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Abstract We examined the seasonality of photosynthesis in 46 evergreen needleleaf (evergreen needleleaf forests (ENF)) and deciduous broadleaf (deciduous broadleaf forests (DBF)) forests across North America and Eurasia. We quantified the onset and end (Start_GPPand End_GPP) of photosynthesis in spring and autumn based on the response of net ecosystem exchange of CO₂to sunlight. To test the hypothesis that snowmelt is required for photosynthesis to begin, these were compared with end of snowmelt derived from soil temperature. ENF forests achieved 10% of summer photosynthetic capacity ∼3 weeks before end of snowmelt, while DBF forests achieved that capacity ∼4 weeks afterward. DBF forests increased photosynthetic capacity in spring faster (1.95% d⁻¹) than ENF (1.10% d⁻¹), and their active season length (End_GPP–Start_GPP) was ∼50 days shorter. We hypothesized that warming has influenced timing of the photosynthesis season. We found minimal evidence for long‐term change in Start_GPP, End_GPP, or air temperature, but their interannual anomalies were significantly correlated. Warmer weather was associated with earlier Start_GPP(1.3–2.5 days °C⁻¹) or later End_GPP(1.5–1.8 days °C⁻¹, depending on forest type and month). Finally, we tested whether existing phenological models could predict Start_GPPand End_GPP. For ENF forests, air temperature‐ and daylength‐based models provided best predictions for Start_GPP, while a chilling‐degree‐day model was best for End_GPP. The root mean square errors (RMSE) between predicted and observed Start_GPPand End_GPPwere 11.7 and 11.3 days, respectively. For DBF forests, temperature‐ and daylength‐based models yielded the best results (RMSE 6.3 and 10.5 days). 

Wildfire impacts on air quality and climate are expected to be exacerbatedby climate change with the most pronounced impacts in the boreal biome.Despite the large geographic coverage, there is limited information onboreal forest wildfire emissions, particularly for organic compounds, whichare critical inputs for air quality model predictions of downwind impacts.In this study, airborne measurements of 193 compounds from 15 instruments,including 173 non-methane organics compounds (NMOG), were used to providethe most detailed characterization, to date, of boreal forest wildfireemissions. Highly speciated measurements showed a large diversity ofchemical classes highlighting the complexity of emissions. Usingmeasurements of the total NMOG carbon (NMOGT), the ΣNMOG wasfound to be 50 % ± 3 % to 53 % ± 3 % of NMOGT, of which, theintermediate- and semi-volatile organic compounds (I/SVOCs) were estimatedto account for 7 % to 10 %. These estimates of I/SVOC emission factorsexpand the volatility range of NMOG typically reported. Despite extensivespeciation, a substantial portion of NMOGT remained unidentified(47 % ± 15 % to 50 % ± 15 %), with expected contributions from morehighly-functionalized VOCs and I/SVOCs. The emission factors derived in thisstudy improve wildfire chemical speciation profiles and are especiallyrelevant for air quality modelling of boreal forest wildfires. Theseaircraft-derived emission estimates were further linked with those derivedfrom satellite observations demonstrating their combined value in assessingvariability in modelled emissions. These results contribute to theverification and improvement of models that are essential for reliablepredictions of near-source and downwind pollution resulting from borealforest wildfires.