Rothamsted Research

where knowledge grows

Atmospheric Chemistry

Atmospheric pollutants, whether anthropogenic or natural, may have detrimental effects on ecosystems including microorganisms, plants, animals and humans.  The main sources for these pollutants are mainly anthropogenic from intensive livestock production (ammonia, NH3), energy and heating production (sulphur dioxide, SO2; nitrogen oxides, NOx) and motor vehicles (nitrogen oxides, NOx).

When these primary pollutants react with the atmosphere they produce secondary pollutants as acids (sulphuric acid, H2SO4; nitric acid, HNO3; and hydrochloric acid, HCl respectively) and as particulates and aerosols (sulphate, SO4; Nitrate, NO3; and ammonium, NH4).  These primary and secondary pollutants are deposited onto the Earth’s surface through wet deposition (precipitation as rain, snow and fog) and dry deposition (direct turbulent deposition over a surface).

The increased burning of fossil fuels (e.g. coal, oil and natural gas) since the beginning of the industrial revolution in the mid eighteenth century, has enhanced emissions of these pollutants in the United Kingdom and parts of NW Europe. In recent times Government legislation implemented since the mid-1950s (Clean Air Act 1956, 1968, repealed 1993; Pollution Prevention and Control Act, 1999) has seen a decrease of industrial and domestic emissions of these pollutants into the atmosphere (RoTAP, 2012).

  Wet Deposition (Precipitation chemistry)

Precipitation (rain, snow, fog etc.) is naturally acidic, with a pH of between 5.0 and 5.6. This is due largely to carbon dioxide in the atmosphere reacting with water to produce weak carbonic acid. Acid deposition can be increased naturally through volcanic eruptions emitting SO2, NOx, Cl and VOCs (volatile organic compounds). As already mentioned these pollutants have been further enhanced by the burning of fossil fuels and react in the atmosphere to produce H2SO4, NO3 and HCl respectively. This results in an increase in the acidity of rain water.

Figure 1. Annual volume weighted mean pH of rainfall at Rothamsted (1992-2012)

Precipitation samples are collected at all ECN sites on a weekly basis and are analysed for conductivity, pH and major ions. All sites have seen the pH of rain water increase, except for Hillsborough, Northern Ireland, and the level of major ions associated with acidity decrease (Morecroft et al, 2009). At Rothamsted the pH of precipitation (Figure 1) has increased from 4.5 to 5.2 between 1992 and 2012; a ten-fold decrease in acidity in twenty years.

Figure 2. Annual volume weighted mean ion concentrations in precipitation collected at Rothamsted

The precipitation chemistry (wet deposition) results (Figure 2) show a decreasing trend in SO4-S, NO3-N, NH4-N and Cl concentrations at Rothamsted over the last twenty years. Despite the regular inputs of N and S in the fertilisers applied to arable crops grown at this site this would indicate that the contributions of these fertilisers to the local atmosphere are relatively small. Concentrations of sulphur (S), nitrate (NO3) and ammonium (NH4), within wet deposition, across the UK have steadily declined. Over a twenty year period, 1986 to 2008, there has been a reduction of approximately 70%, 24% and 35% respectively in S, NO3 and NH4  (RoTAP,2012).

  Dry Deposition of Nitrogen (Atmospheric chemistry)

The main sources of atmospheric nitrogen are ammonia (NH3), from intensive livestock farming and recently motor vehicles with the introduction of catalytic converters (although this has been reduced with the introduction of 3-way catalytic converters), and nitrogen oxides (NOx) comprising nitric oxide (NO) and nitrogen dioxide (NO2) from fossil fuel combustion (e.g. power stations, motor vehicles and domestic heating). In 1970 the UK was emitting approximately 2.6 million tonnes NOx peaking at approximately 2.9 million tonnes in 1989 (Figure 3). This upsurge in emissions was due largely to an increase in road transport. Since 1970 the total emission of NOx for the UK has decreased by 46% with emissions from power stations decreasing by 49% and that from road transport by 61% (Dore et al, 2008; DEFRA, Statistical release 2012). The large decrease in that emitted from road transport (Figure 1) is due to improvements in fuel quality and the introduction of catalytic converters to petrol vehicles in 1993.

Figure 3. UK emissions of NOx as NO2 and that emitted by road transport,domestic heating and power stations
(© Crown 2014 copyright Defra & DECC via naei.defra.gov.uk, licenced under the Open Government Licence (OGL))

Rothamsted is located in a semi-urban area with the resultant large use of domestic heating and motor vehicles. The fuel used (coal, oil, natural gas, petrol and diesel) for these purposes, almost certainly contributes significantly to the local NOx emissions. At Rothamsted we measure concentrations of NOx as NO2 (which includes NO). The Rothamsted data show seasonal peaks and troughs, year on year, as the demand for fuels increase during the autumn/winter months and decline during spring/summer months. The annual means show that, even though there is seasonal variation, NO2 concentrations at Rothamsted have fallen by 50% (Figure 4) from a maximum of 11.64 µg N/m3 (4.4 kg.N.ha-1) in 1996 to 5.87 µg N/m3 (2.2 kg.N.ha-1) in 2012 and that there is a clear downward trend in NO2 levels. This decrease is in line with national statistics (Defra National Statistical release 2012), with UK NOx emissions having fallen by 53% (1996 to 2011) and which are predicted to continue to fall.

Figure 4. Monthly and Annual concentrations of NO2-N measured on the Rothamsted Met Station

NO2 concentrations measured near to the B487 (Redbourn Lane), a secondary but an important arterial road in and out of Harpenden, show that levels are much higher and more variable (Figure 5). The variability is most likely associated with changes in traffic volume due to weather and school holidays. Also, at peak traffic times, levels of NO2 will increase as traffic becomes stationary or moves relatively slowly passed the samplers. Mean annual concentrations show that, even near the road, levels have fallen by 26% from 13.92 µg N/m3 in 1995 to 10.26 µg N/m3 in 2012.

Figure 5. Monthly and Annual concentrations of NO2-N measured beside the B487 (Redbourn Lane), Harpenden, Herts

National Capability

A national capability is a BBSRC-funded resource intended to benefit the scientific community in general. These can be facilities as well as opensource datasets. 

National Capabilities

Rothamsted Insect Survey

Operates two national networks for monitoring insect populations in the UK

North Wyke Farm Platform

Provides the research community access to a range of in situ state-of-the-art instrumentation in hydrologically isolated fields and farms to better address key issues in sustainable agriculture.

PHI-base

A database of interactions between pathogens and their hosts maintained at Rothamsted Research with international input.

Long-Term Experiments

These have been running since the mid 19th Century, provide a unique experimental system and archive of soil and plant samples.