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Water-soluble inorganic ions (WSI), a major component of PM 2.5 , often increased rapidly during the haze event in Beijing. Sulfate (SO 4 2−), Nitrate (NO 3 −), and Ammonium (NH 4 +) are three main components of WSI. Since year 2015, sulfate concentrations in PM 2.5 has gradually decreased owing to the effective control of SO 2 emissions. However, the contribution of nitrate to PM 2.5 has significantly increased during haze events in Beijing at the same time. In this study, a highly time-resolved online analyzer (Monitor for Aerosols and Gases, MARGA) was employed to measure the WSI in PM 2.5 in Beijing from 5 February to 15 November 2017. Three typical haze events during this sampling period were investigated. During heavy pollution episodes in winter, nitrate concentrations increased from 7.5 μg/m3 to 45.6 μg/m3 (45.0% of WSI), while sulfate increased from 4.2 μg/m3 to 20.1 μg/m3 (19.8% of WSI). This indicated that nitrate is more important than sulfate as a driver for the growth of PM 2.5 during the period of heavy air pollution in winter. Nitrate also dominates the increase of WSI in the pollution episodes in autumn, with an average concentration of 52.5 μg/m3, and contributed up to 67% of WSI. The average concentration ratio of NH 4 + to SO 4 2− was higher in autumn (1.02) than that in summer (0.74) and close to that in winter (1.00). This is mainly because the emission control of coal combustion in Beijing and surrounding areas results in an NH 3 -rich and SO 2 -lean atmosphere, which promoted the formation of ammonium nitrate. Our study indicates that nitrate has become the most important component of WSI in PM 2.5 and is driving the rapid growth of PM 2.5 concentrations during heavy pollution episodes in Beijing. Therefore, more efforts shall be made to reduce the nitrogen oxide and ammonia emissions in Beijing and surrounding areas. Unlabelled Image • Nitrate become the main component of PM 2.5 than sulfate during the haze events in Beijing. • High NOx emission, low temperature and low aerosol acidity are the key factors to promote nitrate formation in Beijing. • Molar ratio of NH 4 + to SO 4 2- increased from 1.5 (before 2013) to 3.33 (2017) due to ammonia-rich atmosphere in Beijing. [ABSTRACT FROM AUTHOR]

Gray water footprint (GWF) is a simple and efficient indicator to quantify water pollution that has received extensive attention. Previously, an agricultural GWF was estimated by assuming the nitrogen loss rate throughout the crop cycle, while the field chemicals accompanying water movement during crop growth were not considered. In this study, using field observations of water and nitrogen transport in Nanjing, China, the GWF was calculated at the time steps of a day (GWF d), sub-growth period (GWF s), and whole crop season (GWF w), and its formation mechanism and step effect in rice production were explored. Field drainage and leakage caused nitrogen loss and generated a GWF, with the latter occurring intermittently. The values of GWF d , GWF s , and GWF w from 2015 to 2018 were 0.42, 0.38, and 0.31 m3/kg, respectively, and the step effect exists temporally for the rice GWF. The GWF of paddy rice growth decreased as the measurement period increased and the time step was calculated using GWF d as a reference. Herein, it was determined that the GWF w used in previous studies may have greatly underestimated the adverse impact of the GWF on water quality, particularly in humid regions or years. Therefore, it is essential to identify the formation mechanism and appropriately select the time steps for GWF evaluation during rice production. This research provides methodological references for GWF assessment and reduction in crop production systems. Unlabelled Image • Formation mechanism and step effect were analysed for rice gray water footprint (GWF). • GWF was determined by field water and nitrogen transport and mainly occurred in early stages of crop season. • GWF showed scale effect temporally and reduced with the increase of time step. • Sub growth period is the suitable time step for macro scale rice GWF estimation. [ABSTRACT FROM AUTHOR]