US ATLAS has ceased to use OSG-supplied user certificates, in favor of certificates issued by the CERN certificate authority (CA). US ATLAS encourages you to obtain a personal certificate from the CERN CA (if you don't yet have one), and add it to your existing ATLAS VO membership.
Throughout the transition and beyond, OSG certificates issued by DigiCert will remain valid and honored until their stated expiration dates.
The CERN Knowledge Base page on grid user certificate eligibility shows the CERN affiliation types and stipulations that enable CERN login accounts to request certificates from the CERN CA. You can check your account on CERN's Check certificate eligibility page. More information is available on the WLCG certificates page.
Note: the Chrome browser is not officially supported by CERN, and may return an error when used with the CERN CA service (e.g., 'Key not valid for use in specified state.'). To mitigate these errors, clear any browser cookies associated with the domain 'ca.cern.ch', and then access the CERN CA site again.