Assessing Impacts of the 2022 Winter Olympic Games on Meteorology and of Atmospheric Ammonia Emissions in Taiyuan City Using a Random Forest Model

<p indent="0mm"> To ensure improvement of air quality during the 24th Winter Olympic Games (Beijing 2022), held from 4^th to 20^th February 2022, a series of anthropogenic control measures (motor vehicle restrictions and industrial production curbs) were implemented in Taiyuan city. In order to assess the impacts of the short-term control measures on urban atmospheric ammonia (NH₃), online observations of hourly-resolved gaseous NH₃ and water-soluble ions were monitored using MARGA at an urban site in Taiyuan city from 1 January to 20^th February, 2022. The pollution was characterized and, in parallel, the influence on meteorology and NH₃ emissions were assessed using a random forest model. The results showed that the NH₃ concentration (5.4 ± <sc> 3.3 μg/m³) </sc> during the control period of the Winter Olympic Games (C2: 30^th January to 20^th February) was 36.0% lower than the pre-control period (C1: 1st to 29^th January; 8.4 <sc> ±3.9 μg/m³). </sc> Compared with the C1 period, higher wind speed (1.8 ± <sc>0.8 m/s)</sc> and the atmospheric boundary layer height (508.1 ± <sc>636.7 m)</sc> favored pollutant dispersion during the C2 period, whereas obvious decline in acidic gas concentrations (NO₂ , 47.8%, and SO₂ ,33.2%) during the C2 period could have elevated NH₃ concentration through gas-particle partitioning of NH₃, suggesting that meteorological and chemical factors could have interfered with the assessment of NH₃ impact from emission changes. The results of the random forest model showed that meteorological normalized concentrations of NH₃, NH₄⁺ and TNHx (NH₃+NH₄⁺) decreased by 25.2%, 21.3% and 20.3% during C2, respectively, compared with C1. Due to the combination of the effects of anthropogenic emissions and secondary conversion, TNHx could better assess the real changes of NH₃ caused by anthropogenic emissions. The contributions of anthropogenic emission reduction and meteorological factors to the decrease in TNHx during C2 were 39.5% and 60.5%, respectively, indicating that the reduction of anthropogenic emissions further lowered TNHx concentration under favorable meteorological conditions. The study can provide scientific reference to assess the impact of control measures on environmental air quality in other regions.