The washing effect of precipitation on particulate matter and the pollution dynamics of rainwater in downtown Beijing

The particulate matter (PM) pollution of Beijing fluctuated significantly before and after precipitation events during the rainy period. The rainwater samples were continually collected at five-minute intervals on the roof of a building in downtown Beijing. The PM2.5 was also monitored at same temporal scale. The pH and concentrations of ammonia-N, nitrate-N, phosphorus, sulfur, and heavy metals (Cd, Mn, Fe, Zn, and Cu) in the rainwater samples were measured, and the values were used to assess the effect of washing on PM2.5 and determine the characteristics of the rainwater pollutants. The PM2.5 was negatively correlated to the quantity of accumulated rainfall (R2 value ranged from 0.668 to 0.974), which identified the function of the washing process on PM2.5 pollution. The washing process of rainfall strongly affects PM2.5, which decreased to 10–30 μg m− 3 with 5 mm of rainfall. The analysis of the temporal patterns of ten pollutants in rainwater demonstrated that most of the pollution load was contributed by the first 5 to 10 min of the washing process. The event mean concentration (EMC) of nine inorganic pollutants was used to evaluate the pollution load and identify sources of particle pollution. Sulfur was the primary inorganic element detected in PM, and the average EMC of twelve rainfall events was 8.92 mg L− 1. The EMC of ammonia-N, nitrate-N, and phosphorus after significant PM2.5 pollution was 11.57 mg L− 1, 1.72 mg L− 1, and 0.019 mg L− 1, respectively. The total pollution load of the rainwater collecting area during the rainy reason was also calculated, and the largest heavy metal load of 3.11 mg was attributed to Zn. The pollution loads of heavy metals and the stable relationship between the pollutants indicate the potential urban environmental management policies in Beijing.