Emissions, meteorological and climate impacts on PM_2.5 levels in Southern California using a generalized additive...

Annual fine particulate matter (PM_2.5) mass concentrations in the South Coast Air Basin (SoCAB) of California decreased from around 30 μg/m³ to 11 μg/m³ between 2000 and 2013 but rose from 11 μg/m³ to 13 μg/m³ between 2014 and 2018, raising important questions about the effectiveness of ongoing emission control policies. A two-step generalized additive model (GAM)-least squares approach was developed to explore the effects of emissions, large-scale climate events and meteorological factors on daily PM_2.5 mass concentrations from 2000 to 2019 to quantitatively link impacts of emissions and meteorological on PM_2.5 and to assess factors leading to the increase. The GAM had an R² = 0.99 and root mean square error (RMSE) = 0.7 μg/m³ for the annual average PM_2.5 concentrations. The two-step method had an R² = 0.93 and RMSE = 4.07 μg/m³ for the 98th percentile 24-hr average PM_2.5 concentrations. Variations in both emissions and relative humidity were of high importance compared with other included factors. Interactions of NH₃ emissions with NOx and SO₂ emissions, which lead to ammonium nitrate and sulfate aerosol formation, were the most important factors. Meteorological effects on PM_2.5 explained the majority of the daily PM_2.5 fluctuations. Emission changes (increases in SO₂ and PM_2.5) led to increases in predicted PM_2.5 between 2014 and 2018. Predicted future PM_2.5, using projected emissions and meteorological data from model simulations of representative concentration pathway (RCP) scenarios, are around 12 μg/m³ (annual) and 30 μg/m³ (98th percentile daily), which are both close to the current National Ambient Air Quality Standards (NAAQS) for PM_2.5. Meteorological impacts on the predicted PM_2.5 in future years lead to variations of ±2 μg/m³ for the annual average and ±5 μg/m³ for the 98th percentile daily level. Future climate changes lead to a probable year-to-year variation that will let PM_2.5 levels in some years exceed the standard.