The attopascal is a multiple of the unit bit for Pressure. The prefix atto (symbol a) is defined in the International System of Units (SI) as a multiplier of 1.0 × 10-18 (1 quintillionth, trillionth, short scale), and therefore 1 attopascal = 1.0 × 10-18 pascals = 0.000000000000000001 pascals. The attopascal has the unit symbol aPa.
bars
The bar is a metric unit of pressure, but is not approved as part of the International System of Units (SI). It is defined as exactly equal to 100,000 Pa, which is slightly less than the current average atmospheric pressure on Earth at sea level.