At which location would sunlight have to pass through the thickest amount of atmosphere before reaching earth's surface

At the equator

Explanation:

The thickness and depth of the atmosphere on earth varies from one geographic location  to the other. This thickness not only vary with location but also with time of year.

Equator is warmer than any other part of the world and hence the air mass over the equator is, on average, warmer there than anywhere else on Earth.  Cold air is denser than warm air . However the gravitational force is exerted in equal amount across the world , therefore , the same amount of air molecules over the equator go upward while  the air molecules in the coldest regions on Earth i.e the poles hovers near the surface of earth . Thus, the atmosphere is thickest and deepest at the equator.

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.