Development and application of low-cost monitoring approaches for atmospheric ammonia, acid gases and ammonium aerosols
Tang, Yuk Sim
Ammonia (NH3) is the major alkaline gas in the atmosphere, with around 90 % of the total anthropogenic emissions in Europe coming from agriculture-related sources. Following emission to the atmosphere, the neutralisation reaction between NH3 and the acid gases sulfur dioxide (SO2), nitric acid (HNO3) and hydrochloric acid (HCl) produces secondary inorganic aerosols (ammonium nitrate (NH4NO3), ammonium sulfate ((NH4)2SO4) and ammonium chloride (NH4Cl)). With longer atmospheric lifetimes than the gases, the aerosols also contribute to transboundary pollution problems. The gases and aerosols are removed from the atmosphere by wet (in precipitation) or dry (direct uptake by vegetation and surfaces) deposition processes. Together, they can negatively impact the natural environment through the input of excess acidity and nutrient nitrogen and harm human health through the formation of aerosols that contributes to fine-mode particulate matter (PM2.5). They can also potentially influence climate change from the radiative forcing properties of the aerosols and the inputs of nitrogen that can alter the carbon cycle.Monitoring data are necessary for assessing the spatial and temporal extent of pollution and as evidence to detect changes in pollutant concentrations in response to current and future policies to mitigate emissions of NOx, SO2 and NH3. Combined with models, the concentration data are also used to estimate the different fractions of the total sulfur or nitrogen input and different chemical forms of the pollutants. Since the spatial and temporal patterns and atmospheric behaviours of gases and aerosols differ, measurements therefore need to distinguish between the phases. The development of simple, low-cost, time-integrated air sampling methods and their application in cost-efficient monitoring strategies to assess temporal, spatial and trends in the gas and aerosol pollutants in the UK and across Europe is described. An active diffusion denuder method (DELTA®) and a passive sampler (ALPHA®) are implemented at a large number of sites (> 70) in the UK National Ammonia Monitoring Network (NAMN, established 1996) to measure NH3 with a monthly frequency. An extension of the DELTA® method provided additional, monthly measurements of particulate NH4+ (for the NAMN) and of the acid gases (SO2, HNO3, HCl) and aerosol species (NO3 , SO42-, Cl , Na+, Ca2+, Mg2+) for the UK Acid Gas and Aerosol network (AGANet, established 1999) at a subset of NAMN sites. The close integration of the two networks demonstrated the cost-effectiveness of the DELTA® approach, which provided quality assured, concurrent speciated measurement data on multiple pollutants at multiple sites, and also simplicity of operation by a large network of site operators, some of whom have no technical or scientific background. The DELTA® approach and quality protocol developed in the UK networks was further applied to a pan-European NitroEurope (NEU) DELTA® network (20 countries: 2006 – 2010), with knowledge sharing and collaboration between multiple laboratories and research organisations. Important features in the spatial variability and seasonality in the gas and aerosol components were captured in the UK and European networks. The gases, with shorter lifetimes in the atmosphere were found to be spatially more heterogeneous, with a wider range of concentrations than their aerosol counterparts. Variations on a spatial scale were correlated with distributions and magnitude of emission sources, e.g. NH3 and NH4+ concentrations were highest in intensively farmed areas (e.g. East Anglia in eastern England, NAMN) and countries (e.g. the Netherlands, NEU DELTA®). In the UK, evidence is also presented of the contribution by long-range transboundary sources to enhancement of concentrations of NH4NO3 and (NH4)2SO4.Distinct and contrasting seasonal cycles in the gas and aerosol phase components were established, important for identifying periods of pollution and for targeting abatement measures. The observed variations were attributed to seasonal changes in emission sources, atmospheric interactions and the influence of climate on partitioning between the gases and aerosols. For NH3, peaks in concentrations occur from increased volatilisation promoted by warm, dry conditions (summer) and also from agriculture-related emissions, with a main peak in spring and a smaller peak in autumn. Concentrations of SO2 were higher in winter (increased combustion), except in Southern Europe where the peak period was in summer. HNO3 concentrations were more complex, with small peaks in the seasonal cycle related to traffic and industrial emissions, photochemistry, meteorology and the influence of climate on HNO3:NH4NO3 equilibrium. In comparison, the springtime peak in NH4NO3 was attributed to the reaction of a surplus of NH3 with HNO3 to form NH4NO3 in the aerosol phase under cooler, wetter conditions. A summertime peak in particulate SO42- was observed in Southern Europe, coinciding also with peaks in SO2, NH3 and HNO3 concentrations. While the high HNO3 concentrations suggests increased oxidative capacity for formation of H2SO4 (from SO2) and reaction with NH3 to form (NH4)2SO4, the absence of an NH4+ peak illustrates the larger influence of the more abundant NH4NO3 in controlling the seasonality of particulate NH4+.Important changes in the atmospheric concentrations and partitioning between the different gas and aerosol components were captured. The measurement data highlighted the dominance of NH3 and NH4NO3 in rural air, as the emissions of SO2 and NOx continues to fall, against a backdrop of increasing NH3 emissions in the UK and across Europe since 2013. The observed shift in the form of NH4+ aerosol from the stable (NH4)2SO4 to the semi-volatile NH4NO3 is expected to maintain a larger fraction of the NH3 and HNO3 in the gas phase. NH4NO3 can act as a reservoir and release the gases in warm weather, which may partly explain the observed non-linearity between emissions and measured concentrations of NH3 in the UK data. The current and projected trends in the emissions of the gases SO2, NOx and NH3 suggest that NH3 and NH4NO3 can be expected to continue to dominate the inorganic pollution load over the next decades.