Pyrolysis oil, often referred to as bio-oil when derived from biomass through a fast pyrolysis process, has attracted considerable attention because of the high carbonaceous matter and high heating value compared to the original biomass. Pyrolysis oil provides a viable link between the agriculture/forestry and (petro-) chemical industry. Utilizing existing fossil fuel infrastructure by introducing bio-oil in petroleum refineries to produce renewable hydrocarbon fuels (i.e., repurposing existing assets) is appealing due to its relatively low capital requirement. Our analysis examined the air pollutant emissions for a biorefinery utilizing an ex situ catalytic fast pyrolysis process to produce raw bio-oil, with a design capacity of 2,000 dry metric tons of biomass per day. Such a biorefinery is capable of providing sufficient bio-oil to a 4,200 barrels per day capacity of fluidized catalytic cracking unit if the bio-oil is co-processed at 5% (by volume) feed rate. This research is focused on understanding the potential air pollutant regulatory and permitting implications for a refinery co-processing 5% bio-oil with 95% vacuum gas oil in a fluidized catalytic cracking unit for two scenarios: the first case assumes that an ex situ biorefinery is collocated with a petroleum refinery, and the second one assumes that a petroleum refinery will receive bio-oil from a stand-alone biorefinery. We estimated the potential-to-emit values for 4 biorefinery and FCC sizes and assessed technically feasible emission control options that could help to avoid being subject to prevention of significant deterioration requirements due to petroleum refinery modifications to co-process bio-oil. This research fills gaps and overcomes barriers related to air permitting requirements for co-processing bio-oil in existing refineries for various stakeholders. The insights gained from this analysis can facilitate decision making and help expedite permitting processes for petroleum refineries, which seek opportunities to produce lower carbon fuels in their existing infrastructure.