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Ammonia (NH 3) is a vital precursor of secondary aerosols, which play an important role in urban air pollution. The passive observation of NH 3 in the urban atmosphere showed higher concentrations than conventionally believed; however, the NH 3 dynamics remain limited due to the paucity of active measurements with fast response instruments. In this study, intensive online measurements of ambient NH 3 were conducted using cavity ring-down spectroscopy in late spring and early summer 2020 at the megacities of Beijing and Shanghai in northern and southern China. The results show that the average NH 3 concentration in Beijing (23.1 ± 10.3 ppb) was nearly double that in Shanghai (12.0 ± 5.0 ppb). This spatial difference was confirmed by satellite column observations and bottom-up emission patterns. In addition, there was a significant increase in NH 3 concentrations during the morning in our study. This special diurnal profile was a universal feature, with an occurrence frequency exceeding 50% in both cities. These frequent morning increases were mainly caused by motor vehicle emissions, with an additional contribution from the residual layer breakup for one-quarter of the days in the morning increase. On these days, the NH 3 concentration increased by approximately 20% due to the downward mixing of the residual layer rupture. The unique data observed here could be used to validate the chemistry and transport models at a higher time resolution and improve the understanding of the current status of NH 3 pollution in the urban atmosphere. [Display omitted] • 1 Hz NH 3 observations were conducted at megacities of Beijing and Shanghai. • Atmospheric NH 3 concentrations in urban Beijing were double that in Shanghai. • A morning pulse of NH 3 levels was a common feature at Beijing and Shanghai. • Highly frequent morning increases of NH 3 were mainly caused by vehicle emissions. • Boundary layer rupture further enhanced the rapid increase of NH 3 in the morning. [ABSTRACT FROM AUTHOR]

Abstract: Rainwater and airborne particles were sampled at five sites, including Beijing and Mazhuang Town in the northeast of China, Chongqing in the southwest of China, Shenzhen and Mangdang Mountain in the south of China. The pollution of airborne particles in winter at Chongqing and Beijing were the worst and the following was in summer at Mazhuang town and atmosphere quality in spring at Mangdang Mountain was the best. The ratios of fine particles to coarse particles were high among all the sampling sites. The contribution from soluble components to particulate mass was important and the main ions both on PM10 and PM2.5 were SO4 2 −, NO3 − and NH4 +. Both inorganic and organic soluble components on particles were mainly enriched on fine particles. The average pH values in rainwater at Beijing, Mazhuang Town, Shenzhen and Mangdang Mountain were 6.02, 5.97, 4.72 and 4.81 respectively. A new methodology to determine the neutralizing capacity of airborne particles was established in this study. The pH values and water-soluble ion concentrations in particulate matter extract were measured, then the ionization balance and charge conservation principle were adopted to determine the amount of acid-basic compositions on particle. The amount of H+ in rainwater neutralized by airborne particles per unit mass was used to denote the neutralizing capacity of particles. The neutralizing capacity of airborne particles was lower if both of the concentrations of particles and ratio of fine to coarse particles were higher. The neutralizing capacity of airborne particles was inverse proportion to the acidity of rainwater and the rain intensity. The neutralizing effect of particles accounted for 4%–28% of the reduction of rainwater acidity during wash-out process among different sites, and it was deduced that NH3 was the major neutralizing species especially for rainwater from sites in the northeast of China. [Copyright &y& Elsevier]