It is a puzzle as to why more severe haze formed during the New Year Holiday in 2020 (NYH‐20), when China was in an unprecedented state of shutdown to contain the coronavirus (COVID‐19) outbreak, than in 2019 (NYH‐19). We performed a comprehensive measurement and modeling analysis of the aerosol chemistry and physics at multiple sites in China (mainly in Shanghai) before, during, and after NYH‐19 and NYH‐20. Much higher secondary aerosol fraction in PM_2.5 were observed during NYH‐20 (73%) than during NYH‐19 (59%). During NYH‐20, PM_2.5 levels correlated significantly with the oxidation ratio of nitrogen (r² = 0.77, p < 0.01), and aged particles from northern China were found to impede atmospheric new particle formation and growth in Shanghai. A markedly enhanced efficiency of nitrate aerosol formation was observed along the transport pathways during NYH‐20, despite the overall low atmospheric NO₂ levels.

Agricultural activities are a major source contributing to NH₃ emissions in Shanghai and most other regions of China; however, there is a long-standing and ongoing controversy regarding the contributions of vehicle-emitted NH₃ to the urban atmosphere. From April 2014 to April 2015, we conducted measurements of a wide range of gases (including NH₃) and the chemical properties of PM_2.5 at hourly resolution at a Shanghai urban supersite. This large data set shows NH₃ pollution events, lasting several hours with concentrations 4 times the annual average of 5.3 µg m⁻³, caused by the burning of crop residues in spring. There are also generally higher NH₃ concentrations (mean ± 1 σ) in summer (7.3 ± 4.9 µg m⁻³; n = 2181) because of intensive emissions from temperature-dependent agricultural sources. However, the NH₃ concentration in summer was only an average of 2.4 µg m⁻³ or 41 % higher than the average NH₃ concentration of other seasons. Furthermore, the NH₃ concentration in winter (5.0 ± 3.7 µg m⁻³; n = 2113) was similar to that in spring (5.1 ± 3.8 µg m⁻³; n = 2198) but slightly higher, on average, than that in autumn (4.5 ± 2.3 µg m⁻³; n = 1949). Moreover, other meteorological parameters like planetary boundary layer height and relative humidity were not major factors affecting seasonal NH₃ concentrations. These findings suggest that there may be some climate-independent NH₃ sources present in the Shanghai urban area. Independent of season, the concentrations of both NH₃ and CO present a marked bimodal diurnal profile, with maxima in the morning and the evening. A spatial analysis suggests that elevated concentrations of NH₃ are often associated with transport from regions west–northwest and east–southeast of the city, areas with dense road systems. The spatial origin of NH₃ and the diurnal concentration profile together suggest the importance of vehicle-derived NH₃ associated with daily commuting in the urban environment. To further examine vehicular NH₃ emissions and transport, sampling of the NH₃ concentration was performed in (from the entrance to the exit of the tunnel) and out (along a roadside transect spanning 310 m perpendicular to the tunnel) of a heavily trafficked urban tunnel during the spring of 2014. NH₃ concentrations in the tunnel exit were over 5 and 11 times higher than those in the tunnel entrance and in the ambient air, respectively. Based on the derived mileage-based NH₃ emission factor of 28 mg km⁻¹, a population of 3.04 million vehicles in Shanghai produced around 1300 t NH₃ in 2014, which accounts for 12 % of total NH₃ emissions in the urban area. Collectively, our results clearly show that vehicle emissions associated with combustion are an important NH₃ source in Shanghai urban areas and may have potential implications for PM_2.5 pollution in the urban atmosphere.

Atmospheric trace elements, especially metal species, are an emerging environmental and health concern with insufficient understanding of their levels and sources in Shanghai, the most important industrial megacity in China. Here we continuously performed a 1 year (from March 2016 to February 2017) and hourly resolved measurement of 18 elements in fine particles (PM_2.5) at the Shanghai urban center with an Xact multi-metals monitor and several collocated instruments. Mass concentrations (mean ± 1σ; ng m⁻³) determined by Xact ranged from detection limits (nominally 0.1 to 20 ng m⁻³) to 15 µg m⁻³. Element-related oxidized species comprised an appreciable fraction of PM_2.5 during all seasons, accounting for 8.3 % on average. As a comparison, the atmospheric elements concentration level in Shanghai was comparable with that in other industrialized cities in East Asia but 1 or 2 orders of magnitude higher than at sites in North America and Europe. Positive matrix factorization (PMF) was applied to identify and apportion the sources of the elements in the PM_2.5 mass. Five different factors were resolved (notable elements and relative contribution in parentheses): traffic-related (Ca, Fe, Ba, Si; 46 %), shipping (V, Ni; 6 %), nonferrous metal smelting (Ag, Cd, Au; 15 %), coal combustion (As, Se, Hg, Pb; 18 %) and ferrous metal smelting (Cr, Mn, Zn; 15 %). The contribution from the exhaust and non-exhaust vehicle emissions, i.e., the traffic-related factor shows a strong bimodal diurnal profile with average concentration over 2 times higher during the rush hour than during nighttime. The shipping factor was firmly identified because V and Ni, two recognized tracers of shipping emissions, are almost exclusively transported from the East China Sea and their ratio (around 3.2) falls within the variation range of V ∕ Ni ratios in particles emitted from heavy oil combustion. Interestingly, nearly half of the K was derived from coal combustion with high mineral affinity (elements associated with aluminosilicates, carbonates and other minerals in coal ash). The contributions of nonferrous metal smelting to the trace elements are consistent with a newly developed emission inventory. Although the precipitation scavenging effect on the mass concentration of the trace elements varied among different species and sources, precipitation could effectively lower the concentration of the traffic- and coal combustion-related trace elements. Therefore, water spray to simulate natural types of precipitation could be one of the abatement strategies to facilitate the reduction of ambient PM_2.5 trace elements in the urban atmosphere. Collectively, our findings in this study provide baseline levels and sources of trace elements with high detail, which are needed for developing effective control strategies to reduce the high risk of acute exposure to atmospheric trace elements in China's megacities.

Stable nitrogen isotope composition (δ¹⁵N) offers new opportunities to address the long-standing and ongoing controversy regarding the origins of ambient ammonia (NH₃), a vital precursor of PM_2.5 (particulate matters with aerodynamic diameter equal or less than 2.5 µm) inorganic components, in the urban atmosphere. In this study, the δ¹⁵N values of NH₃ samples collected from various sources were constrained using a novel and robust chemical method coupled with standard elemental analysis procedures. Independent of the wide variation in mass concentrations (ranging from 33 (vehicle) to over 6000 (human excreta) µg m⁻³), different NH₃ sources have generally different δ¹⁵N values (ranging from −52.0 to −9.6 ‰). Significantly high δ¹⁵N values are seen as a characteristic feature of all vehicle-derived NH₃ samples (−14.2 ± 2.8 ‰), which can be distinguished from other sources emitted at environmental temperature (−29.1 ± 1.7, −37.8 ± 3.6, and −50.0 ± 1.8 ‰ for livestock, waste, and fertilizer, respectively). The isotope δ¹⁵N signatures for a range of NH₃ emission sources were used to evaluate the contributions of the different sources within measured ambient NH₃ in Beijing, using an isotope mixing model (IsoSource). The method was used to quantify the sources of ambient NH₃ before, during and after the 2014 Asia-Pacific Economic Cooperation (APEC) summit, when a set of stringent air quality control measures were implemented. Results show that the average NH₃ concentrations (the overall contributions of traffic, waste, livestock, and fertilizer) during the three periods were 9.1 (20.3, 28.3, 23.6, and 27.7 %), 7.3 (8.8, 24.9, 14.3, and 52.0 %), and 12.7 (29.4, 23.6, 31.7, and 15.4 %) µg m⁻³, respectively, representing a 20.0 % decrease first and then a 74.5 % increase in overall NH₃ mass concentrations. During (after) the summit, the contributions of traffic, waste, livestock, and fertilizer decreased (increased) by 56.7 (234.2), 12.0 (−5.0), 39.4 (120.8), and −87.7 % (−70.5 %) when compared with periods before (during) the summit, respectively, signifying that future NH₃ control efforts in megacities like Beijing should prioritize traffic sector as well as livestock breeding. The results show that isotope ratio measurements of NH₃ to be a valuable tool to quantify the atmospheric sources of NH₃ in urban atmospheres.