The given sea level pressure from an ATIS will reflect the local deviation from standard temperature such that your altimeter (which does not correct for temperature) displays field elevation on the ground.
It is important to note that the ATIS/AWOS/ASOS station is directly sampling station pressure, not sea level pressure, which must be derived. The equation used to do this is the hypsometric equation and there is an assumption of temperature in the atmospheric layer between the ground and sea level.
$$(z_2 - z_1) = \dfrac{R\cdot\bar{T}}{g}\ln\left(\dfrac{p_1}{p_2}\right)$$
In this equation $z_1$ is 0 m, $z_2$ is station elevation, $R$ is the gas constant for dry air, $\bar{T}$ is the average temperature between $z_1$ and $z_2$, $p_1$ is sea level pressure (hPa), $p_2$ is station pressure (hPa) and $g$ is acceleration due to gravity. All are known except $p_1$ (assumptions about $\bar{T}$ are made from the lapse rate of the standard atmosphere and station temperature). Solving for $p_1$ yeilds:
$$p_1 = p_2 \exp \left( \dfrac{g z_2}{R \bar{T}} \right)$$