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Biomass burning releases a vast amount of aerosols into the atmosphere, often leading to severe air quality and health problems. Prediction of the air quality effects from biomass burning emissions is challenging due to uncertainties in fire emission, plume rise calculation, and other model inputs/processes. Ensemble forecasting is increasingly used to represent model uncertainties. In this paper, an ensemble forecast was conducted to predict surface PM2.5 during the 2018 California Camp Fire event using the National Oceanic and Atmospheric Administration (NOAA) HYSPLIT dispersion model at 0.1° horizontal resolution. Different combinations of four satellite‐based fire emission data sets (FEER, FLAMBE, GBBEPx, and GFAS), two plume rise schemes (Briggs and Sofiev), various meteorology inputs, and model setup options were used to create the forecast ensemble, for a total of 112 experiments. The performance of each ensemble member and the ensemble mean were evaluated using ground‐based observations, with four statistical metrics and an overall rank. The ensemble spread of the 112 members reached 1,000 μg/m3, highlighting the large uncertainty in wildfire forecast. The ensemble mean displayed the best performance. Each fire emission product contributed to one or more members among the top 10 performers, revealing the forecasting dependence on both the quality of fire emissions data and model representation of emission, transport, and removal processes. In addition, an ensemble size reduction technique was introduced. With the help of this technique, the ensemble size was reduced from 112 to 28 members and still produced an ensemble mean that yielded comparable or even better performance to that of the full ensemble. Key Points: The difference between satellite fire emission data sets is large, which reaches a factor of 10Ensemble forecast with different emission data sets, meteorology, and plume rise schemes can reduce forecast uncertaintyThe new ensemble size reduction technique can save calculation time while improving the performance of the ensemble forecast [ABSTRACT FROM AUTHOR]

This paper presents the results of long‐term observations (1964–2010) of direct solar flux at two stations in the Tatra Mountains in Poland (Central Europe). The measurements obtained from a Linke‐Feussner actinometers are used to estimate broadband (0.29–2.9 μm) and wideband (0.53–0.63 μm) aerosol optical depth (BAOD and WAOD, respectively). Both BAOD and WAOD at Mount Kasprowy Wierch (1,991 m a.s.l.) and in Zakopane (851 m a.s.l.) show similar annual cycles with peaks during spring and summer. Coincidence of actinometer observation at the top of the Tatra Mountains and in Zakopane valley allows determination of AOD in the column between 851 and 1,991 m a.s.l. Annual variabilities of both BAOD and WAOD for this layer are similar to PM10 concentration measurements in Zakopane. Conversely, BAOD and WAOD obtained at both stations show opposing annual variability, which is consistent with the Aerosol Robotic Network and model results in Central Europe. Long‐term data show significant reduction of both BAOD and WAOD in Zakopane (−0.018 ± 0.003 and −0.023 ± 0.003, respectively, per decade) and lower reductions at Mt. Kasprowy Wierch (−0.005 ± 0.002 and −0.009 ± 0.002, respectively, per decade). Trends for BAOD and WAOD defined between 851 and 1,991 m a.s.l. are the same (−0.013 ± 0.004 per decade), which corresponds to the reduction of WAOD and BAOD by about 60% between 1964 and 2010. Conversely, emission data for the Zakopane region obtained from Emissions Database for Global Atmospheric Research inventories shows reduction of SO2 by a factor of 3 and of NOx and PM10 by about 40% between 1988 and 2010. Key Points: Annual cycle AOD as well as long‐range trend varies with altitude in the Tatra MountainsAOD long‐range negative trend is significantly higher in the lower station than at the mountain top indicating reduction of human emissionAOD defined between valley and mountain top shows positive correlation with monthly means PM10 concentration [ABSTRACT FROM AUTHOR]