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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]

Identifying the sources of increased ammonia (NH 3) in the atmosphere has important implications for air quality, ecosystems, and climate. While analysis of the nitrogen isotopic composition (δ15N) can aid in the apportionment of NH 3 emission sources, the widely-used passive collection methods have not been verified for their ability to accurately characterize δ15N-NH 3. In this study, three types of passive samplers (ALPHA, Analyst, and Radiello) were employed and collocated with a denuder based active sampler (DELTA, serves as a reference method) to compare and contrast their ability in collecting atmospheric NH 3 for subsequent concentration and δ15N-NH 3 analysis in urban Beijing during the summer of 2018. The results showed that NH 3 concentrations determined using passive collection samplers that included ALPHA (n = 11), Analyst (n = 12), and Radiello (n = 7) samplers were 13.4%, 14.4%, and 27.5% lower than that of the active sampler (n = 12), respectively. The low NH 3 concentration bias in the passive collection samplers was suggested to be the result of inaccurate effective sampling rates due to incorrect mass transfer correction factors for the environmental conditions of this study. The δ15N-NH 3 values did not show significant differences between passive samplers, with weekly means of −31.0 ± 1.0‰ (n = 4), −28.4 ± 5.5‰ (n = 4), and −29.1 ± 0.9‰ (n = 4) for the ALPHA, Analyst, and Radiello, respectively. Notably, the overall mean δ15N-NH 3 values collected by these passive samplers (−29.5 ± 3.2‰, n = 12) was significantly lower than that of the active sampler (−14.1 ± 1.6‰, n = 4), with a difference of 15.4 ± 3.5‰. The large underestimation of δ15N-NH 3 values by passive samplers can be explained by the diffusive isotope fractionation of NH 3 isotopologues, which is driven by relative mass differences of 14NH 3 and 15NH 3. Overall, these findings highlight a potential low bias in NH 3 concentrations and a substantial uncertainty in previous source interpretation using δ15N-NH 3 as previous studies have almost exclusively characterized δ15N-NH 3 using passive samplers. We suggest that the denuder-based active sampler should be used in the future for characterizing δ15N-NH 3 whether at the source or in an atmospheric environment with mixed sources. This work also has important implications for characterizing δ15N of other gaseous nitrogen compounds. Unlabelled Image • Nitrogen isotopic composition of ammonia (δ15N-NH 3) aids in source apportionment. • Conventional collection methods for δ15N-NH 3 were almost based on passive samplers. • δ15N-NH 3 obtained by passive and active samplers were respective −29.5‰ and −14.1‰. • The low bias of 15.4‰ by passive δ15N-NH 3 was due to diffusive isotope fractionation. • Source apportionment based on passive δ15N-NH 3 might mislead mitigation strategy. [ABSTRACT FROM AUTHOR]

Size-resolved aerosol organic carbon (OC) and water-soluble OC (WSOC) was determined weekly in urban Beijing between 2013 and 2018. The results showed that in winter, the concentrations of WSOC and OC in fine (<2.1 μm) and coarse (>2.1 μm) mode were highest in 2013. During the following years, the concentrations of WSOC and OC were lower but an increasing trend was found from 2014 to 2016, which might be due to the increasing precursor concentrations. From summer 2013 to 2018, the concentrations of OC and WSOC in both fine and coarse mode decreased, which might be affected by decreasing emission intensity and photochemical oxidation. To characterize the seasonality of WSOC, two intensive field observations were further performed in winter 2013 and summer 2018. In winter of 2013, WSOC was mainly enriched in PM 1 (<1 μm) on clean days and lightly polluted days. In addition, the fine mode maximum shifted from 0.43-0.65 μm to 1.1–2.1 μm with the haze development due to the hygroscopic growth of particles. In summer of 2018, the WSOC was mainly concentrated in PM 1 , but in a few samples, the maximum size appeared in coarse mode. This difference might due to the different relative humidity and air mass sources. While the formation process of WSOC in winter was affected by photochemical oxidation below 2.1 μm, in 1.1–2.1 μm, it tended to change from photochemical oxidation to aqueous-phase oxidation. For the coarse mode WSOC formation in winter, however, it was mainly affected by heterogeneous processes. The findings advanced our understanding of the annual trends, size distribution and formation processes of WSOC, with significant implications for air pollution control in Beijing and surrounding regions. Image 1 • Carbonaceous species in both fine and coarse modes decreased from 2013 to 2018. • The size distribution of WSOC shifted from fine to coarse during haze episodes. • In summer, coarse mode WSOC was mainly condensed on sea salt. • In winter, WSOC formation was affected by secondary processes. [ABSTRACT FROM AUTHOR]