Nitrate Aerosol Concentrations Research Articles

Emission, transport, deposition, chemical and radiative impacts of mineral dust during severe dust storm periods in March 2021 over East Asia

A Regional Air Quality Model System (named RAQMS) coupled with a developed dust model driven by WRF was applied to synthetically investigate the emission, transport, deposition, budget, and chemical and radiative effects of mineral dust during the severe dust storm periods of 10–31 March 2021. Model results were validated against a variety of ground, vertical and satellite observations, which demonstrated a generally good model ability in reproducing meteorological variables, particulate matter and compositions, and aerosol optical properties. The first dust storm (DS1), which was the severest one since 2010 was originated from the Gobi Desert in southern Mongolia on 14 March, with the dust emission flux reaching 2785 μg m−2 s−1 and the maximum dust concentration exceeding 18,000 μg m−3 in the dust deflation region. This dust storm resulted in remarkably high hourly PM10 observations up to 7506 μg m−3, 1887 μg m−3, and 2704 μg m−3 in Beijing, Tianjin, and Shijiazhuang on 15 March, respectively, and led to a maximum decrease in surface shortwave radiation up to 313.4 W m−2 (72 %) in Beijing. The second dust storm (DS2) broke out in the deserts of eastern Mongolia, with lower dust emission than the first one. The extinction of shortwave radiation by dust aerosols led to a reduction in photolysis rate and consequently decreases in O3 and secondary aerosol concentrations over the North China Plain (NCP), whereas total sulfate and nitrate concentrations consistently increased due to heterogeneous reactions on dust surfaces over the middle reaches of the Yellow River and the NCP region during DS1. Sulfate and nitrate formation through heterogeneous reactions were enhanced in the dust backflow on 16–17 March by approximately 18 % and 24 % on average in the NCP. Heterogeneous reactions and photolysis rate reduction by mineral dust jointly led to average changes in sulfate, nitrate, ammonium, and secondary organic aerosol (SOA) concentrations by 13.0 %, 13.5 %, −12.3 %, and −4.4 %, respectively, in the NCP region during DS1, larger than the changes in the Yangtze River Delta (YRD). The maximum dry deposition settled in the 7–11 μm size range in downwind land and ocean areas, while wet deposition peaked in the 4.7–7 μm size range in the entire domain. Wet deposition was approximately twice the dry deposition over mainland China except for dust source regions. During 10–31 March, the total dust emission, dry and wet depositions were estimated to be 31.4 Tg, 13.78 Tg and 4.75 Tg, respectively, with remaining 12.87 Tg of dust aerosols (41 % of the dust emission) suspending in the atmosphere or transporting to other continents and oceans.

A case study in the wintertime Vaal Triangle Air-Shed Priority Area on the utility of the nitrogen stable isotopic composition of aerosol nitrate to identify NOx sources

In South Africa, the Highveld region and the Johannesburg-Pretoria megacity are known as global NOx (NOx = NO + NO2) “hotspots” identified by satellite-based instruments. The ultimate sink for atmospheric NOx is conversion to aerosol nitrate. However, measurements of aerosol nitrate concentrations do not provide information on which NOx sources served as nitrate precursors at that location. This complicates efforts to reduce concentrations of particulate matter (PM) in these air quality priority areas. Here, we measured the nitrogen stable isotopic composition of nitrate from daily wintertime collections of coarse mode PM2.5-10 (PM ≤ 10 and >2.5 µm in diameter) at three air quality monitoring stations located in the Vaal Triangle Air-Shed Priority Area (VTAPA). The overall aim of this case study was to evaluate the use of the distinct stable isotopic signatures of various NOx sources to identify their relative contribution to aerosol nitrate across the Highveld. The nitrogen isotopic ratios of aerosol nitrate were similar across the three sites, with greater day-to-day variability than site to site variability. Air mass history was the main driver of the variability in the nitrogen isotopic ratios of aerosol nitrate, with significantly higher isotopic ratios observed for air masses originating from the southwest. Using an isotope mixing model we determined that NOx from coal-burning is the dominant contributor to aerosol nitrate (66%), followed by biomass burning (16%), vehicles (12%), and soil emissions (6%).

Impact of dimethylsulfide chemistry on air quality over the Northern Hemisphere

We implement oceanic dimethylsulfide (DMS) emissions and its atmospheric chemical reactions into the Community Multiscale Air Quality (CMAQv53) model and perform annual simulations without and with DMS chemistry to quantify its impact on tropospheric composition and air quality over the Northern Hemisphere. DMS chemistry enhances both sulfur dioxide (SO2) and sulfate (SO42−) over seawater and coastal areas. It enhances annual mean surface SO2 concentration by +46 pptv and SO42−by +0.33 μg/m3 and decreases aerosol nitrate concentration by −0.07 μg/m3 over seawater compared to the simulation without DMS chemistry. The changes decrease with altitude and are limited to the lower atmosphere. Impacts of DMS chemistry on SO42− are largest in the summer and lowest in the fall due to the seasonality of DMS emissions, atmospheric photochemistry and resultant oxidant levels. Hydroxyl and nitrate radical-initiated pathways oxidize 75% of the DMS while halogen-initiated pathways oxidize 25%. DMS chemistry leads to more acidic particles over seawater by decreasing aerosol pH. Increased SO42−from DMS enhances atmospheric extinction while lower aerosol nitrate reduces the extinction so that the net effect of DMS chemistry on visibility tends to remain unchanged over most of the seawater.

Investigation of dark condition nitrate radical- and ozone-initiated aging of toluene secondary organic aerosol: Importance of nitrate radical reactions with phenolic products

To investigate the reaction mechanisms during the NO3- and O3-initiated aging of aromatic secondary organic aerosol (SOA), the gases and particles formed from the OH-initiated oxidation of toluene were exposed to NO3 radicals and O3 in a laboratory chamber. Hydroxyl radicals for the oxidation of toluene were produced under light conditions through H2O2 photolysis and dark conditions through tetramethylethylene ozonolysis. Aerodyne aerosol mass spectrometer results demonstrated an increase in SOA and nitrate aerosol concentrations only after N2O5 injection, but no clear change after O3 injection. The results of proton transfer reaction-quadrupole interface-time of flight mass spectrometer showed that the signal of m/z 109.07 decreased and the signals of m/z 123.05 and m/z 154.05 increased after N2O5 injection, suggesting that gaseous cresols formed from toluene oxidation are rapidly degraded and gaseous nitrocresols and methylbenzoquinones are newly formed. The results of liquid chromatography-mass spectrometry analysis showed that nitrocresols and nitrosalicylic alcohols were the major products formed after N2O5 injection in the particle phase. To confirm the contribution of cresols on aging, NO3 oxidation of o-cresol was carried out, which resulted in the formation of similar products, i.e. gaseous nitrocresols and particulate nitrosalicylic alcohols. Yield curve analysis using two product model estimated that cresol accounted for ~62% of the increase in SOA during NO3-initiated aging of toluene SOA.

Is water vapor a key player of the wintertime haze in North China Plain?

Abstract. Water vapor has been proposed to amplify the severe haze pollution in China by enhancing the aerosol–radiation feedback (ARF). Observations have revealed that the near-surface PM2.5 concentrations ([PM2.5]) generally exhibit an increasing trend with relative humidity (RH) in the North China Plain (NCP) during 2015 wintertime, indicating that the aerosol liquid water (ALW) caused by hygroscopic growth could play an important role in the PM2.5 formation and accumulation. Simulations during a persistent and heavy haze pollution episode from 5 December 2015 to 4 January 2016 in the NCP were conducted using the WRF-Chem Model to comprehensively quantify contributions of the ALW effect to near-surface [PM2.5]. The WRF-Chem Model generally performs reasonably well in simulating the temporal variations in RH against measurements in the NCP. The factor separation approach (FSA) was used to evaluate the contribution of the ALW effect on the ARF, photochemistry, and heterogeneous reactions to [PM2.5]. The ALW not only augments particle sizes to enhance aerosol backward scattering but also increases the effective radius to favor aerosol forward scattering. The contribution of the ALW effect on the ARF and photochemistry to near-surface [PM2.5] is not significant, being generally within 1.0 µg m−3 on average in the NCP during the episode. Serving as an excellent substrate for heterogeneous reactions, the ALW substantially enhances the secondary aerosol (SA) formation, with an average contribution of 71 %, 10 %, 26 %, and 48 % to near-surface sulfate, nitrate, ammonium, and secondary organic aerosol concentrations. Nevertheless, the SA enhancement due to the ALW decreases the aerosol optical depth and increases the effective radius to weaken the ARF, reducing near-surface primary aerosols. The contribution of the ALW total effect to near-surface [PM2.5] is 17.5 % on average, which is overwhelmingly dominated by enhanced SA. Model sensitivities also show that when the RH is less than 80 %, the ALW progressively increases near-surface [PM2.5] but commences to decrease when the RH exceeds 80 % due to the high occurrence frequencies of precipitation.

Sensitivity of Nitrate Aerosol Production to Vehicular Emissions in an Urban Street

This study investigated the sensitivity of nitrate aerosols to vehicular emissions in urban streets using a coupled computational fluid dynamics (CFD)–chemistry model. Nitrate concentrations were highest at the street surface level following NH3 emissions from vehicles, indicating that ammonium nitrate formation occurs under NH3-limited conditions in street canyons. Sensitivity simulations revealed that the nitrate concentration has no clear relationship with the NOx emission rate, showing nitrate changes of only 2% across among 16 time differences in NOx emissions. NOx emissions show a conflicting effect on nitrate production via decreasing O3 and increasing NO2 concentrations under a volatile organic compound (VOC)-limited regime for O3 production. The sensitivity simulations also show that nitrate aerosol is proportional to vehicular VOC and NH3 emissions in the street canyon. Changes of VOC emissions affect the nitrate aerosol and HNO3 concentrations through changes in the O3 concentration under a VOC-limited regime for O3 production. Nitrate aerosol concentration is influenced by vehicular NH3 emissions, which produce ammonium nitrate effectively under an NH3-limited regime for nitrate production. This research suggests that, when vehicular emissions are dominant in winter, the control of vehicular VOC and NH3 emissions might be a more effective way to degrade PM2.5 problems than the control of NOx.

Computational fluid dynamics simulation of reactive fine particulate matter in a street canyon

We developed a coupled computational fluid dynamics–chemistry model to examine the transport and chemical transformation of reactive aerosols on an urban street. The model was evaluated by comparing the results of simulations with those of observational campaigns in a street canyon in Elche, Spain. The model generally captured the composition of fine particulate matter (PM1) in the street canyon in summer and winter. However, compared with the observed concentration of PM1 in summer, the simulated concentration of PM1 was overestimated by 40%, indicating that the model predicted a weaker canyon vortex. Although the model has some bias, it reasonably reproduced the observed aerosol concentration. We also investigated the diurnal variations and spatial distribution of PM1 and its composition in the street canyon. The simulated sulfate concentrations were mostly affected by boundary transport, showing weak diurnal variations. The nitrate aerosol concentrations exhibited clear sinusoidal diurnal variations following the precursor gas, HNO3, which is mainly formed by photochemical reactions. We also found that nitrate aerosol formation was suppressed by low O3 concentrations under extreme volatile organic compound-limited conditions. The concentrations of PM1, organic carbon, and black carbon followed traffic volume curves, indicating the dominant effect of vehicular emissions on aerosols. Our sensitivity model simulation showed that considering chemical reactions significantly affects the diurnal variations of secondarily produced aerosol concentrations. These results clearly demonstrate that considering chemical production and loss is essential to investigate the diurnal variations of PM1 in street canyons, especially in winter.

The impact of historical land use change from 1850 to 2000 on secondary particulate matter and ozone

Abstract. Anthropogenic land use change (LUC) since preindustrial (1850) has altered the vegetation distribution and density around the world. We use a global model (GEOS-Chem) to assess the attendant changes in surface air quality and the direct radiative forcing (DRF). We focus our analysis on secondary particulate matter and tropospheric ozone formation. The general trend of expansion of managed ecosystems (croplands and pasturelands) at the expense of natural ecosystems has led to an 11 % decline in global mean biogenic volatile organic compound emissions. Concomitant growth in agricultural activity has more than doubled ammonia emissions and increased emissions of nitrogen oxides from soils by more than 50 %. Conversion to croplands has also led to a widespread increase in ozone dry deposition velocity. Together these changes in biosphere–atmosphere exchange have led to a 14 % global mean increase in biogenic secondary organic aerosol (BSOA) surface concentrations, a doubling of surface aerosol nitrate concentrations, and local changes in surface ozone of up to 8.5 ppb. We assess a global mean LUC-DRF of +0.017, −0.071, and −0.01 W m−2 for BSOA, nitrate, and tropospheric ozone, respectively. We conclude that the DRF and the perturbations in surface air quality associated with LUC (and the associated changes in agricultural emissions) are substantial and should be considered alongside changes in anthropogenic emissions and climate feedbacks in chemistry–climate studies.

Composition and Sources of Particulate Matter Measured near Houston, TX: Anthropogenic-Biogenic Interactions

Particulate matter was measured in Conroe, Texas (~60 km north of downtown Houston, Texas) during the September 2013 DISCOVER-AQ campaign to determine the sources of particulate matter in the region. The measurement site is influenced by high biogenic emission rates as well as transport of anthropogenic pollutants from the Houston metropolitan area and is therefore an ideal location to study anthropogenic-biogenic interactions. Data from an Aerosol Chemical Speciation Monitor (ACSM) suggest that on average 64 percent of non-refractory PM1 was organic material, including a high fraction (27%–41%) of organic nitrates. There was little diurnal variation in the concentrations of ammonium sulfate; however, concentrations of organic and organic nitrate aerosol were consistently higher at night than during the day. Potential explanations for the higher organic aerosol loadings at night include changing boundary layer height, increased partitioning to the particle phase at lower temperatures, and differences between daytime and nighttime chemical processes such as nitrate radical chemistry. Positive matrix factorization was applied to the organic aerosol mass spectra measured by the ACSM and three factors were resolved—two factors representing oxygenated organic aerosol and one factor representing hydrocarbon-like organic aerosol. The factors suggest that the measured aerosol was well mixed and highly processed, consistent with the distance from the site to major aerosol sources, as well as the high photochemical activity.

Effects of mineral dust on global atmospheric nitrate concentrations

Abstract. This study assesses the chemical composition and global aerosol load of the major inorganic aerosol components, focusing on mineral dust and aerosol nitrate. The mineral dust aerosol components (i.e., Ca2+, Mg2+, K+, Na+) and their emissions are included in the ECHAM5/MESSy Atmospheric Chemistry model (EMAC). Gas/aerosol partitioning is simulated using the ISORROPIA-II thermodynamic equilibrium model that considers K+, Ca2+, Mg2+, NH4+, Na+, SO42−, NO3−, Cl−, and H2O aerosol components. Emissions of mineral dust are calculated online by taking into account the soil particle size distribution and chemical composition of different deserts worldwide. Presence of metallic ions can substantially affect the nitrate partitioning into the aerosol phase due to thermodynamic interactions. The model simulates highest fine aerosol nitrate concentration over urban and industrialized areas (1–3 µg m−3), while coarse aerosol nitrate is highest close to deserts (1–4 µg m−3). The influence of mineral dust on nitrate formation extends across southern Europe, western USA, and northeastern China. The tropospheric burden of aerosol nitrate increases by 44 % when considering interactions of nitrate with mineral dust. The calculated global average nitrate aerosol concentration near the surface increases by 36 %, while the coarse- and fine-mode concentrations of nitrate increase by 53 and 21 %, respectively. Other inorganic aerosol components are affected by reactive dust components as well (e.g., the tropospheric burden of chloride increases by 9 %, ammonium decreases by 41 %, and sulfate increases by 7 %). Sensitivity tests show that nitrate aerosol is most sensitive to the chemical composition of the emitted mineral dust, followed by the soil size distribution of dust particles, the magnitude of the mineral dust emissions, and the aerosol state assumption.

Characterizing the thermodynamic and chemical composition factors controlling PM2.5 nitrate: Insights gained from two years of online measurements in Hong Kong

In this study, we investigated the seasonal, diurnal, and episodic characteristics of aerosol nitrate concentrations in PM2.5 at a suburban receptor site in Hong Kong using an hourly MARGA sampled dataset. At the site, large spikes in the NO3− concentration have been observed in all seasons, and are easily overlooked in datasets examining 24 h average concentrations. As a key component to PM2.5, nitrate constituted between 5 and 12% of the mass concentration on average per month, but contributed up to 25% during some episodic cases spanning only a few hours. Seasonal variations of PM2.5 nitrate concentrations at the site were driven by temperature and excess [NH4+] in the aerosol, defined as the amount of ammonium in excess of that required for satisfying [NH4+]/[SO42−] = 1.5. The vast majority of winter nitrate data was associated with ammonium-rich aerosols ([NH4+]/[SO42−] > 1.5), with the diurnal variation tracking the availability of excess [NH4+]. Distinctly different than winter conditions, the summer nitrate data was in ammonium-poor regime and tracked nitric acid concentrations, a photochemical tracer. A regression analysis of measured nitrate with the excess [NH4+] shows good correlation in spring, summer and winter (R2: 0.72–0.81), with slopes greater than 0.7 indicating the majority of excess NH4+ is associated with PM2.5 nitrate. It was found that measured nitrate exceeded excess [NH4+] in samples of low excess [NH4+] availability, leading to our finding that nitric acid attaching to sea salt and crustal particles in the fine mode is a non-negligible route (constituting up to ∼20% of the PM2.5 nitrate in this study) to assimilate nitrate into the PM2.5 aerosol. Accounting for both this minor route and the ammonia + nitric acid route may prove useful in modeling efforts to capture PM2.5 nitrate measurement fluctuations, particularly during events of a large influx of alkali particles, such as dust storms.

The MESSy aerosol submodel MADE3 (v2.0b): description and a box model test

Abstract. We introduce MADE3 (Modal Aerosol Dynamics model for Europe, adapted for global applications, 3rd generation; version: MADE3v2.0b), an aerosol dynamics submodel for application within the MESSy framework (Modular Earth Submodel System). MADE3 builds on the predecessor aerosol submodels MADE and MADE-in. Its main new features are the explicit representation of coarse mode particle interactions both with other particles and with condensable gases, and the inclusion of hydrochloric acid (HCl) / chloride (Cl) partitioning between the gas and condensed phases. The aerosol size distribution is represented in the new submodel as a superposition of nine lognormal modes: one for fully soluble particles, one for insoluble particles, and one for mixed particles in each of three size ranges (Aitken, accumulation, and coarse mode size ranges). In order to assess the performance of MADE3 we compare it to its predecessor MADE and to the much more detailed particle-resolved aerosol model PartMC-MOSAIC in a box model simulation of an idealised marine boundary layer test case. MADE3 and MADE results are very similar, except in the coarse mode, where the aerosol is dominated by sea spray particles. Cl is reduced in MADE3 with respect to MADE due to the HCl / Cl partitioning that leads to Cl removal from the sea spray aerosol in our test case. Additionally, the aerosol nitrate concentration is higher in MADE3 due to the condensation of nitric acid on coarse mode particles. MADE3 and PartMC-MOSAIC show substantial differences in the fine particle size distributions (sizes ≲ 2 μm) that could be relevant when simulating climate effects on a global scale. Nevertheless, the agreement between MADE3 and PartMC-MOSAIC is very good when it comes to coarse particle size distributions (sizes ≳ 2 μm), and also in terms of aerosol composition. Considering these results and the well-established ability of MADE in reproducing observed aerosol loadings and composition, MADE3 seems suitable for application within a global model.

Chemically-resolved aerosol eddy covariance flux measurements in urban Mexico City during MILAGRO 2006

Abstract. As part of the MILAGRO 2006 field campaign, the exchange of atmospheric aerosols with the urban landscape was measured from a tall tower erected in a heavily populated neighborhood of Mexico City. Urban submicron aerosol fluxes were measured using an eddy covariance method with a quadrupole aerosol mass spectrometer during a two week period in March, 2006. Nitrate and ammonium aerosol concentrations were elevated at this location near the city center compared to measurements at other urban sites. Significant downward fluxes of nitrate aerosol, averaging −0.2 μg m−2 s−1, were measured during daytime. The urban surface was not a significant source of sulfate aerosols. The measurements also showed that primary organic aerosol fluxes, approximated by hydrocarbon-like organic aerosols (HOA), displayed diurnal patterns similar to CO2 fluxes and anthropogenic urban activities. Overall, 47% of submicron organic aerosol emissions were HOA, 35% were oxygenated (OOA) and 18% were associated with biomass burning (BBOA). Organic aerosol fluxes were bi-directional, but on average HOA fluxes were 0.1 μg m−2 s−1, OOA fluxes were −0.03 μg m−2 s−1, and BBOA fluxes were −0.03 μg m−2 s−1. After accounting for size differences (PM1 vs PM2.5) and using an estimate of the black carbon component, comparison of the flux measurements with the 2006 gridded emissions inventory of Mexico City, showed that the daily-averaged total PM emission rates were essentially identical for the emission inventory and the flux measurements. However, the emission inventory included dust and metal particulate contributions, which were not included in the flux measurements. As a result, it appears that the inventory underestimates overall PM emissions for this location.

Improving ammonia emissions in air quality modelling for France

We have implemented a new module to improve the representation of ammonia emissions from agricultural activities in France with the objective to evaluate the impact of such emissions on the formation of particulate matter modelled with the air quality model CHIMERE. A novel method has been set up for the part of ammonia emissions originating from mineral fertilizer spreading. They are calculated using the one dimensional 1D mechanistic model “VOLT'AIR” which has been coupled with data on agricultural practices, meteorology and soil properties obtained at high spatial resolution (cantonal level). These emissions display high spatiotemporal variations depending on soil pH, rates and dates of fertilization and meteorological variables, especially soil temperature. The emissions from other agricultural sources (animal housing, manure storage and organic manure spreading) are calculated using the national spatialised inventory (INS) recently developed in France. The comparison of the total ammonia emissions estimated with the new approach VOLT'AIR_INS with the standard emissions provided by EMEP (European Monitoring and Evaluation Programme) used currently in the CHIMERE model shows significant differences in the spatiotemporal distributions. The implementation of new ammonia emissions in the CHIMERE model has a limited impact on ammonium nitrate aerosol concentrations which only increase at most by 10% on the average for the considered spring period but this impact can be more significant for specific pollution episodes. The comparison of modelled PM10 (particulate matter with aerodynamic diameter smaller than 10 μm) and ammonium nitrate aerosol with observations shows that the use of the new ammonia emission method slightly improves the spatiotemporal correlation in certain regions and reduces the negative bias on average by 1 μg m−3. The formation of ammonium nitrate aerosol depends not only on ammonia concentrations but also on nitric acid availability, which is often a limiting factor in rural regions in France, and on meteorological conditions. The presented approach of ammonia emission calculation seems suitable for use in chemistry–transport models.

Distributions and regional budgets of aerosols and their precursors simulated with the EMAC chemistry-climate model

Abstract. The new global anthropogenic emission inventory (EDGAR-CIRCE) of gas and aerosol pollutants has been incorporated in the chemistry general circulation model EMAC (ECHAM5/MESSy Atmospheric Chemistry). A relatively high horizontal resolution simulation is performed for the years 2005–2008 to evaluate the capability of the model and the emissions to reproduce observed aerosol concentrations and aerosol optical depth (AOD) values. Model output is compared with observations from different measurement networks (CASTNET, EMEP and EANET) and AODs from remote sensing instruments (MODIS and MISR). A good spatial agreement of the distribution of sulfate and ammonium aerosol is found when compared to observations, while calculated nitrate aerosol concentrations show some discrepancies. The simulated temporal development of the inorganic aerosols is in line with measurements of sulfate and nitrate aerosol, while for ammonium aerosol some deviations from observations occur over the USA, due to the wrong temporal distribution of ammonia gas emissions. The calculated AODs agree well with the satellite observations in most regions, while negative biases are found for the equatorial area and in the dust outflow regions (i.e. Central Atlantic and Northern Indian Ocean), due to an underestimation of biomass burning and aeolian dust emissions, respectively. Aerosols and precursors budgets for five different regions (North America, Europe, East Asia, Central Africa and South America) are calculated. Over East-Asia most of the emitted aerosols (precursors) are also deposited within the region, while in North America and Europe transport plays a larger role. Further, it is shown that a simulation with monthly varying anthropogenic emissions typically improves the temporal correlation by 5–10% compared to one with constant annual emissions.

The contribution of ship emissions to air pollution in the North Sea regions

As a consequence of the global distribution of manufacturing sites and the increasing international division of labour, ship traffic is steadily increasing and is becoming more and more important as an origin of air pollution. This study investigates the impact of ship emissions in coastal areas of the North Sea under conditions of the year 2000 by means of a regional chemistry transport model which runs on a sufficiently high resolution to study air pollution in coastal regions. It was found that northern Germany and Denmark in summer suffer from more than 50% higher sulphate, nitrate and ammonium aerosol concentrations due to contributions from ships. The implementation of a sulphur emission control area (SECA) in the North Sea, as it was implemented at the end of 2007, directly results in reduced sulphur dioxide and sulphate aerosol concentrations while nitrate aerosol concentrations are slightly increased.

Causes of the elevated nitrate aerosol levels during episodic days in Taichung urban area, Taiwan

The purpose of this study is to explore the possible reasons accounting for elevated nitrate aerosol levels during high particulate days (HPD) in Taichung urban area of central Taiwan. To achieve this goal, simultaneous measurements of particulate and gaseous pollutants were carried out from September 2004 to April 2005 using an annular denuder system (ADS). The formation rate of NO 2 to nitrate aerosol, calculated using the relevant chemical reactions, was employed to interpret enhanced nitrate aerosol concentrations during HPD. The observations showed that nitrate concentration during HPD was 14 times higher than that during low particulate days (LPD). The average formation rate during HPD was 4.0% h −1, which was 3.1 times higher than that during LPD. The quantitative analysis showed that the formation rate was mainly influenced by temperature and relative humidity. Lower temperature and higher relative humidity led much nitrate aerosol formation in HPD. Moreover, the residence time analysis of air masses staying over the studied area showed that the slow-motion air retained high nitrate concentrations due to more nitrate aerosol converted from the precursors in NOx-rich areas.

The interpretation of spikes and trends in concentration of nitrate in polar ice cores, based on evidence from snow and atmospheric measurements

Abstract. Nitrate is frequently measured in ice cores, but its interpretation remains immature. Using daily snow surface concentrations of nitrate at Halley (Antarctica) for 2004–2005, we show that sharp spikes (>factor 2) in nitrate concentration can occur from day to day. Some of these spikes will be preserved in ice cores. Many of them are associated with sharp increases in the concentration of sea salt in the snow. There is also a close association between the concentrations of aerosol nitrate and sea salt aerosol. This evidence is consistent with many of the spikes in deposited nitrate being due to the conversion or trapping of gas-phase nitrate, i.e. to enhanced deposition rather than enhanced atmospheric concentrations of NOy. Previously, sharp spikes in nitrate concentration (with concentration increases of up to a factor 4 seen in probably just one snowfall) have been assigned to sharp production events such as solar proton events (SPEs). We find that it is unlikely that SPEs can produce spikes of the kind seen. Taken together with our evidence that such spikes can be produced depositionally, we find that it is not possible to track past SPEs without carrying out a new multi-site and multi-analyte programme. Seasonal and interannual trends in nitrate concentration in cores from any single site cannot be interpreted in terms of production changes until the recycling of nitrate from central Antarctica to coastal Antarctica is better quantified. It might be possible to assess the interannual input of NOy to the Antarctic lower troposphere by using a network of cores to estimate variability in the total annual deposition across the continent (which we estimate to be 9±2×107kg/a – as NO3−), but it will first have to be established that the outflow across the coast can be ignored.

Modeling urban and regional aerosols—Application of the CMAQ-UCD Aerosol Model to Tampa, a coastal urban site

The University of California at Davis (UCD) aerosol module, an internally mixed, sectional aerosol model with dynamic mass transfer between the gas and particle phases, has been coupled to the Community Multiscale Air Quality (CMAQ) model. This paper describes the application of the CMAQ-UCD model to simulate air quality in Tampa, a large city with a population of 2M on the west coast of Florida, USA. Modeled aerosol size and composition distributions are evaluated against size-segregated ambient measurements of SO 4 2 - , NH 4 + , NO 3 - , Na + , and Cl - collected at three Tampa-area sites during May 2002, and against semi-continuous HNO 3 and total aerosol SO 4 2 - , NH 4 + , NO 3 - , and Cl - measurements collected at a single site. Sea-salt emissions over the open ocean and the surf zone are parameterized as a function of modeled wind speed and relative humidity. Modeled total aerosol sulfate and ammonium concentrations and size distributions agree with measurements, with an overall normalized mean bias (NMB) of 2% and −23% and normalized mean error (NME) of 46% and 38%, respectively, and correctly identifying the size bin in which the peak concentration is observed. Sea-salt size distributions are also simulated well, with the distribution dominated by the coarse mode and total aerosol sodium and chloride NMB of −2% and 17% and NME of 32% and 38%. Though the model correctly identifies that nitrate is predominantly in the coarse ( D p > 2.5 μ m ) size sections, aerosol nitrate concentrations are underpredicted by a factor of two. The availability of highly time-resolved measurements provides a unique opportunity to evaluate the model's partitioning of total nitrate and the simulation of chloride depletion as a function of particle size.

Evaluation of a three‐dimensional chemical transport model (PMCAMx) in the eastern United States for all four seasons

A three‐dimensional chemical transport model (PMCAMx) is used to simulate particulate matter (PM) mass and composition in the eastern United States during the four seasons of the year (July 2001, October 2001, January 2002, and April 2002). The model predictions are evaluated against daily average PM2.5 measurements taken throughout the eastern United States by the IMPROVE and STN monitoring networks and the EPA Supersites program. During the spring and summer the model reproduces the measured daily average PM2.5 concentrations with an error of less than 50%, two thirds of the time. The PM2.5 error is less than 30% for 43% of the measurements during these seasons. For the fall and winter the PM2.5 predictions are within 50% of the measurements for 51% of the data points and within 30% for 34% of the time. The performance of the model in reproducing sulfate, organic mass, elemental carbon, and total PM2.5 concentrations varies from average to good depending on the season. Uncertainties in ammonia emissions during the fall cause errors in the corresponding ammonium predictions, while the ammonia emissions inventory appears to be satisfactory during the other seasons. The ability of the model to reproduce the aerosol nitrate concentrations in the spring and summer is limited by difficulties in simulating the heterogeneous nighttime formation rate of nitric acid. During the summer and fall the model performance in reproducing the organic PM concentrations and diurnal patterns is good. The predicted organic PM during the summer is on average 60% primary and 40% secondary. The secondary contribution to organic PM drops to around 20% during the winter. The used average EC emission rate of approximately 0.55 ktons d−1 (0.45 ktons d−1 during the weekends) is consistent with the observed EC concentrations. The nighttime chemistry of NOx determines the PM nitrate concentrations during most days in the winter, spring, and fall and its mathematical description needs improvement. The good agreement between the predicted and observed temporal profiles for most species suggests a reasonable understanding and depiction in the model of the corresponding processes. Additional strengths and limitations of current modeling approaches for this modeling domain and for the different seasons are further discussed.