The PUMA is the smallest geographic unit available in the 2012 ACS. It is possible to combine PUMAs to identify larger geographic areas, such as counties, if the boundaries of the PUMAs align with the boundaries of the county. However, because most cities have fairly irregular boundaries that do not always follow Census Tract boundaries, it is very difficult to perfectly identify a city. The IPUMS-USA variable CITY (which is currently being constructed for the 2012 ACS sample) uses PUMAs to determine whether or not a household is located within a city like Chicago, however, it is not likely that the frequency of CITY will accurately give the population of Chicago because many PUMAs straddle Chicago’s borders making it impossible to determine if the household is inside the City limits. Depending on how much of the PUMA falls outside of the city limits, it may still be acceptable to use the PUMA to identify residents of the City. You can check the 2012 population of Chicago against the aggregated populations of the PUMAs that fall within Chicago to see if they are reasonably similar. But there is no way to precisely identify the city of Chicago in the 2012 ACS sample.