The online version of the Regional Air Qual-ity Deterministic Prediction System (RAQDPS) is a chem-ical weather forecast system that has been employed op-erationally by Environment and Climate Change Canada(ECCC) since 2009. It is run twice per day to produce72 h forecasts of hourly 10 km abundance fields of threekey predictands, NO2, O3, and PM2.5total mass, as well asother gas-phase chemical species, PM2.5chemical compo-nents, and dry and wet deposition for Canada, the contigu-ous U.S., and northern Mexico. The forecasts of NO2, O3,and PM2.5are needed to calculate the Air Quality Health In-dex (AQHI), which is used to communicate current and fore-casted pollutant levels to the Canadian public. Version 023of the RAQDPS (RAQDPS023) went into service at ECCCin December 2021 and was replaced by the RAQDPS025in June 2024. This paper provides the first full descrip-tion of any version of the online RAQDPS. After givinga brief history of the ECCC operational air quality fore-casting program, we provide a comprehensive descriptionof the RAQDPS023 forecast system as well as shorter de-scriptions of several upstream and downstream forecast andanalysis systems. The latter include two upstream opera-tional meteorological forecast systems that were based onversion 5.1.0 of the ECCC Global Environmental Multiscale(GEM) numerical weather prediction model, one which useda global configuration, the Global Deterministic PredictionSystem (GDPS 8.0.0), and the other which used a regionalconfiguration, the Regional Deterministic Prediction System(RDPS 8.0.0). An emissions processing system, an Update-able Model Output Statistics-based system for bias-correctedstation-specific pollutant concentration forecasts (UMOS-AQ), and a regional objective analysis system for surfacepollutant concentration fields, the Regional Deterministic AirQuality Analysis system (RDAQA 2.0.0), are also described. The RAQDPS023 itself consisted of version 3.1.0.0 of theGEM-Modelling Air quality and CHemistry (GEM-MACH)chemistry module, which was embedded with one-way cou-pling within GEM 5.1.0, its meteorological host model. Themeteorological configuration of the RAQDPS023 closelyfollowed that of the RDPS 8.0.0. Details covered in thispaper include a summary of the dynamical representationsand physical parameterizations used in the three GEM-basedforecast systems, which are highly harmonized, the chemi-cal species and parameterizations used in the MACH chem-istry module, including gas-phase, aqueous-phase, and in-organic heterogeneous schemes and associated numericalsolvers, system inputs, including both anthropogenic and nat-ural emissions of chemical species, system outputs, and runconfiguration, strategies, and timings. One simplification inaddition to the use of the condensed ADOM-2 gas-phasechemistry scheme that was made to reduce RAQDPS023 ex-ecution time for operational deployment was to represent the particulate matter (PM) size distribution with only twoaerosol particle size bins, one corresponding to particle di-ameters in the 0-2.5 mu m range ("fine particles" or PM2.5) andthe other to the 2.5-10 mu m range ("coarse fraction" or PMcf).A second simplification was to represent the chemical com-position of PM2.5with only nine chemical components, anda third simplification was to use a longer time step (900 s)for the time integration of atmospheric chemistry than thetime step used for time integration of atmospheric dynamicsand physics (300 s). Even so, activating the MACH moduleincreased RAQDPS023 run time by a factor of 4. 4 on aver-age compared to meteorology only, partly due to the cost ofthe integration of chemistry but partly to the increased costof the GEM dynamical core due to the advection with im-posed shape preservation and mass conservation of 57 addi-tional chemical tracers. The role of the RAQDPS-FW023, asecond chemical weather forecast system that was identicalto the operational RAQDPS023 (or RAQDPS-OP023) exceptfor the addition of near-real-time biomass burning emissions,is also described. Biomass burning emissions for Canada andthe U.S. estimated from satellite measurements were first cal-culated by the Canadian Forest Fire Emissions PredictionSystem (CFFEPS) version 4.1 before each RAQDPS-FW023run was launched. Outputs from the two RAQDPS versionswere then used to produce forecasts of wildfire smoke trans-port and diffusion. The paper closes by summarizing the keyupgrades made to the RAQDPS025, the current version ofthe ECCC operational chemical weather forecast system, andthen describing some possible future improvements and up-dates. A companion paper by Moran et al. (2026) presents theresults of a comprehensive, five-year performance evaluationof prospective and retrospective annual air quality simula-tions made with the RAQDPS023.

The GEM-MACH air quality model has been used by Environment and Climate Change Canada in its operational Regional Air Quality Deterministic Prediction System (RAQDPS) since November 2009. The RAQDPS is run twice daily to produce 48-h forecasts of hourly surface O3, NO2, and PM2.5 concentration fields over North America. In the past decade there have been 20 upgrades of varying magnitude made to the RAQDPS, and it is now possible to examine the evolution of RAQDPS performance skill over the near-decadal period from January 2010 to June 2019. A set of quality-controlled near-real-time hourly measurements of O3, NO2, and PM2.5 surface concentrations for North America has been used for this evaluation. Results of three selected analyses that focus on time trends in performance skill are presented in this study along with a discussion of the impacts of some of the major model upgrades.

Since the last ITM in October 2016, the Canadian operational Regional Air Quality Deterministic Prediction System (RAQDPS) has been ported to a new high-performance computing system and has been updated to use a new meteorological initialization method, a new meteorological “piloting” model, a new and faster version of the GEM-MACH code, and a new set of input emissions files and to produce an expanded set of output fields. These updates are briefly described and some examples are given of their impact on RAQDPS forecast performance, including improved NO2 forecasts and a large reduction (~10 ppbv) in summertime ozone overpredictions for the eastern United States.

The Regional Deterministic Air Quality Analysis (RDAQA) is a mapping of surface air pollutant concentration which combines numerical forecasts from the Regional Air Quality Deterministic Prediction System (RAQDPS) and hourly AQ observational data from monitoring surface networks over North America. These include Canadian measurement networks operated by the provinces, territories, and some municipalities and those networks covering the continental United States under the umbrella of EPA's national AIRNow program. The model forecasts and observations are combined based on an optimal interpolation algorithm. The current RDAQA has a horizontal spatial resolution of 10 km and is issued every hour. It provides our best knowledge of the current state of the atmosphere for surface pollutant concentration for chemical species like: ozone, NO2, NO, SO2, PM2.5 , PM10. The RDAQA also provide a mapping of the Air Quality Health Index (AQHI) in quasi-real time and can be used by meteorologists in Environment Canada's regional forecast offices to inform the public on a daily basis about the health risk associated to short term exposure to air pollution. The RDAQA products available over an extended period of time can also be used by Health Canada and health consortium partners to study how Air Quality and other multiple environmental factors are linked to a wide range of health outcomes. A description of the RDAQA will be presented as well as data access and future plans.

FireWork is an on-line, one-way coupled meteorology–chemistry model based on near-real-time wildfire emissions. It was developed by Environment and Climate Change Canada to deliver operational real-time forecasts of biomass-burning pollutants, in particular fine particulate matter (PM2.5), over North America. Such forecasts provide guidance for early air quality alerts that could reduce air pollution exposure and protect human health. A multi-year (2013–2016) analysis of FireWork forecasts over a five-month period (May to September) was conducted. This work used an archive of FireWork outputs to quantify wildfire contributions to total PM2.5 surface concentrations across North America. Different concentration thresholds (0.2 to 28 µg/m3) and averaging periods (24 h to five months) were considered. Analysis suggested that, on average over the fire season, 76% of Canadians and 69% of Americans were affected by seasonal wildfire-related PM2.5 concentrations above 0.2 µg/m3. These effects were particularly pronounced in July and August. Futhermore, the analysis showed that fire emissions contributed more than 1 µg/m3 of daily average PM2.5 concentrations on more than 30% of days in the western USA and northwestern Canada during the fire season.

A first regional assessment of the impact of shipping emissions on air pollution in the Canadian Arctic and northern regions was conducted in this study. Model simulations were carried out on a limited-area domain (at 15 km horizontal resolution) centred over the Canadian Arctic, using the Environment and Climate Change Canada's on-line air quality forecast model, GEM-MACH (Global Environmental Multi-scale – Modelling Air quality and CHemistry), to investigate the contribution from the marine shipping emissions over the Canadian Arctic waters (at both present and projected future levels) to ambient concentrations of criteria pollutants (O3, PM2.5, NO2, and SO2), atmospheric deposition of sulfur (S) and nitrogen (N), and atmospheric loading and deposition of black carbon (BC) in the Arctic. Several model upgrades were introduced for this study, including the treatment of sea ice in the dry deposition parameterization, chemical lateral boundary conditions, and the inclusion of North American wildfire emissions. The model is shown to have similar skills in predicting ambient O3 and PM2.5 concentrations in the Canadian Arctic and northern regions, as the current operational air quality forecast models in North America and Europe. In particular, the model is able to simulate the observed O3 and PM components well at the Canadian high Arctic site, Alert. The model assessment shows that, at the current (2010) level, Arctic shipping emissions contribute to less than 1 % of ambient O3 concentration over the eastern Canadian Arctic and between 1 and 5 % of ambient PM2.5 concentration over the shipping channels. Arctic shipping emissions make a much greater contributions to the ambient NO2 and SO2 concentrations, at 10 %–50 % and 20 %–100 %, respectively. At the projected 2030 business-as-usual (BAU) level, the impact of Arctic shipping emissions is predicted to increase to up to 5 % in ambient O3 concentration over a broad region of the Canadian Arctic and to 5 %–20 % in ambient PM2.5 concentration over the shipping channels. In contrast, if emission controls such as the ones implemented in the current North American Emission Control Area (NA ECA) are to be put in place over the Canadian Arctic waters, the impact of shipping to ambient criteria pollutants would be significantly reduced. For example, with NA-ECA-like controls, the shipping contributions to the population-weighted concentrations of SO2 and PM2.5 would be brought down to below the current level. The contribution of Canadian Arctic shipping to the atmospheric deposition of sulfur and nitrogen is small at the current level, < 5 %, but is expected to increase to up to 20 % for sulfur and 50 % for nitrogen under the 2030 BAU scenario. At the current level, Canadian Arctic shipping also makes only small contributions to BC column loading and BC deposition, with < 0.1 % on average and up to 2 % locally over the eastern Canadian Arctic for the former, and between 0.1 % and 0.5 % over the shipping channels for the latter. The impacts are again predicted to increase at the projected 2030 BAU level, particularly over the Baffin Island and Baffin Bay area in response to the projected increase in ship traffic there, e.g., up to 15 % on BC column loading and locally exceeding 30 % on BC deposition. Overall, the study indicates that shipping-induced changes in atmospheric composition and deposition are at regional to local scales (particularly in the Arctic). Climate feedbacks are thus likely to act at these scales, so climate impact assessments will require modelling undertaken at much finer resolutions than those used in the existing radiative forcing and climate impact assessments.

Table S.1a Comparison of modelled O 3 with observations at Northern sites.Site Lat Lon Mn-obs (ppbv) Mn-mod (ppbv) MB (ppbv) NMB (%) URMSE (ppbv) nURMSE (%) r

Elevated levels of PM2.5 have been observed in North America in all seasons, underlining the need for year-round air-quality (AQ) forecasts. Wintertime AQ forecasting, however, poses unique challenges given well-known seasonal variations in emissions, meteorology, chemistry, and removal processes. In the case of PM2.5, both systematic and episodic overpredictions have been noted in the wintertime for current AQ forecast models, including Environment Canada’s GEM-MACH15 AQ forecast model. GEM-MACH15 is the regional forecasting configuration of the multi-scale, in-line AQ model GEM-MACH. Performance evaluations of GEM-MACH15 predictions for several recent winter periods have pointed to several sources of forecast error, including the spatial and temporal allocation of primary PM2.5 emissions and the treatment of vertical diffusion. This paper describes recent improvements to input emissions for the GEM-MACH15 modelling system and their resulting impacts on forecast performance.

During the summer of 2004, the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) coordinated the largest field study related to air quality so far. Over 500 scientists from 5 different countries, 11 planes, 1 ship, 2 satellites and numerous ground sites were involved in characterizing the emissions of particulate matter (PM), ozone and their precursors, their chemical transformations, and their removal during transport to and over the North Atlantic and into Europe. The study was organized around three main objectives: characterizing the regional air quality in eastern North America; assessing the export of smog related air pollution from eastern North America and its evolution as it travels towards Europe; and assessing the possible effects of the PM associated with the smog on our climate.