A computational study demonstrating the potential application of armchair graphene nanoribbons as ultrasensitive chemical detectors is presented. To this end, we propose the use of lithium adatoms, serving as surface anchoring sites, to allow for aromatic contaminant chemisorption that alters the all-carbon substrate electronic properties. The corresponding variations in the electronic transport characteristics, which are evaluated using a divide and conquer approach based on density functional theory, suggest device sensitivities as low as 10^-5-10^-9 ppbv. The microscopic understanding of the contaminant adsorption process and its influence on the electronic and transport properties of graphene nanoribbons gained in this study may assist in the rational design of ultrasensitive chemical detectors based on low-dimensional graphene derivatives.