Leaf enclosure measurements for determining volatile organic compound emission capacity from Cannabis spp.

The legal commercialization of Cannabis for recreational and medical use in certain US states has effectively created a new and nearly unregulated cultivation industry. Within the city limits of Denver, Colorado, there are now more than 600 registered Cannabis spp. cultivation facilities (CCFs) for recreational and medical uses, each containing thousands of plants. Ambient measurements collected inside growing operations pre-legalization have found concentrations as high as 50–100 ppbv of terpenes; a group of highly reactive biogenic volatile organic compounds (BVOCs) and known precursors for the formation of ozone and particulate matter (PM). Due to its illicit nature there has been insufficient experimental data produced to determine Cannabis spp. emission rates. This study used, for the first time, an enclosure chamber and live Cannabis spp. plants during a 90-day growing period consisting of four different strains of Cannabis spp.: Critical Mass, Lemon Wheel, Elephant Purple, and Rockstar Kush. These measurements enabled characterization of terpenes and estimates of emission capacity (EC, μgC g−1 hr−1) at standard conditions. During peak growth, the percentages of individual BVOC emissions were dominated by β-myrcene (18–60%), eucalyptol (17–38%), and d-limonene (3–10%) for all strains. Our results showed large variability in the rate and composition of terpene emissions across different strains. For the Critical Mass and Lemon Wheel, the dominant terpenoid was eucalyptol (32% and 38%), and it was β-myrcene (60% and 45%) for the Elephant Purple and Rockstar Kush. Critical Mass produced the highest terpene emission capacity (8.7 μgC g−1 hr−1) and Rockstar Kush the lowest (4.9 μgC g−1 hr−1). With 600 CCFs in Denver, and assuming 10,000 plants per CCF, an emission capacity of 8.7 μgC g−1 hr−1 would more than double the existing rate of BVOC emissions to 520 metric ton year−1. Using Maximum Incremental Reactivity (MIR) values the total ozone formation potential from all these emitted species could produce 2100 metric tons year−1 of ozone, and based on published secondary organic aerosols yields 131 metric tons year−1 of PM. It is likely that the ECs calculated here are lower than those achieved in CCFs where growing conditions are optimized for rapid growth and higher biomass yields. Further studies including a greater number of the 620 available Cannabis spp. strains and a wider range of treatments are needed to generate a representative dataset. Such a dataset could then better enable assessments of the potential impacts of this new industry on indoor and regional air quality.