Ammonia (NH3) emission reduction is key to limiting the deadly PM2.5 pollution globally. However, studies of long-term source apportionment of vertical NH3 are relatively limited. On the basis of the one-year measurements of weekly vertical profiles of δ 15N–NH3 at 5 heights (2, 15, 102, 180, and 320 m) on a 325-m meteorological tower in urban Beijing, we found that vertical profiles of NH3 concentrations generally remained stable with height. δ 15N–NH3 increased obviously as a function of height in cold seasons (with heating) and decreased in warm seasons (with fertilization), indicating a stronger human-induced seasonal variation via regional transport at higher altitudes. Relatively stable δ 15N–NH3 near the ground surface suggested the strong local emission. The results of isotopic mixing model (SIAR) indicate that source apportionment using measured δ 15N–NH3 only would overestimate the contribution of agricultural emissions to NH3. By using an estimation of initial δ 15N–NH3, we found that nonagricultural sources contributed ∼72% of NH3 on average. Our study suggests that (i) both persistent nonagricultural and periodic agricultural emissions drive atmospheric NH3 concentration and its vertical distribution in urban Beijing; and (ii) source apportionment based on measured δ 15N–NH3 only likely underestimates fossil fuel source contribution, if the combined NH x isotope effects are not considered.

This study investigates the concentrations and δ 15 N values of NH 3 in Beijing during and after the 2020 COVID-19 lockdown. Higher NH 3 concentrations and lower δ 15 N-NH 3(measured) were observed at most sites in 2020 compared to 2017. Except for a site inside a tunnel, NH 3 concentrations did not increase significantly after the lockdown had ended compared to those during the lockdown, while δ 15 N-NH 3(measured) increased by 2.1–9.9‰. Nonagricultural sources (fossil fuel and urban waste) overall contributed 81% and 62% of NH 3 at on-road (tunnel interior) and nonroad (CAU) sites in 2020, respectively, comparable to those in 2017 (without significant difference). The contribution of nonagricultural sources slightly increased after the lockdown compared to the contribution during the lockdown at the nonroad site and hardly changed at the tunnel interior site. Our results suggest that (1) unfavorable meteorological conditions, especially lower boundary layer heights and changes in regional transport patterns, might play a more important role than reduced anthropogenic emissions in the temporal variations of Beijing NH 3 and (2) the effect of reduced anthropogenic emissions, during the COVID-19 outbreak or with the future implementation of emission control strategies, on atmospheric NH 3 can be better demonstrated by isotope-based source apportionment of NH 3 , rather than only by changes in NH 3 concentrations.

Air pollution especially of PM 2.5 pollution is a serious problem in Beijing. In order to quantify the effect of a festival in which pollution was expected to be reduced, we collected and analyzed PM 2.5 samples in urban Beijing during the 2015 Spring Festival (from February 9 th to March 6 th 2015). We divided the sampling period into three phases: non-haze, haze and firework days. The average concentration of PM 2.5 was highest on firework days at 248.9 µg m -3 , followed by haze days (199.9 µg m -3 ) and non-haze days (90.8 µg m -3 ). The air quality of non-haze days during the holiday was better than that during non-holiday periods. 2-/K + and Cl -/K + effectively distinguished the characteristics of PM 2.5 between firework events and haze days. Ion balance calculations indicated that the acidity of PM 2.5 from firework days was higher than that from haze and non-haze days. A method using enrichment factors (EF) found that crustal elements (EF < 10 on all three types of days) included Ca, Al, Fe, Na, Co, Ni, P, Ti, and V; firework elements (EF > 10 on firework days, significantly higher than haze days) were made up of Ba, Cr, Cu, Mg, Pb, S, Si, and Zn; common anthropogenic pollution elements (EF > 10 in all three types of days) were As, Cd, Cu, Pb, S, Sb, Zn. Differences in chemical characteristics indicated that holidays such as the Spring Festival can affect air pollution patterns in two ways: a decrease in the population and vehicles but an increase in activities such as firework displays.

Reducing atmospheric ammonia (NH3) emissions is critical to mitigating poor air quality. However, the contributions of major agricultural and non-agricultural source emissions to NH3 at receptor sites remain uncertain in many regions, hindering the assessment and implementation of effective NH3 reduction strategies. This study conducted simultaneous measurements of the monthly concentrations and stable nitrogen isotopes of NH x (gaseous NH3 plus particulate NH4 +) at 16 sites across China. Ambient NH x concentrations averaged 21.7 ± 19.6 μg m–3 at rural sites, slightly higher than those at urban (19.2 ± 6.0 μg m–3) and three times of those at background (7.0 ± 6.9 μg m–3) sites. Based on revised δ15N values of the initial NH3, source apportionment results indicated that non-agricultural sources (traffic and waste) and agricultural sources (fertilizer and livestock) contributed 54 and 46% to NH3 at urban sites, 51 and 49% at rural sites, and 61 and 39% at background sites, respectively. Non-agricultural sources contributed more to NH3 at rural and background sites in cold than warm seasons, arising from traffic and waste, but were similar across seasons at urban sites. We concluded that non-agricultural sources need to be addressed when reducing ambient NH3 across China, even in rural regions.

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