The scientific development of Environment and Climate Change Canada's new ventilation index

Environment and Climate Change Canada (ECCC) has updated its ventilation forecast guidance through the development of a new ventilation index. This index was created using output from a Lagrangian stochastic model (LSM), with ventilation defined as inversely proportional to modeled ground-level concentrations from a surface-layer point source. The index is calibrated to the LSM output via parameterizations involving 10-m wind speeds, friction velocity, and Richardson number ($\mathrm{Ri}$). The resulting Calibrated Ventilation Index (CVI) ranges from 0 (worst) to 100 (best), where a $\pm 33$-unit change corresponds to a doubling or halving of ventilation. Testing and development spanned a wide range of atmospheric stabilities, wind speeds, and land-use classes, guided by LSMs utilizing diverse parameterizations of surface layer turbulence, idealized and NWP-based meteorological fields, as well as output from a Gaussian plume model. The CVI shows a strong dependence on $\mathrm{Ri}$ within the $-0.1<\mathrm{Ri}<0.1$ unstable-to-stable transition region, though uncertainty remains under very stable ($\mathrm{Ri}>0.25$) conditions. A qualitative evaluation in the Province of Quebec during one winter (December 2020 through March 2021) compared hindcast CVI values on evenings with and without ECCC air quality advisories. The CVI successfully resolved subtle ventilation differences under highly stable conditions (coefficient of variation of $\mathrm{CV}=1.51$), whereas ECCC's legacy index lacked sensitivity-compressing its hindcasts against its lower bound ($\mathrm{CV}=0.56$). The CVI is intended for local surface-layer dispersion guidance, not elevated stacks or open burning with high vertical momentum. ECCC uses the CVI to produce 48-hour forecasts across Canada four times daily at 2.5-km spatial and hourly temporal resolution.Implications: Environment and Climate Change Canada has developed a new Calibrated Ventilation Index (CVI) with the goal of protecting public health through improved guidance around the impacts of open air biomass burning activities. Index development was based on Lagrangian stochastic modeling and offers a scientifically robust and calibrated tool for forecasting surface-level atmospheric ventilation. Its implementation will enable more reliable guidance for open-air burning and other near-surface activities involving atmospheric dispersion. ECCC's CVI forecasts are now operational with guidance provided at high spatial and temporal resolution across much of Canada.