Cross-validation of satellite and ground-level measurements

Each month we highlight one of our Scoping Studies, each funded through our Collaboration Building Workshops. This month we welcome a blog post from Dr Anna Font (Imperial College London), who is working with STFC colleagues in RAL Space and the Hartree Centre to improve our understanding of ammonia concentrations through satellite and ground-level measurements.

Ammonia (NH3) is mostly emitted to the atmosphere from agricultural and farming activities including the use of fertilizers, manure, cattle and dairy farming, among others. NH3 is an important precursor of fine particles due to its reaction with available acids (i.e. nitric and sulfuric acid) to form ammonium nitrate and ammonium sulphate. Airborne fine particles are a significant human health thread and are associated with cardiovascular and respiratory diseases. Further, gaseous ammonia and ammonium compounds are deposited into the ecosystems damaging sensitive habitats. In the UK, agricultural activities represent more than 80% of the atmospheric ammonium emissions and there are no regulations in place to limit these emissions. The Clean Air Strategy published in 2018 aims to reduce NH3 emissions in the UK making available a code of good agricultural practice (COGAP).

Hotspots of ammonia have been identified by means of satellite measurements from both CrIS and the IASI instruments onboard the Suomi NPP and MetOp satellites, respectively. In Europe these include areas in the UK and neighbouring countries such as France, Belgium and The Netherlands. NH3 emissions from agricultural fields in north-west Europe have been associated with particle episodes which accumulate on a regional scale especially during springtime in south-east England. In the UK, hotspots of NH3 are observed in the intense agricultural regions in southern England. 

In the UK, a network of 85 sites distributed in the country have been measuring ammonia at the surface level since mid-1990s. Despite the network provides a good coverage of the UK land, there is a large heterogeneity of concentrations related to the large variation in emission sources at ground level as it can be seen in Figure 1. Concentrations are reported as monthly means. Recently, on-line concentrations of NH3 are available at two rural locations in the country comprising one agricultural site in Scotland and one in south-east England; and also in urban areas comprising London, Manchester and Birmingham. On-line techniques report high time resolve data of NH3 concentrations, usually at hourly basis.

Figure 1. Monthly concentrations of NH3 gas as measured by the Defra UKEAP National Ammonia Monitoring Network in 2019. Only active samplers (delta) are shown in this graph. Data from UK-air website.

Satellite observations are column-integrated and have larger footprints compared to the ground-level measurements. Little is known if satellite retrievals of NH3 concentrations are representative of ground-level measurements in the UK and how they are related. Despite the mistime and misdistance errors which are introduced by comparing measurements of a very reactive species such as NH3 that are not perfectly collocated in time and space, such comparison of column-integrated and ground-level measurements is still important to validate satellite products.

The SAQN workshop on last autumn 2020 was the perfect platform to establish collaboration links between scientists working with satellite data, in-situ observations and big data specialists. The project is establishing links between STFC RAL space, STFC Hartree Centre and Imperial College London. With this project we are aiming to evaluate the ability of satellite observations of NH3 to reproduce temporal variability of surface air concentrations across the UK to before estimate long-term changes in air pollution to assess the efficacy of air quality policies in recent years. The integration of satellite data and ground-level measurements will help us identify hot-spots of NH3 in the UK and monitor possible changes over time.