Characteristics of ammonia, acid gases, and PM_2.5 for three typical land-use types in the North China Plain

Air pollution is one of the most serious environmental problems in China due to its rapid economic development alongside a very large consumption of fossil fuel, particularly in the North China Plain (NCP). During the period 2011–2014, we integrated active and passive sampling methods to perform continuous measurements of NH3, HNO3, NO2, and PM2.5 at two urban, one suburban, and two rural sites in the NCP. The annual average concentrations of NH3, NO2, and HNO3 across the five sites were in the ranges 8.5–23.0, 22.2–50.5, and 5.5–9.7 μg m−3, respectively, showing no significant spatial differences for NH3 and HNO3 but significantly higher NO2 concentration at the urban sites. At each site, annual average concentrations of NH3 and NO2 showed increasing and decreasing trends, respectively, while there was no obvious trend in annual HNO3 concentrations. Daily PM2.5 concentrations ranged from 11.8 to 621.0 μg m−3 at the urban site, from 19.8 to 692.9 μg m−3 at the suburban site, and from 23.9 to 754.5 μg m−3 at the two rural sites, with more than 70 % of sampling days exceeding 75 μg m−3. Concentrations of water-soluble ions in PM2.5 ranked differently between the non-rural and rural sites. The three dominant ions were NH4 +, NO3 −, and SO4 2− and mainly existed as (NH4)2SO4, NH4HSO4, and NH4NO3, and their concentrations averaged 48.6 ± 44.9, 41.2 ± 40.8, and 49.6 ± 35.9 μg m−3 at the urban, suburban, and rural sites, respectively. Ion balance calculations indicated that PM2.5 was neutral at the non-rural sites but acidic at the rural sites. Seasonal variations of the gases and aerosols exhibited different patterns, depending on source emission strength and meteorological conditions. Our results suggest that a feasible pathway to control PM2.5 pollution in the NCP should target ammonia and acid gases together

Study of the presence of ionic constituents in the gas and particulate phase of the atmosphere and their scavenging mechanisms by wet and dry deposition

The aim of this research was to investigate the presence of acidic and alkaline constituents in the gas and particulate phase of the atmosphere at an urban environment, Thessaloniki (Greece), and their removal mechanisms, in the prevailing weather conditions, by wet and dry deposition. More precisely, the concentrations of 8 ionic constituents (Cl-, NO3 -, SO4 2-, NH4 +, Na+, K+, Ca2+ and Mg2+) were determined in every phase (gases, particles, rainwater and dust). In addition, the concentrations of 8 trace metals (Al, Cd, Cr, Fe, Mn, Ni, Pb and Zn) were also determined in the case of rain and dust samples. The gas phase compounds (HCl, HNO3, SO2 and NH3) were measured (24hrs samples) by an annular denuder system equipped with a back up filter for the collection of particles with diameter # 5 μm (PM5), while the sampling of wet and dry deposits was performed by using a conventional wet/dry collector. The gas phase compounds and the particles were collected for a period of 16 months, the rainwater and the dust deposits were sampled for 27 and 24 months respectively. The distribution of the common constituents’ concentration of the gaseous and particulate phase and their seasonal variation was evaluated. Furthermore, the presence of several volatile or non-volatile compounds (for example: NH4NO3, NH4Cl and (NH4)2SO4/NH4HSO4) in the particulate phase was investigated. This investigation revealed the formation of NH4NO3 under cold conditions (T < 15 ° C), while NH4Cl is not expected to be formed neither in the warm nor in the cold period. On the other hand, both forms of ammonium sulfate ((NH4)2SO4/NH4HSO4) are probably present in the atmosphere, depending on the atmospheric concentration levels of ammonia. Possible emissions sources were evaluated using statistical methods for particles, rain and dust data. ##3 and CaCO3 were found to be the main neutralizing agents of the atmospheric acidity, by calculating the Neutralization Factors (NF) and applying the Multiple Linear Regression Analysis (MLR). The study of the air masses’ origin, by using the HYSPLIT model, in the case of rainwater and particle samples, showed a possible contribution of long range transported pollutants. In order to investigate the atmospheric scavenging mechanisms of ionic constituents and trace elements we determined the wet and dry deposition fluxes (dry deposition represents the sum of gaseous and particle fluxes), their relative contribution to the total atmospheric deposition, as well as the influence of the precipitation amount to the prevailing mechanism. The contribution of gases to the total dry deposition was significant, while the prevailing removal process for the majority of the constituents was found to be wet deposition. Finally, it was feasible to calculate the dry deposition velocities of major particles (NO3 -, SO4 2-, NH4 + and Cl-) by using the dust fluxes. Their seasonal variation was evaluated, while the comparison of these results with literature data (obtained by models or using direct measurement of dry deposition with several surrogate techniques) showed a good agreement with values estimated by models