Ambient observations indicating an increasing effectiveness of ammonia control in wintertime PM2.5 reduction in Central China

Ammonia emission reduction is increasingly being considered one of the control measures to mitigate wintertime fine particulate matter (PM2.5) pollution. Three wintertime observations from 2012 to 2018 in Wuhan, China, were analyzed to examine the effectiveness of ammonia control in wintertime PM2.5 reduction based on the critical total ammonia concentration (CTAC, i.e., the inflection point of effective ammonia control for PM2.5 mass reduction based on the asymmetric response of PM2.5 to ammonia control). The CTAC gradually approached 0% (immediate effectiveness), with values of -26% in 2012, -23% in 2015, and -9% in 2018. At the observed ambient conditions, there were significant positive correlations of the CTAC with sulfate and total nitrate changes, in contrast to the negative correlation of the CTAC with total ammonia change. An approximately 10% total ammonia reduction could offset the decline in CTAC attributed to a 30-40% sulfate or 20-30% total nitrate reduction in Wuhan. This study indicates that the combined control of SO2 + NOx (NO+NO2) remains the preferred way to reduce inorganic particles in Central China at present, despite a tendency of the ambient chemical state moving towards effective ammonia control. However, as the CTAC approaches 0%, the effectiveness of ammonia and NOx reduction measures targeting wintertime PM2.5 can greatly exceed that observed during the 2012-2018 period in Central China.