How much force is exerted by the atmosphere on 1.00km^2 of land at sea level

"Normal" atmospheric pressure is 101.325 kilopascals

= 101,325 pascals

= 101,325 newtons/meter².

1 km² = (1,000 m) x (1,000 m) = 1 million m²

Force = (101,325 N/m²) x (1 million m²)

= 101,325,000,000 Newtons

= 1.013 x 10¹¹ Newtons

= 101.3 Giganewtons.

That's the weight of all the air sitting on 1 km² of land.