Moisturize, Moisturize, Moisturize
It is absolutely necessary to always moisturize the skin, whether during harmattan or not. Moisture keeps the skin going; it nourishes it, makes it supple and helps retain its elasticity. During the harmattan season, the weather is harsher- you skin dries up as often as possible- which requires you to always use a moisturizer often, you should always carry one around with you as well. Make sure to choose one that deeply moisturizes without being greasy. Ensure it has an SPF of at least 15 or 20
Foundation
A light liquid foundation should be worn in moderation during harmattan, try as much as possible to use foundation of very good quality. Preferably, use one that includes sun protection. An illuminating or bronzing powder could work as well.
Concealer
If you wear your foundation light during this period, you can apply a concealer on your foundation, this covers imperfections. This also keeps you from applying too much product while you achieve a perfect coverage.
There are other factors you should consider as well: