because it cannot download/copy files via http. Instead, wget or curl can be used.