W: GPG error: http://ppa.launchpad.net/webupd8team/atom/ubuntu xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY C2518248EEA14886
W: The repository 'http://ppa.launchpad.net/webupd8team/atom/ubuntu xenial InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
ubuntu1604, sudo apt-get update 时出现的错误如上！
原因：无法下载 http://ppa.launchpad.net/webupd8team/atom/ubuntu xenial ，主要和这个软件(webupd8team)的源有关系.
解决方法：将对应的PPA删除掉即可.
找到上述无法下载的对应PPA目录，即webupd8team，安全起见，用mv命令将该文件添加后缀.bak即可
1.cd /etc/apt/sources.list.d
2.sudo mv webupd8team-ubuntu-atom-xenial.list webupd8team-ubuntu-atom-xenial.list.ba
3.sudo apt-get update