Answer
$1.10\text{ g/L}$
Work Step by Step
We will use the formula:
$\text{Density}=\frac{\text{Mass}}{\text{Volume}}$
First let's calculate the mass of the original air
$\text{Mass}=1.00\times 10^6\text{ L}\times 1.20\text{ g/L}=1.20\times 10^6\text{ g}$
Then we determine the new density:
$\text{Density}=\frac{1.20\times 10^6\text{ g}}{1.09\times 10^6\text{ L}}\approx 1.10\text{ g/L}$
