Field investigations into the potential influence of ammonia and acidic gases on concentrations and surface - Atmosphere exchange of aerosols

INTRODUCTIONThere is ample evidence in literature that aerosol is not only deposited but seems to be emitted under certain conditions (e.g. Gallagher et al., 1997). Processes that could potentially lead to production of new particles or the growth of existing particles include the interaction of ammonium aerosol with ammonia and acidic gases. As a result of the differences in surface / atmosphere exchange rates and mechanisms of ammonia, acidic gases and aerosols the surface flux of ammonia can disturb the equilibria of NH 4 NO 3 and NH 4 Cl leading to aerosol evaporation or condensation.Investigations into the growth and production of aerosols in situ in the atmosphere (gas-to-particle conversion, gtpc) are important in many respects. The partitioning of total NH x between the aerosol and the gas phase influences the rate of the surface / atmosphere exchange of NH x (e.g., if gtpc provides an efficient sink of gaseous NH 3 , this could enhance emission) and the range of transport (as a secondary and slowly deposition species NH 4 + aerosol is transported over longer distances than NH 3 ). Aerosols play an important role in global and climatic processes, both directly through radiative forcing and indirectly by acting as cloud condensation nuclei, and it is therefore of importance to investigate and quantify the processes of their formation and destruction. Except for coagulation, the growth mechanisms of aerosols and the production and here gtpc of ammonia could provide one of the key mechanisms.Theoretical studies have been performed to model the concentration profiles of NH 3 , HNO 3 , HCl, NH 4 NO 3 and NH 4 Cl as modified by their interaction. Quantitative predictions, however, are extremely difficult because the equilibrium products and chemical time-scales involved are largely unknown and highly dependent on the composition and size distribution of the aerosol. Moreover, there are very few data sets of simultaneous surface / atmosphere fluxes of aerosols and gaseous species involved.MEASUREMENTSA joint field campaign was performed in May / June 1996 near Elspeet, NL, over heathland and focused on the simultaneous measurements of all parameters necessary to assess the mechanisms and quantify effects of ammonia / aerosol interactions. During the 4 week campaign the usual micrometeorological parameters were measured (friction velocity, sensible and latent and soil heat fluxes, net radiation, PAR, temperature, relative humidity, wind direction). Wet denuders (ECN, NL) were used at 2 - 3 heights to separate gases and aerosol species. The gases were collected in a suitable stripping solution on the denuder walls, whereas the aerosols passing through the denuders were collected using a steam jet aerosol collector (SJAC). Both collection solutions (containing gases and aerosols) were analyzed online for NH 4 + using a membrane-diffusion-conductivity system and for SO 4 2 - , Cl - and MN 3 - using online anion chromatography (IC). This way continuous measurements of the gradients of particulate ammonium, nitrate, chloride and sulphate were obtained together with fluxes of NH 3 , HCl, HNO 3 and SO 2 . Backup was provided by gradient measurements using batch techniques such as automated rotating wet denuders and filter packs.Particle number concentrations were obtained with an aerodynamic particle sizer (D p = 0.5 - 15 μm) and scanning mobility particle sizer (SMPS, 0.1 - 0.8 μm) the latter of which was used for short periods to measure gradients. Aerosol number fluxes (0.1 - 3 μm) were measured by eddy correlation technique using a fast response optical particle counter (PMS, ASASP-x), but channels above 0.5 μm had to be discarded due to limited counting statistics. Some 12-hr runs with a 7 stage cascade impactor (Berner) provided the chemical composition of the different particle sizes.FIRST RESULTSThe results provide a unique dataset for intercomparison studies of different (partially novel) techniques of measuring the deposition rate of aerosol as well as to study the origin of and composition of particles at this fairly polluted Dutch site. In particular, it gives the opportunity to study the interactions of aerosols with NH 3 and acidic gases: daily variations in the aerosol sizes, formation of ultra-fine aerosol, the attainment of phase equilibria and the influence of gtpc on NH 3 flux measurements.In contrast to previous measurements which had shown considerable NH 3 emission at this heathland site NH 3 was found to be mainly deposited with very short spells of morning emission. Nitric acid and hydrochloric acid were continuously deposited. And the concentrations of both acidic gases and aerosols were fairly variable, indicating the presence of near by sources. NH 4 + and SO 4 2 - aerosols were both found in a very similar size range (average mass median diameter, MMD = 0.25 μm), whereas NO 3 - aerosol was considerably larger (0.60 μm). Ions of probably maritime origin (Na + , Ca 2 + ) showed a MMD of 1.2 - 1.5 μm and chloride one of 0.83 μm. The eddy correlation flux measurements show a strong diurnal variation of the deposition velocity (V d ) with typical values of 2 - 5 mm s - 1 during day- and below 0.2 mm s - 1 at stable and calm night-time conditions. Over the size range in which the number concentration was high enough for data analysis (0.1 - 0.5 μm) the deposition velocity increases monotonously, which is in agreement with other measurements (Gallagher et al., 1997) but in contrast to model results (Slinn, 1982).The analysis of the data with respect to gas-particle interactions is currently ongoing and the results will be presented at the conference.