INICIAR EL PROCESO: LOS TALLERES DE PROSPECTIVA

of ammonia and other SIA precursors? What impact would a reduction in UK and non- UK ammonia emissions have on PM concentrations?

Ammonia is clearly the precursor to particulate ammonium (and is not present to any significant degree as undissociated dissolved ammonia). NH3 is the most abundant alkaline

gas in the atmosphere (A2.2.3), and because of its dominant role in neutralising acids, is the most abundant secondary cation in UK atmospheric aerosol (H+ being dominant in high acidity atmospheres, such as parts of the US)3. It is efficiently taken up into any acidic aqueous particles and NH3 uptake serves to regulate the acidity of aerosol and cloud

droplets (which, in turn, influences the solubility and oxidation rate of species such as SO2).

3

Under heavily marine-influenced conditions, Na+ and K+ may dominate the coarse mode, but NH3 will still dominate the accumulation mode (and largely PM2.5)

In NAME it has been found that in-cloud SO42-production is very dependent on NH3 control

of droplet acidity, which is strongly dependent on the chemical scheme. A 30% increase or decrease in ammonia could lead to change in particulate sulphate of over 1 µgm-3. Where it occurs in the atmosphere, NH3 may also play a role in the ternary nucleation of sulphate

particles. As with ammonium sulphate ((NH4)2SO4), the equilibrium thermodynamics of

ammonium nitrate (NH4NO3) under dry and moist conditions are extremely well established.

The existence of NH4NO3 in aerosol particles is strongly favoured at low temperature and

high relative humidity, and can therefore be influenced by normal daily cycles in humidity and temperature, and loading will vary strongly with altitude (as discussed in Annex B) Whilst at a fundamental level the processes by which NH3 impact SIA are well known, the

strong dependencies of NH4NO3 equilibration and vertical temperature and RH gradients

lead to high sensitivity to accurate representation of the boundary layer height and its physical properties. The non-linear aerosol nitrate response to reductions in SO2 and NOx

occurs through changes in gas phase concentrations and thermodynamic changes favourable production of NH4NO3. Reductions in NH3 can induce large sensitivities for

predicted particulate sulphate resulting from the effect of NH3 on balancing acidity of in-cloud

production when accounted for in models. This has important implications for the representation of cloudiness and the coarseness of model grid resolution. Indeed, there is a growing body of evidence that domain size and grid resolution are important to resolving SIA processes. Along with the non-linearities in equilibration, the dynamics of aerosol microphysical processes will be important in determining whether a grid resolution is sufficiently fine. In addition, NH3 emissions possess a large spatial variability, and it

influences SIA formation through direct reaction on a short timescale (A1.2.2.3). Emission grid resolution should be sufficiently high to resolve the spatial variability. The MADE/SORGAM aerosol model coupled to RADM2 in WRF-Chem was found to underestimate sulphate by a factor of 2, and overestimate nitrate and ammonium by a factor of 2 compared with the EMEP surface stations in February 2007. The discrepancies were attributed to missing aqueous sulphate production and the thermodynamic treatment in the model.

UK PTM studies have found varying responses to projected emissions reductions, finding reduction of SO2, NOx and NH3 of 64%, 75% and 96% can lead to a reduction of 2 gm−3 of

SIA. A second UK PTM perturbation study attributed a larger sensitivity of PM2.5 NO3 to 30%

EU emissions reductions in NOx and NH3 than to UK reductions of the same magnitude.

European studies have found greater sensitivity to EU NOx emissions, though it is unclear

whether this is a regional difference in sensitivity. WRF-CMAQ studies have found less of a sensitivity to NH3 emissions, but it is unclear whether this is a result of primary sulphate

emissions (without any sensitivity to NH3) being used as reported in US studies. It should be

noted that NH3 is very heterogeneous in its emissions and is taken up directly and rapidly

without a need for atmospheric reaction. As such, coarse representation of emissions can directly translate to inaccuracy in SIA production unlike emissions of SO2 and NOx.

It is unclear how accurately some models can represent aqueous phase processing and impact of oxidant concentration perturbations in future scenarios. Uncertainties in the spatial variation of the emissions are not quantified and reported. However, the NAEI “data quality confidence ratings” for each pollutant map in order of decreasing mapping quality for the major pollutants is reported as: SO2 > NOx > NMVOCs > PM10 > NH3 (A1.1.2.3; A1.2.2.3).

There are few evaluations of the relative SIA precursor contributions using reliable models and none that comprehensively map the possible parameter space.

A treatment of its emissions at high spatial resolution (i.e. much finer than 50 km x 50 km) is therefore ideally required for its role in SIA formation to be fully represented. An NH3 emission perturbation sensitivity study is required, using different model

treatments) of SIA evaluated against UK measurements, where possible. This should be rolled into the multi-pollutant analysis recommended above. NH3 is only one of

many contributory species in a non-linear interactive system. The study should reflect the current uncertainty in emission spatial distribution as well as future uncertainties in emissions and environmental conditions (see section A for a discussion of the emission uncertainties).