Coarse Mode Nitrate Research Articles

A seasonal analysis of aerosol NO3− sources and NOx oxidation pathways in the Southern Ocean marine boundary layer

Abstract. Nitrogen oxides, collectively referred to as NOx (NO + NO2), are an important component of atmospheric chemistry involved in the production and destruction of various oxidants that contribute to the oxidative capacity of the troposphere. The primary sink for NOx is atmospheric nitrate, which has an influence on climate and the biogeochemical cycling of reactive nitrogen. NOx sources and NOx-to-NO3- formation pathways remain poorly constrained in the remote marine boundary layer of the Southern Ocean, particularly outside of the more frequently sampled summer months. This study presents seasonally resolved measurements of the isotopic composition (δ15N, δ18O, and Δ17O) of atmospheric nitrate in coarse-mode (> 1 µm) aerosols, collected between South Africa and the sea ice edge in summer, winter, and spring. Similar latitudinal trends in δ15N–NO3- were observed in summer and spring, suggesting similar NOx sources. Based on δ15N–NO3-, the main NOx sources were likely a combination of lightning, biomass burning, and/or soil emissions at the low latitudes, as well as oceanic alkyl nitrates and snowpack emissions from continental Antarctica or the sea ice at the mid-latitudes and high latitudes, respectively. Snowpack emissions associated with photolysis were derived from both the Antarctic snowpack and snow on sea ice. A combination of natural NOx sources, likely transported from the lower-latitude Atlantic, contribute to the background-level NO3- observed in winter, with the potential for a stratospheric NO3- source evidenced by one sample of Antarctic origin. Greater values of δ18O–NO3- in spring and winter compared to summer suggest an increased influence of oxidation pathways that incorporate oxygen atoms from O3 into the end product NO3- (i.e. N2O5, DMS, and halogen oxides (XO)). Significant linear relationships between δ18O and Δ17O suggest isotopic mixing between H2O(v) and O3 in winter and isotopic mixing between H2O(v) and O3/XO in spring. The onset of sunlight in spring, coupled with large sea ice extent, can activate chlorine chemistry with the potential to increase peroxy radical concentrations, contributing to oxidant chemistry in the marine boundary layer. As a result, isotopic mixing with an additional third end-member (atmospheric O2) occurs in spring.

The importance of alkyl nitrates and sea ice emissions to atmospheric NO<sub><i>x</i></sub> sources and cycling in the summertime Southern Ocean marine boundary layer

Abstract. Atmospheric nitrate originates from the oxidation of nitrogen oxides (NOx=NO+NO2) and impacts both tropospheric chemistry and climate. NOx sources, cycling and NOx to nitrate formation pathways are poorly constrained in remote marine regions, especially the Southern Ocean, where pristine conditions serve as a useful proxy for the pre-industrial atmosphere. Here, we measured the isotopic composition (δ15N and δ18O) of atmospheric nitrate in coarse-mode (>1 µm) aerosols collected in the summertime marine boundary layer of the Atlantic Southern Ocean from 34.5 to 70∘ S and across the northern edge of the Weddell Sea. The δ15N–NO3- decreased with latitude from −2.7 ‰ to −42.9 ‰. The decline in δ15N with latitude is attributed to changes in the dominant NOx sources: lightning at the low latitudes, oceanic alkyl nitrates at the mid-latitudes and photolysis of nitrate in snow at the high latitudes. There is no evidence of any influence from anthropogenic NOx sources or equilibrium isotope fractionation. Using air mass back trajectories and an isotope mixing model, we calculate that oceanic alkyl nitrate emissions have a δ15N signature of -21.8±7.6 ‰. Given that measurements of alkyl nitrate contributions to remote nitrogen budgets are scarce, this may be a useful tracer for detecting their contribution in other oceanic regions. The δ18O–NO3- was always less than 70 ‰, indicating that daytime processes involving OH are the dominant NOx oxidation pathway during summer. Unusually low δ18O–NO3- values (less than 31 ‰) were observed at the western edge of the Weddell Sea. The air mass history of these samples indicates extensive interaction with sea-ice-covered ocean, which is known to enhance peroxy radical production. The observed low δ18O–NO3- is therefore attributed to increased exchange of NO with peroxy radicals, which have a low δ18O, relative to ozone, which has a high δ18O. This study reveals that the mid- and high-latitude surface ocean may serve as a more important NOx source than previously thought and that the ice-covered surface ocean impacts the reactive nitrogen budget as well as the oxidative capacity of the marine boundary layer.

Measurement report: Vertical distribution of atmospheric particulate matter within the urban boundary layer in southern China – size-segregated chemical composition and secondary formation through cloud processing and heterogeneous reactions

Abstract. Many studies have recently been done on understanding the sources and formation mechanisms of atmospheric aerosols at ground level. However, vertical profiles and sources of size-resolved particulate matter within the urban boundary layer are still lacking. In this study, vertical distribution characteristics of size-segregated particles were investigated at three observation platforms (ground level, 118 m, and 488 m) on the 610 m high Canton Tower in Guangzhou, China. Size-segregated aerosol samples were simultaneously collected at the three levels in autumn and winter. Major aerosol components, including water-soluble ions, organic carbon, and elemental carbon, were measured. The results showed that daily average fine-particle concentrations generally decreased with height. Concentrations of sulfate and ammonium in fine particles displayed shallow vertical gradients, and nitrate concentrations increased with height in autumn, while the chemical components showed greater variations in winter than in autumn. The size distributions of sulfate and ammonium in both seasons were characterized by a dominant unimodal mode with peaks in the size range of 0.44–1.0 µm. In autumn, the nitrate size distribution was bimodal, peaking at 0.44–1.0 and 2.5–10 µm, while in winter it was unimodal, implying that the formation mechanisms for nitrate particles were different in the two seasons. Our results suggest that the majority of the sulfate and nitrate is formed from aqueous-phase reactions, and we attribute coarse-mode nitrate formation at the measurement site to the heterogeneous reactions of gaseous nitric acid on existing sea-derived coarse particles in autumn. Case studies further showed that atmospheric aqueous-phase and heterogeneous reactions could be important mechanisms for sulfate and nitrate formation, which, in combination with adverse weather conditions such as temperature inversion and calm wind, led to haze formation during autumn and winter in the Pearl River Delta (PRD) region.

Significant impacts of heterogeneous reactions on the chemical composition and mixing state of dust particles: A case study during dust events over northern China

The impact of heterogeneous reactions on the chemical components and mixing state of dust particles are investigated by observations and an air quality model over northern China between March 27, 2015 and April 2, 2015. Synergetic observations were conducted using a polarization optical particle counter (POPC), a depolarized two-wavelength Lidar and filter samples in Beijing. During this period, dust plume passed through Beijing on March 28, and flew back on March 29 because of synoptic weather changes. Mineral dust mixed with anthropogenic pollutants was simulated using the Nested Air Quality Prediction Modeling System (NAQPMS) to examine the role of heterogeneous processes on the dust. A comparison of observations shows that the NAQPMS successfully reproduces the time series of the vertical profile, particulate matter concentration, and chemical components of fine mode (diameter ≤ 2.5 μm) and coarse mode (2.5 μm < diameter ≤ 10 μm) particles. After considering the heterogeneous reactions, the simulated nitrate, ammonium, and sulfate are in better agreement with the observed values during this period. The modeling results with observations show that heterogeneous reactions are the major mechanisms producing nitrate reaching 19 μg/m3, and sulfate reaching 7 μg/m3, on coarse mode dust particles, which were almost 100% of the coarse mode nitrate and sulfate. The heterogeneous reactions are also important for fine mode secondary aerosols, for producing 17% of nitrate and 11% of sulfate on fine mode dust particles, with maximum mass concentrations of 6 μg/m3 and 4 μg/m3. In contrast, due to uptake of acid gases (e.g. HNO3 and SO2) by dust particles, the fine mode anthropogenic ammonium nitrate and ammonium sulfate decreased. As a result, the total fine mode nitrate decreased with a maximum of 14 μg/m3, while the total fine mode sulfate increased with a maximum of 2 μg/m3. Because of heterogeneous reactions, 15% of fine mode secondary inorganic aerosols and the entire coarse mode nitrate and sulfate were internally mixed with dust particles. The significant alterations of the chemical composition and mixing state of particles due to heterogeneous reactions are important for the direct and indirect climate effects of dust and anthropogenic aerosols.

Sea salt emission, transport and influence on size-segregated nitrate simulation: a case study in northwestern Europe by WRF-Chem

Abstract. Sea salt aerosol (SSA) is one of the major components of primary aerosols and has significant impact on the formation of secondary inorganic particles mass on a global scale. In this study, the fully online coupled WRF-Chem model was utilized to evaluate the SSA emission scheme and its influence on the nitrate simulation in a case study in Europe during 10–20 September 2013. Meteorological conditions near the surface, wind pattern and thermal stratification structure were well reproduced by the model. Nonetheless, the coarse-mode (PM1 − 10) particle mass concentration was substantially overestimated due to the overestimation of SSA and nitrate. Compared to filter measurements at four EMEP stations (coastal stations: Bilthoven, Kollumerwaard and Vredepeel; inland station: Melpitz), the model overestimated SSA concentrations by a factor of 8–20. We found that this overestimation was mainly caused by overestimated SSA emissions over the North Sea during 16–20 September. Over the coastal regions, SSA was injected into the continental free troposphere through an “aloft bridge” (about 500 to 1000 m above the ground), a result of the different thermodynamic properties and planetary boundary layer (PBL) structure between continental and marine regions. The injected SSA was further transported inland and mixed downward to the surface through downdraft and PBL turbulence. This process extended the influence of SSA to a larger downwind region, leading, for example, to an overestimation of SSA at Melpitz, Germany, by a factor of ∼ 20. As a result, the nitrate partitioning fraction (ratio between particulate nitrate and the summation of particulate nitrate and gas-phase nitric acid) increased by about 20 % for the coarse-mode nitrate due to the overestimation of SSA at Melpitz. However, no significant difference in the partitioning fraction for the fine-mode nitrate was found. About 140 % overestimation of the coarse-mode nitrate resulted from the influence of SSA at Melpitz. In contrast, the overestimation of SSA inhibited the nitrate particle formation in the fine mode by about 20 % because of the increased consumption of precursor by coarse-mode nitrate formation.

Illustrating the benefit of using hourly monitoring data on secondary inorganic aerosol and its precursors for model evaluation

Abstract. Secondary inorganic aerosol, most notably ammonium nitrate and ammonium sulphate, is an important contributor to ambient particulate mass and provides a means for long range transport of acidifying components. The modelling of the formation and fate of these components is challenging. Especially, the formation of the semi-volatile ammonium nitrate is strongly dependent on ambient conditions and the precursor concentrations. For the first time an hourly artefact free data set from the MARGA instrument is available for the period of a full year (1 August 2007 to 1 August 2008) at Cabauw, the Netherlands. This data set is used to verify the results of the LOTOS-EUROS model. The comparison showed that the model underestimates the SIA levels. Closer inspection revealed that base line values appear well estimated for ammonium and sulphate and that the underestimation predominantly takes place at the peak concentrations. For nitrate the variability towards high concentrations is much better captured, however, a systematic relative underestimation was found. The model is able to reproduce many features of the intra-day variability observed for SIA. Although the model captures the seasonal and average diurnal variation of the SIA components, the modelled variability for the nitrate precursor gas nitric acid is much too large. It was found that the thermodynamic equilibrium module produces a too stable ammonium nitrate in winter and during night time in summer, whereas during the daytime in summer it is too unstable. We recommend to improve the model by verification of the equilibrium module, inclusion of coarse mode nitrate and to address the processes concerning SIA formation combined with a detailed analysis of the data set at hand. The benefit of the hourly data with both particulate and gas phase concentrations is illustrated and a continuation of these measurements may prove to be very useful in future model evaluation and improvement studies. Based on our findings we propose to implement a monitoring strategy using three levels of detail within the Netherlands.

Acidic species and chloride depletion in coarse aerosol particles in the US east coast

To investigate the interactions of water-soluble acidic species associated with coarse mode aerosol particles (1.8–10 μm) and chlorine depletion, ten sets of size-segregated aerosol samples were collected by a 10-stage Micro-Orifice Uniform Deposit Impactor (MOUDI) in Newark, New Jersey on the U.S. east coast. The samples were grouped into two categories according to the air-mass back trajectories and mass ratios of sodium to magnesium and calcium: Group I was primarily impacted by marine air mass and Group II was dominated by the continental air mass. In Group I, the concentrations of coarse mode nitrate and chloride depletion showed a strong correlation ( R 2 = 0.88). Without considering other cations, nitrate was found to account for all of the chloride depletion in coarse particles for most samples. The association of coarse mode nitrate with sea-salt particles is favored when the mass ratio of sodium to calcium is approximately equal to or greater than unity. Excess sulfate accounts for a maximum of 33% of chloride depletion in the coarse particles. Regarding chloride depletion in the different particle sizes, excess nitrate and sulfate account for 89% of the chloride depletion in the particle size range of 1.8–3.2 μm in the sample from July 13–14; all of the determined dicarboxylic acids and mono-carboxylic acids cannot compensate for the rest of the chloride depletion. In Group II, high percentages of chloride depletion were not observed. With nitrate being dominant in chlorine depletion observed at this location, N-containing species from pollution emissions may have profound impact on atmospheric composition through altering chlorine chemistry in this region.

Size distribution and source analysis of ionic compositions of aerosols in polluted periods at Xinken in Pearl River Delta (PRD) of China

Size-resolved chemical composition of aerosol was investigated during the PRIDE intensive field campaign in October 2004 at Xinken, at Pearl River Delta (PRD), China. Xinken is a rural/coastal site located south of Guangzhou. Size-segregated particles were sampled using Micro-Orifice Uniform Deposit Impactors (MOUDI) in order to characterize regional aerosol pollution. The meteorological conditions during the measurements were stable with low wind speed, high temperature and high RH, leading to high concentrations of fine particles, especially due to secondary components. The average concentrations of PM 1.8 and PM 10 were 51±19 and 84±27 μg m −3, respectively, with a ratio of PM 1.8/PM 10 0.64±0.09, which indicates that fine particles dominated in PM 10. Sulfate, nitrate and ammonium were major ionic components. Aerosol mass size distributions showed either two or three modes. Land–sea circulation brought back aged aerosols to the sampling site leading to an increase of the concentrations of sodium and chloride in aerosols. The “condensation mode” of sulfate, which was formed by gas-to-particle conversion, was in the size range of 0.32–0.56 μm, and the “droplet mode” of sulfate was observed in the size range of 0.56–1.0 or 1.0–1.8 μm. Nitrate was distributed evenly over fine and coarse particles. The analysis of measured and calculated equilibrium constant of NH 4NO 3 indicates that dissociation of NH 4NO 3 in daytime is favored, but that at night ammonium nitrate is stable and constitutes the fine mode nitrate, while the coarse mode nitrate may be formed by the reaction of HNO 3 and sea-salt or soil components. Chemical mass closures showed reasonable agreement.

Factors affecting levels of aerosol sulfate and nitrate on the Western Mediterranean coast

The present study was carried out between October 2003 and September 2004 at an urban background station near the Mediterranean coast in southeastern Spain. The sulfate and nitrate content was determined in 332 PM10 and PM2.5 daily samples. The results show that the seasonal variation of nitrate measured in PM10 does not correspond with what has been observed at other locations in the Iberian Peninsula, where the minima are reached during the summer months due to decomposition of ammonium nitrate at high temperatures. The recorded levels of PM10 nitrate were slightly higher in summer due to an increase in the concentrations of coarse mode nitrate. On the contrary, both the concentrations and the percentages of nitrate in PM2.5 were lowest from June to September. The sulfate levels in both PM10 and PM2.5 were maxima in summer because the oxidation rate of SO 2 increases with solar radiation. An elevated correlation (0.72 < r < 0.92) between the monthly average concentrations and percentages of sulfate and solar radiation has been found. We have also investigated the influence that Saharan dust intrusions and high pollution episodes have upon the sulfate and nitrate concentrations. Both types of events increased NO 3 − and SO 4 2− levels, particularly the high pollution episodes.

Observations of fine and coarse particle nitrate at several rural locations in the United States

Nitrate comprises an important part of aerosol mass at many non-urban locations during some times of the year. Little is known, however, about the chemical form and size distribution of particulate nitrate in these environments. While submicron ammonium nitrate is often assumed to be the dominant species, this assumption is rarely tested. Properties of aerosol nitrate were characterized at several IMPROVE monitoring sites during a series of field studies. Study sites included Bondville, Illinois (February 2003), San Gorgonio Wilderness Area, California (April and July 2003), Grand Canyon National Park, Arizona (May 2003), Brigantine National Wildlife Refuge, New Jersey (November 2003), and Great Smoky Mountains National Park, Tennessee (July/August 2004). Nitrate was found predominantly in submicron ammonium nitrate particles during the Bondville and San Gorgonio (April) campaigns. Coarse mode nitrate particles, resulting from reactions of nitric acid or its precursors with sea salt or soil dust, were more important at Grand Canyon and Great Smoky Mountains. Both fine and coarse mode nitrate were important during the studies at Brigantine and San Gorgonio (July). These results, which complement earlier findings about the importance of coarse particle nitrate at Yosemite and Big Bend National Parks, suggest a need to more closely examine common assumptions regarding the importance of ammonium nitrate at non-urban sites, to include pathways for coarse mode nitrate formation in regional models, and to consider impacts of coarse particle nitrate on visibility. Because coarse particle nitrate modes often extend well below 2.5 μm aerodynamic diameter, measurements of PM 2.5 nitrate in these environments should not automatically be assumed to contain only ammonium nitrate.

A model evaluation of coarse-mode nitrate heterogeneous formation on dust particles

The significance of the heterogeneous formation of particulate nitrate, rarely included in chemistry-transport models (CTMs), is investigated. According to several field campaigns, particulate nitrate in a coarse particle (2–10 μm) can be obtained by reaction of nitric acid with calcium and magnesium carbonates. This process significantly contributes to the total nitrate mass concentration over land, particularly in summer. We introduce this heterogeneous reaction in the CHIMERE CTM and analyze its impact on total nitrate, nitric acid and aerosol particles, by statistical comparison of one-year long simulations with EMEP measurements over Europe. Several values of the nitric acid uptake coefficient are tested in order to investigate the sensitivity to this parameter, and we propose an optimal value for our study. The model performance in simulating nitrates and PM 10 significantly improves when the heterogeneous nitrate formation is included. This process also greatly improves the simulation of coarse nitrate by comparison with field campaign measurements near Paris city.

Chemical characteristics of aerosols transported from Asia to the East China Sea: an evaluation of anthropogenic combined nitrogen deposition in autumn

Deposition of terrestrial aerosols is one of the major means of fertilizing the oligotrophic ocean waters. In order to understand the biogeochemical significance of aerosols transported from the Asian continent, and deposited over the East China Sea, we studied their chemical properties by making continuous measurements on board R/V Hakuho Maru in autumn, a season of lower dust events. Outflows of anthropogenic substances were observed over the East China Sea. The average concentration of non-sea-salt (nss)-SO 4 2− in aerosols was 8.7±6.8 μg m −3 , a value typical for polluted atmosphere of a highly industrialized urban area in South China. Fine mode NH 4 + mainly occurred as (NH 4 ) 2 SO 4 and/or NH 4 HSO 4 while in coarse mode it was formed from intermodal coagulation of ammonium particles in fine and coarse aeorosols. On the other hand, the mean concentration of NO 3 − (1.8±1.2 μg m −3 ) was lower than that in the urban atmosphere. Non-sea-salt SO 4 2− were derived from combustion sources and marine biological processes, the latter accounting to 0–38%. Nitrate in coarse mode was associated with Ca 2+ and Na + perhaps due to replacement reactions. Our estimates of the annual atmospheric aerosol deposition fluxes of NH 4 + and NO 3 − to the East China Sea were 270 Gg N yr −1 and 160 Gg N yr −1 , respectively. These atmospheric inputs are comparable to the riverine inputs of the Changjiang River. The total combined nitrogen deposition could account for a biological fixation of about 2.5 Tg C (approximately 0.1–9% of the new production) in the East China Sea annually, which could actually be larger since the autumn season experience minimal dust storms.

Study on size distribution of total aerosol and water-soluble ions during an Asian dust storm event at Jeju Island, Korea.

Soil dust particles transported from loess regions of the Asian continent, called Asian dust, highly influences the air quality of north-eastern Asia and the northern Pacific Ocean. In order to investigate the effects of these dust storms on the chemical composition of atmospheric aerosol particles with different size, measurements of size distributions of total aerosol and major ion species were carried out on Jeju Island, Korea during April 2001. Juju Island was chosen for the study because the levels of emissions of anthropogenic air pollutants are very low. A 5-stage cascade impactor was used to sample size-fractionated aerosol particles. Samples were analyzed for major water-soluble ions using Dionex DX-120 ion chromatograph. The average mass concentration of total aerosol was found to be 24.4 and 108.3 microg m(-3) for non-Asian dust and Asian dust periods, respectively. The total aerosol size distribution, measured during the non-Asian dust period, was bimodal, whereas the coarse particles dominated the size distribution of total aerosol during the Asian dust period. It was found that SO4(2-), NH4+ and K+ were mainly distributed in fine particles, while Cl-, NO3-, Na+, Mg2+ and Ca2+ were in coarse particles. Although SO4(2-) was mainly distributed in fine particles, during the Asian dust period, the concentrations in coarse particles were significantly increased. This indicates heterogeneous oxidation of SO2 on wet surfaces of basic soil dust particles. The NH4+ was found to exist as (NH4)2SO4 in fine particles, with a molar ratio of NH4+ to SO4(2-) of 2.37 and 1.52 for non-Asian dust and Asian dust periods, respectively. Taking into account the proximity of the sampling site to the sea, and the observed chloride depletion, coarse mode nitrate, during the non-Asian dust period, is assumed to originate from the reaction of nitric acid with sodium chloride on the surfaces of sea-salt particles although the chloride depletion was not shown to be large enough to prove this assumption. During the Asian dust period, however, chloride depletion was much smaller, indicating coarse nitrate particles were mainly produced by the reaction of nitric acid with surfaces of basic soil particles. Most chloride and sodium components were shown to originate from sea-salt particles. Asian dust aerosols, arriving at Jeju Island, contained considerable amounts of sea-salt particles as they passed over the Yellow Sea. Ca2+ was shown to be the most abundant species in Asian dust particles.

Size distributions and formation of ionic species in atmospheric particulate pollutants in Beijing, China: 1—inorganic ions

Abstract To study the formation of atmospheric sulfate, nitrate and chloride particulates in Beijing, China, 14 sets of measurements of the concentrations of SO 2 , HNO 3 , NH 3 and size-segregated particles were made in the summer of 2001 and the spring of 2002. Because of the very different humidity levels in Beijing in the summer and the spring, significant differences in the size characteristics and in the formation mechanisms of these ions were found. In the summer, the major fraction of sulfate was present in the fine mode with a mass median aerodynamic diameter (MMAD) of 0.7±0.1 μm. Its formation was attributed to in-cloud processing, which was supported by the observation of a large mole ratio of the sulfate at 0.54–1.0 μm to SO 2 , with a maximum value of 3.0. The major fraction of nitrate sometimes appeared in the fine mode with an MMAD of 0.7±0.1 μm and sometimes in the coarse mode with an MMAD of 6.0±1.5 μm. The coarse mode nitrate was associated with Ca 2+ . The major peak of chloride, in most cases, appeared in the coarse mode. Chloride was expected to have formation mechanisms similar to nitrate formation in the coarse mode. In the spring, the major fraction of sulfate was present in the fine mode with an MMAD of 0.45±0.05 μm and its formation was ascribed to non-cloud heterogeneous processes. A small mole ratio of the fine mode sulfate to SO 2 , with a maximum value of only 0.06, was observed. Nitrate and chloride generally have a similar MMAD as sulfate in the fine mode, suggesting that both may originate from atmospheric processes similar to sulfate formation. Alternatively, they could be formed by gas condensation onto existing sulfate particles. Overall, much more efficient sulfate formation by cloud processing led to a higher sulfate concentration in the summer than in the spring, although the SO 2 concentration followed the reverse trend in Beijing.

Formation of nitrate and non-sea-salt sulfate on coarse particles

The formation of nitrate and sulfate on coarse particles and its relationships with meteorological conditions and relative abundance of sea-salt and soil particles were studied. Size distributions of particulate SO 2− 4, NO − 3, NH + 4 , Cl −, Na +, K +, Mg 2+ and Ca 2+ were measured using a MOUDI cascade impactor for 11 days in July to December 1997 at a coastal site in Hong Kong. Most of the nitrate and a small fraction of sulfate were distributed in the coarse mode. Since Hong Kong is influenced by both marine and continental aerosols, the distribution of coarse mode nitrate and sulfate depends on the weather conditions. Evidence of nitrate formation on both sea-salt and soil compounds was found in this study. Chloride depletion of sea-salt by both nitrate and sulfate formation was also observed. When Hong Kong was under prevailing easterly wind accompanied with high relative humidity, Na + was the dominant cation species and significant chloride depletion (74–88%) from coarse mode sea-salt aerosols was observed. Nitrate accounted for 65% of chloride depletion. During a heavily polluted period when there were high concentrations of both fine (18 μg m −3) and coarse mode (3 μg m −3) sulfate, significant amount of sulfate was associated with sea-salt particles and accounted for 11–29% of the chloride depletion. At least 16% of chloride depletion was not accounted for by nitrate and sulfate alone in this episode. When Hong Kong was influenced by a northeasterly monsoon, Ca 2+ was the dominant species in the coarse mode. Soil particles compete with sea-salt particles for acidic gases. Chloride depletion was below 50% and most of the nitrate was found to be associated with Ca 2+ in soil particles.

Studying the effects of calcium and magnesium on size-distributed nitrate and ammonium with EQUISOLV II

A chemical equilibrium code was improved and used to show that calcium and magnesium have a large yet different effect on the aerosol size distribution in different regions of Los Angeles. In the code, a new technique of solving individual equilibrium equations was developed. The technique, the analytical equilibrium iteration (AEI) method, gives the same solutions (to at least 7 decimal places) as the previous technique used, the mass-flux iteration (MFI) method, but consumes 13–48 times less computer time. The model was also updated to include treatment of potassium, calcium, magnesium, and carbonate. Previously, it treated only nitrate, ammonium, chloride, sulfate, and sodium. Predictions from the updated code, EQUISOLV II, were compared with data from an eight-stage Berner impactor at Long Beach, Claremont, and Riverside during the Southern California Air Quality Study. When any equilibrium solver is applied between the gas phase and multiple aerosol size bins, unique solutions are possible only when solids (e.g., NH 4NO 3) that form from two gas-phase species are absent. For this study, unique solutions were possible only when the relative humidity exceeded 62%, and only cases in this regime are discussed. Base-case predictions of nitrate and ammonium matched observations well in most size bins of every case. When Ca and Mg were removed from calculations, coarse-mode nitrate decreased at Long Beach, as expected, to maintain charge balance. At Riverside, removing Ca and Mg had the opposite effect, increasing coarse-mode nitrate, shifting it from the accumulation mode. The reason is explained in terms of mean mixed activity coefficients. At Claremont, the charge-balance and activity-coefficient effects nearly canceled each other.

Size distributions of particulate sulfate, nitrate, and ammonium at a coastal site in Hong Kong

Size distributions of particulate sulfate, ammonium, and nitrate were measured at a coastal site in Hong Kong over 22 days using a MOUDI cascade impactor. Sulfate, ammonium, and nitrate were trimodally distributed in the size range of 0.052–10 μm. Sulfate and ammonium were the dominant species in fine particles ( D p<1.8 μm), co-existing in the condensation (0.20 μm) mode and droplet (0.57 μm) modes. The droplet mode dominated the distributions of these two ions. Higher concentrations of droplet mode sulfate were associated with high relative humidity and low clouds. Transport of aerosols from southern China by strong northerly wind also increased the sulfate concentration in Hong Kong. Mode diameters of coarse sulfate and ammonium were about 4–6 μm. Nitrate was mainly distributed in the coarse particles with mode diameter of 3.95±0.69 μm. Coarse mode nitrate was formed by the reaction of gas-phase nitric acid with sea-salt or soil particles. Little fine mode nitrate was found.

Measurement of aerosol size distributions for nitrate and major ionic species

Nitrate particle size distributions were determined at the Nitrogen Species Methods Comparison Study at Claremont, CA during September 1985 with the Berner nine stage cascade impactor. Particle bounce was minimized by an inert fluorocarbon grease substrate. Interference from nitric acid was eliminated with a vertical acid gas denuder designed to prevent large particle loss. Samples were taken with and without the grease or denuder to test the techniques. Volatile nitrate loss was less than 10%, based on comparisons with fine nitrate from parallel filter samplers. Aerosol size distributions for the major ionic species were determined under varying atmospheric conditions and good ion balances achieved. Nitrate was bimodal during both day and night-time periods. Fine mode nitrate was associated with ammonium while coarse mode nitrate was associated with both ammonium and sodium. Extensive evidence including size distributions, temporal associations and species balances is given for the reaction of nitric acid and sea salt to produce hydrochloric acid and coarse mode nitrate. Sulfate was primarily in two submicrometer modes. Strong acid was associated with the smaller sulfate mode.