Apportioning Atmospheric Ammonia Sources across Spatial and Seasonal Scales by Their Isotopic Fingerprint.

Mitigating ammonia (NH3) emissions is a significant challenge, given its well-recognized role in the troposphere, contributing to secondary particle formation and impacting acid rain. The difficulty arises from the highly uncertain attribution of atmospheric NH3 to specific emission sources, especially when accounting for diverse environments and varying spatial and temporal scales. In this study, we established a refined δ15N fingerprint for eight emission sources, including three previously overlooked sources of potential importance. We applied this approach in a year-long case study conducted in urban and rural sites located only 40 km apart in the Shandong Peninsula, North China Plain. Our findings highlight that although atmospheric NH3 concentrations and seasonal trends exhibited similarities, their isotopic compositions revealed significant distinctions in the primary NH3 sources. In rural areas, although agriculture emerged as the dominant emission source (64.2 ± 19.5%), a previously underestimated household stove source also played a considerably greater role, particularly during cold seasons (36.5 ± 12.5%). In urban areas, industry and traffic (33.5 ± 15.6%) and, surprisingly, sewage treatment (27.7 ± 11.3%) associated with high population density were identified as the major contributors. Given the relatively short lifetime of atmospheric NH3, our findings highlight the significance of the isotope approach in offering a more comprehensive understanding of localized and seasonal influences of NH3 sources compared to emissions inventories. The refined isotopic fingerprint proves to be an effective tool in distinguishing source contributions across spatial and seasonal scales, thereby providing valuable insights for the development of emission mitigation policies aimed at addressing the increasing NH3 burden on the local atmosphere.

Vehicular Emissions Enhanced Ammonia Concentrations in Winter Mornings: Insights from Diurnal Nitrogen Isotopic Signatures.

A general feature in the diurnal cycle of atmospheric ammonia (NH3) concentrations is a morning spike that typically occurs around 07:00 to 10:00 (LST). Current hypotheses to explain this morning's NH3 increase remain elusive, and there is still no consensus whether traffic emissions are among the major sources of urban NH3. Here, we confirmed that the NH3 morning pulse in urban Beijing is a universal feature, with an annual occurrence frequency of 73.0% and a rapid growth rate (>20%) in winter. The stable nitrogen isotopic composition of NH3 (δ15N-NH3) in winter also exhibited a significant diurnal variation with an obvious morning peak at 07:00 to 10:00 (-18.6‰, mass-weighted mean), higher than other times of the day (-26.3‰). This diurnal pattern suggests that a large fraction of NH3 in the morning originated from nonagricultural sources, for example, power plants, vehicles, and coal combustion that tend to have higher δ15N-NH3 emission signatures relative to agricultural emissions. In particular, the contribution from vehicular emissions increased from 18% (00:00 to 07:00) to 40% (07:00 to 10:00), while the contribution of fertilizer sources to NH3 was reduced from 15.8% at 00:00 to 07:00 to 5.2% at 07:00 to 10:00. We concluded that NH3 concentrations in winter mornings in urban Beijing were indeed enhanced by vehicle emissions, which should be considered in air pollution regulations.

Revisiting the dynamics of gaseous ammonia and ammonium aerosols during the COVID-19 lockdown in urban Beijing using machine learning models.

The concentration of atmospheric ammonia (NH3) in urban Beijing substantially decreased during the COVID-19 lockdown (24 January to 3 March 2020), likely due to the reduced human activities. However, quantifying the impact of anthropogenic interventions on NH3 dynamics is challenging, as both meteorology and chemistry mask the real changes in observed NH3 concentrations. Here, we applied machine learning techniques based on random forest models to decouple the impacts of meteorology and emission changes on the gaseous NH3 and ammonium aerosol (NH4+) concentrations in Beijing during the lockdown. Our results showed that the meteorological conditions were unfavorable during the lockdown and tended to cause an increase of 8.4 % in the NH3 concentration. In addition, significant reductions in NOx and SO2 emissions could also elevate NH3 concentrations by favoring NH3 gas-phase partitioning. However, the observed NH3 concentration significantly decreased by 35.9 % during the lockdown, indicating a significant reduction in emissions or enhanced chemical sinks. Rapid gas-to-particle conversion was indeed found during the lockdown. Thus, the observed reduced NH3 concentrations could be partially explained by the enhanced transformation into NH4+. Therefore, the sum of NH3 and NH4+ (collectively, NHx) is a more reliable tracer than NH3 or NH4+ alone to estimate the changes in NH3 emissions. Compared to that under the scenario without lockdowns, the NHx concentration decreased by 26.4 %. We considered that this decrease represents the real decrease in NH3 emissions in Beijing due to the lockdown measures, which was less of a decrease than that based on NH3 only (35.9 %). This study highlights the importance of considering chemical sinks in the atmosphere when applying machine learning techniques to link the concentrations of reactive species with their emissions.

Is fertilization the dominant source of ammonia in the urban atmosphere?

It was previously believed that ammonia (NH3) has a short residence time in the atmosphere and cannot be transported far from its sources. In late March, however, this study observed a severe NH3 episode in urban Beijing when fertilizer was intensively applied on the North China Plain, with the highest hourly concentrations of 66.9 µg m-3 throughout the year. The stable nitrogen isotopic composition of NH3 (δ15N-NH3) during this episode (-37.0 to -20.0‰) fell in the range of endmembers of fertilizer and livestock, suggesting the long-range transport of NH3 from agricultural to urban regions. Based on a Bayesian isotope mixing model, the contribution of agriculture (fertilization) to urban NH3 concentrations was apportioned as 43.5% (26.0%) on polluted days. However, these contributions were reduced to 29.1% (12.8%) when nitrogen isotope fractionation between NH3 and ammonium was considered. In contrast to the limited contribution of agricultural sources, we found that nonagricultural emissions, particularly vehicles, dominate the source of NH3 in urban Beijing, even during the fertilization period. This finding indicated that nonagricultural sources should be considered when designing a control strategy for NH3 to reduce haze pollution in the urban atmosphere.

[Concurrent Collection of Ammonia Gas and Aerosol Ammonium in Urban Beijing During National Celebration Days Utilizing an Acid-Coated Honeycomb Denuder in Combination with a Filter System].

Since 2013, the Chinese government implemented the Air Pollution Prevention and Control Action Plan. As a result, the atmospheric concentrations of sulfate reduced significantly, whereas the nitrate concentrations remain relatively high due to the excess of ammonia (NH3). To date, there is no official observation network monitoring NH3 concentrations in China. Previous studies have focused on NH3 or ammonium (NH4+) separately. These limitations hinder a complete understanding of their dynamic changes due to the rapid gas-to-particle conversion. In this study, the concentrations of NH3 and NH4+ were measured concurrently in urban Beijing during autumn 2019 utilizing an acid-coated denuder-filter combination with a time resolution from 2 h (PM2.5>35 µg·m-3) to 5 h (PM2.5<35 µg·m-3). The mean concentrations of NH3 and NH4+ during the study were (4.1±2.9)µg·m-3 and (1.7±1.4) µg·m-3, respectively. The temporal patterns of NH3 or NH4+ coincided with that of PM2.5, CO, and NO2 throughout the sampling period. The diurnal distributions of NH3 were bimodal, both on polluted (PM2.5>75 µg·m-3) and non-polluted (PM2.5<75 µg·m-3) days, peaking at 21:30-05:30 and 05:30-08:30, respectively. The NH3 concentrations on polluted days were relatively lower during 17:30-21:30, which may be related to higher wind speeds. In contrast to NH3, NH4+ had an obvious peak during 17:30-21:30 due to the formation of ammonium nitrate. The meteorological conditions favor the gas-to-particle conversion on polluted days, resulting in a lower NH3/NH4+ ratio of 0.8. However, this value may reach 2.8 on non-polluted days. The concentrations of NH3, CO, NO2, SO2, and PM2.5 in the emission control period showed a significant increase greater than or comparable to those in the non-control period by 54.2%, 40.4%, 33.3%, 0%, and 49.4%, respectively. This result shows that the stagnant conditions offset the benefit of emission control actions implemented during and before the National Celebration Day.

[Source Apportionment of Atmospheric Ammonia: Sensitivity Test Based on Stable Isotope Analysis in R Language].

Ammonia (NH3) is an important precursor of fine particles and nitrogen deposition. It is critical to identify and quantify the sources of NH3 before the implementation of a mitigation strategy. Stable isotope analysis in R (SIAR) has potential with regard to the source apportionment of NH3, but its reliability is closely related to the signatures (δ15N-NH3) of emission sources. Based on SIAR, we found that the agricultural contribution varied significantly with mean δ15N-NH3 values of endmember input. In contrast, both the contributions of fossil fuel and NH3 slip showed low sensitivity against the change of endmember input. Moreover, the agricultural contribution changed by about 20% due to the variations in agricultural endmember mean values. Such a change is five times that due to the variations in endmember standard deviation values. Notably, regardless of the number of input sources tested, "non-agricultural source" was the dominant source of NH3 during hazy days in January 2013 in Beijing. Since various agricultural sources showed large variations in δ15N-NH3, future studies should focus on the endmember signatures of agricultural sources to further reduce the uncertainty in SIAR-based NH3 source apportionment.

[Concurrent Measurement of Wet and Bulk Deposition of Trace Metals in Urban Beijing].

To characterize the dry and wet deposition of atmospheric trace elements in urban Beijing, both active and passive samplers were used to collect bulk and wet sedimentation samples between May 2014 and April 2015.The concentrations of 19 trace elements (Na, Mg, Al, K, Ca, V, Cr, Mn, Fe, Cu, Zn, As, Se, Mo, Cd, Sb, Tl, Th, and U) in the samples were analyzed by inductively coupled plasma mass spectrometry (ICP-MS). The results show that the concentrations of metals in bulk deposition samples[7160.68 µg·L-1 (Ca)-0.02 µg·L-1 (Th)] were generally higher than those in wet deposition samples[4237.74 µg·L-1 (Ca)-0.01 µg·L-1 (Th)], but the enrichment factors of each metal in the two kinds of samples were less different. Of note, the enrichment factors of Cu, As, Tl, Zn, Cd, Se, and Sb were all larger than 100, thus indicating that these heavy metals were mainly from anthropogenic sources. The statistical analysis of the air mass trajectory shows that the precipitation chemistry in urban Beijing is mainly affected by southward air flows. The air mass originating from the southwest region always had higher concentrations of Ca, Mg, Fe, Al, Cu, Mo, U, and Th, whereas the air mass from the south had higher concentrations of K, Zn, Mn, Sb, Cd, and Tl. During the observation period, the bulk deposition fluxes of metals varied from 3591.35 mg·(m2·a)-1 (Ca)-0.01 mg·(m2·a)-1 (Th), and wet deposition fluxes varied from 1847.78 mg·(m2·a)-1 (Ca)-0.01 mg·(m2·a)-1 (Th). The dry deposition fluxes of the 19 metals varied from 1743.57 mg·(m2·a)-1 (Ca)-0.01 mg·(m2·a)-1 (Th). The particle size has important implications in the evaluation of the relative importance of dry deposition versus wet deposition during the scavenging of trace elements in air.