Ambient measurements of monoterpenes near Cannabis cultivation facilities in Denver, Colorado

Abstract Colorado was one of the first US states to legalize the industrial-scale cultivation of Cannabis spp. for recreational purposes. In March 2018, there were 609 indoor Cannabis cultivation facilities (CCFs) in operation in Denver County with a recorded 550,000 mature plants (higher than 8 inches) under cultivation at any given time. It is known that cultivation of Cannabis spp. produces emissions of a group of highly reactive hydrocarbons, monoterpenes. There have been limited studies that have quantified mixing ratios of emitted monoterpenes in air outside CCFs. A field campaign was conducted in August 2016 in Denver County focused on six different CCF clusters near the intersection of interstate highways I-25 and I-70 during which a total of 150 ambient air samples were collected. Monoterpene mixing ratios near CCFs were ∼408 ± 203 pptv; 4–8 times higher than samples collected from a “background” site located at the Denver City Park (75 ± 25 pptv). The composition of samples taken near CCFs were dominated by d-limonene (30%), β-myrcene (20%), and α-pinene (15%), which is similar to previously reported emission factors for Cannabis spp. Since β-myrcene was only detected in leaf enclosure studies, indoor CCF observations and ambient samples near CCFs and not detected at a background site, this particular compound could be used as a tracer for the Denver Cannabis production industry. The monoterpene speciation in ambient measurements varied across Denver suggesting differences in emissions between different Cannabis spp., or different growth stages. Given the observed variabilities in both composition and emission rates, it is critical for the accuracy of emissions inventories to develop strain specific emission factors. This information, coupled with detailed information on each CCF, would greatly reduce the uncertainties currently present in monoterpene emission estimates for the Cannabis industry and its potential impact on air quality.