Tea leaves are sensitive to their environment, and certain elements can quickly alter their quality. Air, humidity, heat, and light are the main factors to monitor. When exposed to air, the leaves gradually oxidize and lose their freshness. Humidity can deteriorate them even faster, while heat and light accelerate the degradation of their aromas.
In everyday life, there's no need to look for complicated solutions. If you consume your tea within two weeks of opening it, storing it at room temperature in a tightly sealed container, away from light, is perfectly sufficient.
The refrigerator can be useful for longer storage, especially for matcha.