Gas-aerosol Equilibrium Research Articles

Speciation of the major inorganic salts in atmospheric aerosols of Beijing, China: Measurements and comparison with model

In the winter and summer of 2013–2014, we used a sampling system, which consists of annular denuder, back-up filter and thermal desorption set-up, to measure the speciation of major inorganic salts in aerosols and the associated trace gases in Beijing. This sampling system can separate volatile ammonium salts (NH4NO3 and NH4Cl) from non-volatile ammonium salts ((NH4)2SO4), as well as the non-volatile nitrate and chloride. The measurement data was used as input of a thermodynamic equilibrium model (ISORROPIA II) to investigate the gas–aerosol equilibrium characteristics. Results show that (NH4)2SO4, NH4NO3 and NH4Cl were the major inorganic salts in aerosols and mainly existed in the fine particles. The sulfate, nitrate and chloride associated with crustal ions were also important in Beijing where mineral dust concentrations were high. About 19% of sulfate in winter and 11% of sulfate in summer were associated with crustal ions and originated from heterogeneous reactions or direct emissions. The non-volatile nitrate contributed about 33% and 15% of nitrate in winter and summer, respectively. Theoretical thermodynamic equilibrium calculations for NH4NO3 and NH4Cl suggest that the gaseous precursors were sufficient to form stable volatile ammonium salts in winter, whereas the internal mixing with sulfate and crustal species were important for the formation of volatile ammonium salts in summer. The results of the thermodynamic equilibrium model reasonably agreed with the measurements of aerosols and gases, but large discrepancy existed in predicting the speciation of inorganic ammonium salts. This indicates that the assumption on crustal species in the model was important for obtaining better understanding on gas–aerosol partitioning and improving the model prediction.

Time-resolved inorganic chemical composition of fine aerosol and associated precursor gases over an urban environment in western India: Gas-aerosol equilibrium characteristics

Inorganic ionic constituents (Na+, NH4+, K+, Mg2+, Ca2+, Cl−, NO3− and SO42−) of PM2.5 and associated trace gases (NH3, HNO3 and HCl) were measured simultaneously by Ambient Ion Monitor – Ion Chromatograph (AIM-IC) system with a time resolution of one hour at an urban location in semi-arid region of western India during summer and winter. The average NH3, HNO3 and HCl concentrations were 11.6 ± 5.0, 2.9 ± 0.8 and 0.15 μg m−3, respectively, during winter. During summer, NH3 and HNO3 concentrations were of similar magnitude, whereas HCl concentration was less than ∼0.03 μg m−3. NH3 concentration exhibited a distinct diurnal variation during both seasons. However, HNO3 did not show a specific diurnal trend during the observation period in both seasons. The data obtained were used to study gas-aerosol equilibrium characteristics using a thermodynamic equilibrium model, ISORROPIA II. The results suggest that NH3 exists in equilibrium between measured fine-mode particle and gas phase with a systematic bias of ∼14%, whereas HCl and HNO3 deviate significantly from the modelled data. These observations have implications on thermodynamic equilibrium assumptions used for estimating various aerosol parameters such as liquid water content, pH, etc., thus causing significant bias in chemical transport model results over the study region.

Diffusion-Limited Versus Quasi-Equilibrium Aerosol Growth

Condensation of gas-phase material onto particulate matter is the predominant route by which atmospheric aerosols evolve. The traditional approach to representing formation of secondary organic aerosols (SOAs) is to assume instantaneous partitioning equilibrium of semivolatile organic compounds between gas and particle phases. Growth occurs as the vapor concentration of the species increases owing to gas-phase chemistry. The fundamental mathematical basis of such a condensation growth mechanism (quasi-equilibrium growth) has been lacking. Analytical solutions for the evolution of an organic aerosol size distribution undergoing quasi-equilibrium growth and irreversible diffusion-limited growth are obtained for open and closed systems. The quasi-equilibrium growth emerges as a limiting case for semivolatile species condensation when the rate of change of the ambient vapor concentration is slow compared with the rate of establishment of local gas-aerosol equilibrium. The results suggest that the growth mechanism in a particular situation might be inferred from the characteristics of the evolving size distribution. In certain conditions, a bimodal size distribution can occur during the condensation of a single species on an initially unimodal distribution. Copyright 2012 American Association for Aerosol Research

Modelling the partitioning of ammonium nitrate in the convective boundary layer

Abstract. An explanatory model study is presented on semi-volatile secondary inorganic aerosols on three clear days in May 2008 during the IMPACT campaign at the Cabauw tower in the Netherlands. A single column model in combination with the equilibrium aerosol model ISORROPIA is used. This model uses surface observations from IMPACT and calculates the gas-aerosol partitioning of ammonium nitrate. The calculated gas-aerosol equilibrium overestimates the gas phase fraction during daytime, and overestimates the aerosol phase fraction during night-time. This discrepancy can partly be solved when the approach of the gas-aerosol equilibrium is forced to proceed with a delay timescale of up to two hours. Although it is shown that the delay itself has a small effect, the most important effect is caused by the mixing of air from higher altitudes at which the equilibrium is shifted to the aerosol phase. Thus, vertical mixing is shown to have a significant influence on the calculated partitioning at the surface. On some occasions, the correspondence to the observed partitioning improves dramatically. Even though gas-aerosol partitioning of ammonium nitrate is not instantaneous, observations show that a different equilibrium in the upper boundary layer causes aerosol ammonium nitrate concentrations to increase with altitude. Our model calculates similar vertical gradients depending on the assumed speed of gas-aerosol equilibrium. The calculated optical properties of the aerosol show a similar behaviour. The aerosol optical properties depend on the aerosol size distribution both directly, because light scattering depends on particle size, and indirectly, because the equilibration timescale depends on the aerosol sizes. Future studies should therefore focus on a fully size-resolved treatment of the gas-aerosol partitioning. Finally, coarser-resolution models may treat the gas-aerosol equilibrium of ammonium nitrate by calculating the equilibrium with a temperature and humidity sampled at a different altitude. We found that the equilibrium at an altitude of 200 m (night) up to 600 m (day) is representative for the partitioning of ammonium nitrate at the surface in the beginning of May 2008.

Size dependence of in situ pH in submicron atmospheric particles in Hong Kong

Some studies have reported that small submicron atmospheric particles are more acidic than large submicron particles; other studies demonstrated a reversed trend. In this study, the size dependence of in situ pH in submicron particles in Hong Kong was investigated. The equivalent ratios of [NH 4 +] measured to [SO 4 2−] measured in submicron particles were found to be generally less than unity and size dependent, suggesting the possibility of incomplete gas–aerosol equilibrium. The Aerosol Inorganic Model-II (AIM-II) model using measured ionic compositions with the gas–aerosol partitioning disabled was used to estimate the in situ pH in different sized particles. The estimated in situ pH of different sized submicron atmospheric particles was between −2.5 and 1.5 and it generally decreases with increasing submicron particle size. At such low in situ pH, the estimated HSO 4 − equivalent concentrations were 3.2 times (on average) of those of H + in different sized particles. The trends of the size dependence of the [NH 4 +] measured to [SO 4 2−] measured ratio and pH under different regimes of relative humidity are discussed.

Importance of mineral cations and organics in gas-aerosol partitioning of reactive nitrogen compounds: case study based on MINOS results

Abstract. The partitioning of reactive nitrogen compounds between the gas and the aerosol phase, as observed during the MINOS (Mediterranean INtensive Oxidant Study) campaign in Crete, Greece, in July and August 2001, has been studied with three thermodynamic gas-aerosol equilibrium models (EQMs) of different chemical complexity: ISORROPIA, which is limited to the ammonium-sulfate-nitrate-sodium-chloride-water-system; SCAPE2, which also includes mineral elements (calcium, magnesium and potassium); and EQSAM2, which additionally accounts for organic acids. The different EQMs are constrained by measured gas (g) and aerosol (a) concentrations: Total ammonia (NH3(g) and NH4(a)+), total nitrate (HNO3(g) and NO3(a)-), total sulfate (H2SO4(g) and SO4(a)2-), total chloride (HCl(g) and Cl-(a)), sodium (Na+(a)), calcium (Ca2+(a)), magnesium (Mg2+(a)), potassium (K+(a)) and organic acids (a). Although the three EQMs differ considerably in particular aspects, their application at the same level of complexity yields comparable results for the equilibrium composition and phase partitioning of ammonia and nitric acid, i.e. within the range of measurement uncertainties (~10%). Their application at different levels of complexity, however, gives rise to substantial differences for the gas-aerosol partitioning of reactive nitrogen compounds. Our results show that only if (soluble) mineral components and (lumped) organic acids are accounted for, the observed gas-aerosol partitioning of ammonia and nitric acid can be accurately reproduced for air pollution episodes characterized by a complex chemical mixture, typical for the Mediterranean lower atmosphere.

A new inorganic atmospheric aerosol phase equilibrium model (UHAERO)

Abstract. A variety of thermodynamic models have been developed to predict inorganic gas-aerosol equilibrium. To achieve computational efficiency a number of the models rely on a priori specification of the phases present in certain relative humidity regimes. Presented here is a new computational model, named UHAERO, that is both efficient and rigorously computes phase behavior without any a priori specification. The computational implementation is based on minimization of the Gibbs free energy using a primal-dual method, coupled to a Newton iteration. The mathematical details of the solution are given elsewhere. The model computes deliquescence behavior without any a priori specification of the relative humidities of deliquescence. Also included in the model is a formulation based on classical theory of nucleation kinetics that predicts crystallization behavior. Detailed phase diagrams of the sulfate/nitrate/ammonium/water system are presented as a function of relative humidity at 298.15 K over the complete space of composition.

Comparison of thermodynamic predictions for in situ pH in PM 2.5

In situ aerosol pH is important in the study of atmospheric chemistry because many heterogeneous atmospheric chemical processes are pH dependent, but it is not easy to measure the in situ pH of PM 2.5 due to the low liquid water content of aerosol particles. In situ aerosol pH is usually estimated by thermodynamic modeling. This study compared the in situ pH of PM 2.5 in Hong Kong estimated by different thermodynamic methods such as AIM-II (Aerosol Inorganics Model-II) with the gas–aerosol partitioning calculation disabled, HCl/Cl −, HNO 3/NO 3 − and NH 3/NH 4 + gas–aerosol equilibrium equations, and two thermodynamic gas–aerosol equilibrium models (ISORROPIA and SCAPE2). In the first method, actual measured particulate composition was used as input while only the total (gas+aerosol) composition was used in the other methods. AIM-II with the gas–aerosol partitioning calculation disabled gives the lowest in situ aerosol pH, 3.5, 2.8, 3.1, 2.7 and 4.3 unit (on average) less than that estimated by HCl/Cl −, HNO 3/NO 3 − and NH 3/NH 4 + gas–aerosol equilibrium equations, and by SCAPE2 and ISORROPIA, respectively. The differences in estimated in situ pH are discussed in terms of the extent of the gas–aerosol equilibrium achieved, especially the NH 3/NH 4 + equilibrium. Although the estimated equilibrium concentrations deviated from the measured NH 4 + in PM 2.5 by only 20–35%, the estimated in situ pH is very sensitive to the deviation. It is possible that approaches based on the assumption of the gas–aerosol equilibrium may not be suitable for estimating in situ aerosol pH in the atmosphere.

Heterogeneous processes involving nitrogenous compounds and Saharan dust inferred from measurements and model calculations

Experimental data on aerosol chemical composition and gaseous concentrations in various African ecosystems have been obtained under the IGAC DEBITS AFRICA (IDAF) program. In this paper, data covering a complete wet and dry season (1996 and 1998) in the semiarid savanna of the Sahelian region of Niger are presented. The analysis of the aerosol chemical composition and the gas phase concentrations at the Banizoumbou station indicates two strong signatures: a nitrogenous component composed of nitric acid, ammonia, particulate ammonium, and nitrates; and a terrigenous component originating from semiarid and desert soils (calcium, carbonates, magnesium, potassium, sulfate). To further investigate the interactions between gas and particles and to help interpret the IDAF experimental data, these data are analyzed using a gas aerosol equilibrium model (Simulating Composition of Atmospheric Particles at Equilibrium (SCAPE)). The model is found to accurately represent the mean aerosol composition for the dry and the wet season of the studied region. It is found that heterogeneous processes involving terrigenous compounds are important and play a major role in partitioning semivolatile species, such as nitric acid, between the gas and aerosol phases. The important role of these heterogeneous processes in the atmospheric chemistry in the Sahelian region is discussed. To compare results obtained in the semiarid savanna of Niger and other African ecosystems, SCAPE model is also applied to humid savanna and forest using IDAF and Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO) measurements.

Behaviors of volatile inorganic components in urban aerosols

A multicomponent gas–aerosol equilibrium model (Kim et al., 1993a,b; Kim and Seinfeld, 1995) was used to explain the behaviors of water content and other volatile species in the aerosols observed in polluted air mass in Central Japan. It was found that gas–aerosol equilibrium was attained after long-range transport of polluted air mass (e.g., 50 km) from emission source area, while it was not completed in large emission source areas. The present model predicted with high accuracy the gas–aerosol equilibrium of ammonium, nitrate and chloride at remote sites. The correlation coefficient was R=0.98 for ammonia and more than R=0.86 for gaseous nitric acid. It was R=0.94 for gaseous hydrochloric acid, which meant significant chlorine deficit under high-temperature and low humidity conditions was also predicted accurately. The predicted water content was consistent with that calculated by the semi-theoretical Winkler's formula (Aerosol Sceince, 13, 1973, 373–387). At RH=90% the water content attained almost the same weight as that of dry aerosol, while at about RH=60% it was less than 10%. In contrast, temperature dependency of the water content was weak except for very high air temperature conditions in summer. Finally, it emphasized the superiority of the multicomponent approach for gas–aerosol equilibrium, compared with the binary-component approach.

Low molecular weight carboxylic acids in an urban atmosphere: Winter measurements in São Paulo City, Brazil

Atmospheric gas and particle-phase carboxylic acids were measured during July 1996, Winter, in an urban area of São Paulo, a highly polluted Latin American city. Ion chromatography and capillary electrophoresis techniques were used to determine the species. As oxalic (36.2±21.4%), pyruvic (15.0±7.9%), β-hydroxy-butyric (9.15±9.00%) and glycolic (3.55±2.26%) acids were determined in aerosol particles, formic and acetic acids were determined both in the gaseous (4.36±2.70 and 3.66±2.63 ppbv, respectively) and particulate phases (17.8±12.4 and 18.2±9.8%, respectively). Approximately 98% of the total acetic and formic acids were in the gas-phase and the gas–aerosol equilibrium was influenced by high levels of relative humidity. Gaseous formic-to-acetic ratios fell in the 0.94–1.85 range. Photochemical production appeared to be a very likely source of the gaseous acetic and formic acid levels found in this investigation. Direct emissions, mainly motor exhaust of vehicles also contributed to their presence in air. The observed amounts of formic and acetic acids in the particle phase were comparable with those observed in other urban sites. Results from aerosol particles indicated lower concentrations of the carboxylic acids at night, but their diurnal and nocturnal variation were similar. Using a correlation matrix, it was possible to suggest some sources for the carboxylic acids in the particulate phase. During daytime, vehicular emission appeared to be the primary source of acetic acid, whereas formic and pyruvic acids should be formed photochemically. Moreover, emissions from biogenic primary sources appeared to be an important contribution to atmospheric concentrations of formic and glycolic acids. Presumably, the photooxidation of pyruvic and glycolic acids gave rise to the oxalic acid. No source for acetic and pyruvic acids at nighttime was possible to suggest. However, direct vehicular and biogenic emissions might be major sources of TOC in nocturnal measurements. Oxalic acid might result from vehicular emission, glycolic acid from biogenic emission and formic acid from both sources.

Factors influencing the seasonal variation in particulate nitrate at Cheju Island, South Korea

The seasonal variation in measured particulate nitrate at Cheju island, South Korea, is analyzed and shown to exhibit high concentrations from February to June and in October, and low values from July to September. Total nitrate concentrations, which are not monitored, are estimated with a gas–aerosol equilibrium model with two-size bins. The total nitrate concentrations are shown to be maximum in June and minimum in August, and these differences are associated with air–mass trajectories. The fraction of particulate to total nitrate is estimated and shown to be sensitive to the total nitrate concentrations and the coarse-mode composition that is dominated by non-volatile species. The particulate nitrate fractions are found to vary within a relatively narrow range around the annual-mean value, indicating that the particulate nitrate concentrations behave similarly to the total nitrate concentrations. Exceptions are found for April, June and July. For April the largest particulate nitrate fraction is found in association with very high concentrations of nss-calcium, an indicator of mineral aerosol. The smallest particulate nitrate fractions occur in June and July and are the result of a combination of low concentrations of nss-calcium, sea salt and total nitrate, and high concentration of nss-sulfate.

Analysis of aerosol composition at Cheju Island, Korea, using a two-bin gas-aerosol equilibrium model

Aerosol chemical composition data measured at Cheju Island, Korea from March 1992 to December 1994 are analyzed with a gas-aerosol equilibrium model. The aerosol composition is divided into fine and coarse modes, with each mode assumed to be in equilibrium with the gas phase. The model represents the mean aerosol composition. The particulate ammonium is found to exist mostly in the fine mode to neutralize nss-sulfate, and the particulate nitrate and chloride are present mostly in the coarse mode and compete with each other to share sea salt and mineral cations. This means little formation of ammonium nitrate and chloride. Sensitivity analyses suggest that the aerosol in this region will be more deficit in chloride due to dechlorination and ammonium nitrate associated with the fine mode may become important.

Time scales to achieve atmospheric gas-aerosol equilibrium for volatile species

Time scales to achieve gas-aerosol equilibrium for volatile atmospheric species are systematically studied. It is found that equilibration between submicron aerosol and the gas phase is attainable on a time scale comparable to that of typical ambient gas and aerosol dynamics. However, the time required for the coarse aerosol to reach equilibrium is predicted to be sufficiently long that volatile species in atmospheric coarse aerosol particles may generally exist in nonequilibrium transition states. Variation of the gas-phase concentrations of volatile compounds is generally controlled by equilibration with the fine-mode particles. Volatile compounds in the gas phase can approach the equilibrium state on a much shorter time scale than that of the gas-aerosol system. Sensitivity analysis is performed to study how the equilibration times depend on relative humidity, initial gas-phase concentrations, aerosol mass concentration, aerosol size, temperature, accommodation coefficient, and different chemical compositions. The equilibration time increases with increasing particle size, or decreasing accommodation coefficient or temperature. Transport calculations performed for typical ambient aerosols confirm the results obtained from hypothetical cases. Equilibrium for ammonia between the gas phase and submicron marine aerosol is also examined.

Atmospheric Gas-Aerosol Equilibrium II. Analysis of Common Approximations and Activity Coefficient Calculation Methods

The gas-aerosol equilibrium model, SCAPE, presented in Part I is used to evaluate the sensitivity of thermodynamic calculations of aerosol composition to common approximations and the choice of activity coefficient estimation method. The treatment of weak electrolytes, associated ammonia, NH3(aq), and bisulfate ion, HSO4 −, is analyzed. Comparisons of the three multi-component activity coefficient estimation methods are carried out with a variety of data. On the basis of the sensitivity analysis results, recommendations are provided for treating these electrolytes and for selecting an activity coefficient estimation method in atmospheric gas-aerosol equilibrium calculations. The two gas-aerosol equilibrium models, SCAPE and AIM, are compared. Remaining questions in gas-aerosol equilibrium are highlighted.

High-volume sampling of airborne polycyclic aromatic hydrocarbons using glass fibre filters and polyurethane foam

A sampling system for collection of polycyclic aromatic hydrocarbons (PAH) in air is described. PAH were collected on glass fibre filters and plugs of polyurethane (PUR) foam by high-volume sampling. The analyses were carried out by high-resolution gas chromatography. The efficiency of these filter media has been investigated, and polyurethane foam seems to be efficient for the collection of selected PAH. The system has been used for PAH measurements in background, suburban and urban areas for average concentrations ranging from 20 to about 1500 ng m −3. A comparison of the profiles of PAH species obtained at the different locations shows that they are similar, and also that the relative concentrations of some of the compounds vary within only a few per cent. The distributions of the gas phase PAH found on the PUR plugs and in the particles collected on the glass fibre filters, correspond well with the results of a study on the gas-aerosol equilibrium made in Belgium.