Dry Nitrogen Deposition Research Articles

Regional evaluation and estimates of atmospheric nitrogen deposition for United States hydrologic units and ecoregions

Atmospheric nitrogen deposition is one of the major contributors to aquatic nitrogen loading in many regions of the United States. In this work, we evaluate the performance of the Comprehensive Air Quality Model with Extensions (CAMx) in predicting dry and wet deposition of nitrogen-containing species in different regions of the US. We compare our estimates to the measurements and model-measurement estimates available from the National Atmospheric Deposition Program (NADP). In addition, we provide estimates of the mass of oxidized and reduced nitrogen depositing to all levels of hydrologic units and ecoregions for the year 2011. Our estimated deposition fluxes align with the trends seen in recent analyses, including higher reduced nitrogen in the Upper Midwest and higher oxidized nitrogen in the Eastern United States and urban areas. Normalized mean bias factor and normalized mean absolute error factor are significantly higher over the Great Lakes. The dry deposition scheme based on Zhang (2001; 2003) showed substantially better performance for oxidized nitrogen deposition throughout the United States than the scheme based on Weseley (1989) and Slinn and Slinn (1980). Normalized mean bias factor and normalized mean absolute error factor are low in the central US for oxidized nitrogen dry deposition. This work provides accessible estimates of nitrogen deposition based on dry and wet deposition for reduced and oxidized species for water quality and ecosystem managers to use when updating the atmospheric loading in their models.

Spatial Variability Analysis of Wheat Nitrogen Yield Response: A Case Study of Henan Province, China

The overapplication of nitrogen to wheat reduces profits and has adverse environmental consequences. Machine learning techniques are employed to identify the factors that hold the most potential in improving nitrogen recommendations. The database used in our analysis consisted of a formula fertilization project, the second soil census of China, and cultivated land fertility evaluation. The results showed that the wheat nitrogen yield response was mainly concentrated around 1300–2400 kg/ha in Henan Province, with the highest values observed in the southern and eastern regions and the lowest in the northern region. The soil nitrogen content, nitrogen fertilizer, wet nitrogen deposition, dry nitrogen deposition, and soil nitrogen supply capacity were mainly concentrated around 0.65–1.30 g/kg, 173–203 kg N/ha, 15–39 kg N/(ha yr), 7–49 kg N/(ha yr), and 105–150 kg N/ha, respectively. When comparing the spatial distribution of the soil nitrogen content, nitrogen fertilizer, and nitrogen deposition, we found that the soil nitrogen supply capacity emerged as the predominant factor controlling wheat grain production. Soil types, precipitation, and temperature demonstrated a notable positive correlation with the soil nitrogen supply capacity. Given this background, it would be unreasonable to rely solely on the soil nitrogen content as the standard for fertilizer management. Particularly in high-yield fields, our focus should be on preserving optimal soil fertility by placing greater emphasis on the soil nitrogen supply capacity rather than simply reducing fertilizer application during wheat growth.

Cloud-based data mapper (CDM): application for monitoring dry deposition of reactive nitrogen

Spatiotemporal monitoring of reactive nitrogen atmospheric deposition is essential for understanding its impact on sensitive ecosystems and quantifying cumulative effects. However, the sparsity of direct surface flux measurements combined with barriers in dissemination are major limiting factors in providing this information to decision makers and non-experts in a timely manner. This work addresses both aspects of this information gap by, 1) utilizing satellite-derived reactive nitrogen dry deposition data products that can be used by decision-makers to supplement the sparse direct surface flux measurements and 2) fill in measurement gaps. Therefore, we have developed a Reactive Nitrogen Flux Mapper (RNFM) component of the interactive Cloud-based Data Mapper (CDM) for providing easy access of satellite-derived reactive nitrogen (defined here as nitrogen dioxide (NO2) and ammonia (NH3)) dry deposition flux spatial maps/data to decision-makers/stakeholders over North America. The RNFM component of CDM has a Graphical User Interface (GUI) that allows users to specify the geographical regions and time periods for computing the average fluxes on the fly using an integrated cloud-based computing platform. The CDM architecture is flexible and can be upgraded in the future to take advantage of upstream satellite data directly on cloud platforms to provide results in near real-time.

Atmospheric dry deposition of nitrogen and phosphorus in Lake Qinghai, Tibet Plateau

To investigate the fluxes and potential sources of the atmospheric dry deposition (ADD) of nitrogen (N) and phosphorous (P) in the Lake Qinghai area, atmospheric deposition dusts were collected monthly at the eastern coast of Lake Qinghai from October 2017 to September 2018. N and P species in the dusts were determined. Results indicated that the annual ADD flux (ADDF) of dusts was 215.64 g m−2 yr−1, 48% of which occurred in March and April. The annual ADDF of N was 7.98 kg ha−1 yr−1, 49% of which was water-soluble total nitrogen, including 23%, 16%, and 10% of water-soluble NH4+-N, NO3−-N, and organic N, respectively. The annual ADDF of the total P was 1.72 kg ha−1 yr−1, only 6% of which was water-soluble. The dusts as well as the N and P in the dusts primarily originated from western Inner Mongolia and northern Gansu, driven by the winter monsoon. However, relatively high concentrations of N and P in the dusts in summer might be related to the southeast Asian monsoon from southern Gansu and northern Sichuan. Overall, the ADDF of the total N was about half of the atmospheric wet deposition flux (AWDF) of N, while the ADDF of the total P was approximately equal to the AWDF of P. However, most of the P in the dusts was not water-soluble and thus not readily bioavailable. Therefore, N and P wet deposition provides most of the readily available nutrients to phytoplankton in Lake Qinghai.

Forest–atmosphere exchange of reactive nitrogen in a remote region – Part I: Measuring temporal dynamics

Abstract. Long-term dry deposition flux measurements of reactive nitrogen based on the eddy covariance or the aerodynamic gradient method are scarce. Due to the large diversity of reactive nitrogen compounds and high technical requirements for the measuring devices, simultaneous measurements of individual reactive nitrogen compounds are not affordable. Hence, we examined the exchange patterns of total reactive nitrogen (ΣNr) and determined annual dry deposition budgets based on measured data at a mixed forest exposed to low air pollution levels located in the Bavarian Forest National Park (NPBW), Germany. Flux measurements of ΣNr were carried out with the Total Reactive Atmospheric Nitrogen Converter (TRANC) coupled to a chemiluminescence detector (CLD) for 2.5 years. The average ΣNr concentration was 3.1 µg N m−3. Denuder measurements with DELTA samplers and chemiluminescence measurements of nitrogen oxides (NOx) have shown that NOx has the highest contribution to ΣNr (∼51.4 %), followed by ammonia (NH3) (∼20.0 %), ammonium (NH4+) (∼15.3 %), nitrate NO3- (∼7.0 %), and nitric acid (HNO3) (∼6.3 %). Only slight seasonal changes were found in the ΣNr concentration level, whereas a seasonal pattern was observed for the contribution of NH3 and NOx. NH3 showed highest contributions to ΣNr in spring and summer, NOx in autumn and winter. We observed deposition fluxes at the measurement site with median fluxes ranging from −15 to −5 ngNm-2s-1 (negative fluxes indicate deposition). Median deposition velocities ranged from 0.2 to 0.5 cm s−1. In general, highest deposition velocities were recorded during high solar radiation, in particular from May to September. Our results suggest that seasonal changes in composition of ΣNr, global radiation (Rg), and other drivers correlated with Rg were most likely influencing the deposition velocity (vd). We found that from May to September higher temperatures, lower relative humidity, and dry leaf surfaces increase vd of ΣNr. At the measurement site, ΣNr concentration did not emerge as a driver for the ΣNrvd. No significant influence of temperature, humidity, friction velocity, or wind speed on ΣNr fluxes when using the mean-diurnal-variation (MDV) approach for filling gaps of up to 5 days was found. Remaining gaps were replaced by a monthly average of the specific half-hourly value. From June 2016 to May 2017 and June 2017 to May 2018, we estimated dry deposition sums of 3.8 and 4.0 kgNha-1a-1, respectively. Adding results from the wet deposition measurements, we determined 12.2 and 10.9 kgNha-1a-1 as total nitrogen deposition in the 2 years of observation. This work encompasses (one of) the first long-term flux measurements of ΣNr using novel measurements techniques for estimating annual nitrogen dry deposition to a remote forest ecosystem.

Inorganic nitrogen and phosphorus in Western European aerosol and the significance of dry deposition flux into stratified shelf waters

Dry deposition of nitrogen (N) and phosphorus (P) from the aerosol phase represents a potential source of nutrients to marine surface waters. To investigate the significance of this deposition pathway, aerosol samples were collected from Penlee Point Atmospheric Observatory in SW England, UK, over a 6-month period (February to July 2015) covering the spring bloom. Samples were analysed for nitrate, ammonium and phosphate and the dry deposition flux of these nutrients calculated to assess its potential impact on primary production in nearby surface seawater. Aerosol-derived N and P deposition fluxes ranged from 2.7 to 620 μmol N m−2 d−1 and 0.16–1.6 μmol P m−2 d−1, respectively. Air mass back trajectory analysis indicated that the highest N fluxes were associated with polluted European air masses, highlighting a significant anthropogenic influence on N-content of aerosols. The N:P ratios of aerosol fluxes and water column concentrations indicated that P deposition was unlikely to be biologically significant in the region. In contrast, aerosol deposition was a significant episodic source of new N to marine phytoplankton after the onset of water column stratification. Carbon fixation estimates indicated that the maximum proportion of new primary production sustained by aerosol-N deposition was 22.4%, a factor of ten higher than the study average. These data suggest that enhanced N-deposition from polluted continental air masses could sustain pulses of surface ocean biological productivity during periods of dissolved N depletion.

Dominant contribution of nitrogen compounds in precipitation chemistry in the Lake Victoria catchment (East Africa)

This work provides a complete chemical characterization of rains collected in the tropical rural site of Mbita (Kenya) on the shores of Lake Victoria (annual rainfall 1259.3 mm). We present a wet nitrogen deposition budget including inorganic and organic dissolved nitrogen in relation with atmospheric sources of gases and particles, precipitation rate and air mass transport. A unique 2 yr monitoring data set (2017–2019), providing 183 rain samples was collected and analyzed according to international standards (WMO/GAW). Considering that precipitation represents the largest contributor of water to the Lake Victoria (80%), this study gives new insights in the seasonality of nutrients wet deposition (WD) inputs in the unique natural resource represented by Lake Victoria and its catchment. Four main contributions to the chemical composition of precipitation, were identified: (a) a 28% terrigenous contribution related to crustal and biomass sources, (b) a 14% marine contribution related to Indian ocean air masses intrusion, (c) a 15% organic contribution due to volatile organic carbon emissions from biomass burning and vegetation and (d) a predominant nitrogenous contribution of 39% due to livestock and fertilizers, biomass burning and neighboring agricultural fires. Ammonium and nitrate volume weighed mean concentrations are 36.75 and 8.88 μeq l−1, respectively. Rain in Mbita is alkaline (pH = 5.8) highlighting neutralization by heterogeneous chemistry. Total nitrogen WD is 8.54 kgN ha−1 yr−1, 58 760 tN yr−1 for the entire lake, with 26% attributed to dissolved organic nitrogen. A total atmospheric deposition of 15 kgN ha−1 yr−1 is estimated taking into account dry deposition estimate from literature, showing that the Lake Victoria ecosystem is exposed to eutrophication. An extensive and regular monitoring of wet and dry nitrogen deposition is highly recommended both in-shore and off-shore to help improving the efficiency of nitrogen use in agricultural areas and reduce nitrogen losses around Lake Victoria.

Long-term atmospheric inorganic nitrogen deposition in West African savanna over 16 year period (Lamto, Côte d’Ivoire)

We present a long term assessment trend of atmospheric inorganic nitrogen deposition in Sub Saharan Africa (2000–2015) using observational and model data. This work proposes a compilation of International Network to study Deposition and Atmospheric chemistry in Africa wet and dry nitrogen deposition fluxes collected at the wet savanna site of Lamto (Côte d’Ivoire). Total deposition calculation takes in account: (a) gaseous (NO2, NH3, HNO3) dry deposition fluxes estimated by considering nitrogen compound concentrations at the monthly scale and modeling average monthly dry deposition velocities, (b) particulate PM10 (pNO3 −, pNH4 +) dry deposition fluxes calculated using the same inferential method and (c) wet deposition (WD) fluxes including ions concentration measurements (NO3 −, NH4 +) in rainwater combined with rainfall amount. We demonstrate for the first time the monthly and annual decreasing trends for dry nitrogen deposition of N-NO2 (−2.33% month−1 and −2.54% yr−1) and N-NH3 (−2.55% month−1 and −2.89% yr−1), but increasing trends for dry deposition of N-HNO3 (+1.00% month−1) and WD of N-NO3 − (+1.67% month−1 and +2.13% yr−1) and N-NH4 + (+2.33% month−1 and +3.36% yr−1). Dry season N-NO2 deposition flux decline shows agreement with long term trend in NOx emissions by biomass burning. Increasing trends for wet N deposition signals a gradual increase of nitrogen fertilizers use in agricultural practices in the Lamto area. Results also show no significant trend in total N deposition over the 16 year study period explained by the compensation of decreasing and increasing trends for dry and wet N deposition, respectively. However, at the annual scale, the mean total N deposition flux is estimated to 10.3 ± 1.2 kgN ha−1 yr−1 over the 16 year period, indicating an increase of 8% compared to the period 2000–2007.

Concentrations and biosphere–atmosphere fluxes of inorganic trace gases and associated ionic aerosol counterparts over the Amazon rainforest

Abstract. The Amazon rainforest presents a unique, natural laboratory for the study of surface–atmosphere interactions. Its alternation between a near-pristine marine-influenced atmosphere during the wet season and a vulnerable system affected by periodic intrusions of anthropogenic pollution during the dry season provides an opportunity to investigate some fundamental aspects of boundary-layer chemical processes. This study presents the first simultaneous hourly measurements of concentrations, fluxes, and deposition velocities of the inorganic trace gases NH3, HCl, HONO, HNO3, and SO2 as well as their water-soluble aerosol counterparts NH4+, Cl−, NO2-, NO3- and SO42- over the Amazon. Species concentrations were measured in the dry season (from 6 October to 5 November 2017), at the Amazon Tall Tower Observatory (ATTO) in Brazil, using a two-point gradient wet-chemistry instrument (GRadient of AErosols and Gases Online Registration, GRAEGOR) sampling at 42 and 60 m. Fluxes and deposition velocities were derived from the concentration gradients using a modified form of the aerodynamic gradient method corrected for measurement within the roughness sub-layer. Findings from this campaign include observations of elevated concentrations of NH3 and SO2 partially driven by long-range transport (LRT) episodes of pollution and the substantial influence of coarse Cl− and NO3- particulate on overall aerosol mass burdens. From the flux measurements, the dry season budget of total reactive nitrogen dry deposition at the ATTO site was estimated as −2.9 kg N ha-1a-1. HNO3 and HCl were deposited continuously at a rate close to the aerodynamic limit. SO2 was deposited with an average daytime surface resistance (Rc) of 28 s m−1, whilst aerosol components showed average surface deposition velocities of 2.8 and 2.7 mm s−1 for SO42- and NH4+, respectively. Deposition rates of NO3- and Cl− were higher at 7.1 and 7.8 mm s−1, respectively, reflecting their larger average size. The exchange of NH3 and HONO was bidirectional, with NH3 showing emission episodes in the afternoon and HONO in the early morning hours. This work provides a unique dataset to test and improve dry deposition schemes for these compounds for tropical rainforest, which have typically been developed by interpolation from conditions in temperate environments. A future campaign should focus on making similar measurements in the wet season in order to provide a complete view of the annual pattern of inorganic trace gas and coarse aerosol biosphere–atmosphere exchange over tropical rainforest.

Assessment of atmospheric pollutant dispersion from mobile sources in Antarctica: a case study of Vecherny Oasis

ABSTRACT The first results of high spatial resolution modeling of the dispersion of atmospheric pollutant emissions from mobile sources in Antarctica are presented. Options for assessing the impacts of vehicles on air quality in Antarctica are discussed using the Belarusian Antarctic Station at Vecherny Oasis, Enderby Land, as an example. Sources of input data and collection and processing of these data are described. Surface air concentrations of nitrogen dioxide (NO2), and concentrations and dry atmospheric deposition levels of respirable suspended particulate matter (PM10) are estimated for different averaging periods and scenarios of the volume and spatial allocation of emission. Nowadays, the number of vehicles in the oasis is small. However, by the estimates, the maximum hourly concentrations of PM10 and NO2 at certain locations and under certain conditions could reach perceptible share of the air quality guideline value. Some other Antarctic stations have significant vehicle population, and assessment of their air impacts will be especially important. Suggestions for improvements of the quantitative assessment of impacts of mobile emission sources in Antarctica are proposed.

Dry deposition of reactive nitrogen to different ecosystems across eastern China: A comparison of three community models

The assessment of nitrogen ecosystem loads mostly use the method of sampling observation combined with numerical model to estimate the spatial distribution pattern of nitrogen dry deposition flux. The selection of models is important which directly affects the reliability of the deposition flux results. In this study, the performance of three widely used models (WRF-Chem, EMEP, CMAQ) are compared. The dry deposition fluxes of typical active nitrogen components over eastern China showed uncertainties by a factor of 0.5 ~ 2 between the oxidized nitrogen (OXN) results of the three models and the observation network while the reduced nitrogen (RDN) simulation results are underestimated by a quarter of the observation reports. These three models show different results on four typical ecosystems: simulation of EMEP got the highest for OXN dry deposition flux on each ecosystem (urban 14.94 ± 4.92kgN ⋅ ha−1 ⋅ yr−1, cropland/grassland 5.53 ± 5.11kgN ⋅ ha−1 ⋅ yr−1, forest 4.75 ± 4.32kgN ⋅ ha−1 ⋅ yr−1, water bodies 1.48 ± 1.53kgN ⋅ ha−1 ⋅ yr−1); WRF-Chem has the highest value of RDN on the urban (8.91 ± 6.44kgN ⋅ ha−1 ⋅ yr−1) and water bodies (1.01 ± 1.44kgN ⋅ ha−1 ⋅ yr−1) while EMEP is highest in cropland/grassland (3.42 ± 3.43kgN ⋅ ha−1 ⋅ yr−1) and forest (2.34 ± 1.94kgN ⋅ ha−1 ⋅ yr−1). CMAQ is in medium range for both OXN and RDN simulations on each ecosystem. Compare with the critical loads, CMAQ generates more exceeded critical load areas than WRF-Chem and EMEP on cropland/grassland and forests ecosystem. For water bodies, WRF-chem and CMAQ showed higher exceeding critical load areas than EMEP. In summary, EMEP generally underestimates while the CMAQ and WRF-Chem model would overestimate the impacts on the ecosystems. So, policy implementation needs special attention accounting the difference of simulation effect with different models.

Fluxes of nitrogen oxides above a subtropical forest canopy in China.

Dry deposition of Nitrogen (N) in forests is commonly estimated from inferential method and/or throughfall measurements, with inevitable uncertainty. In this study, we applied an aerodynamic gradient method to directly measure the nitrogen oxides (NOx) flux above the canopy of a subtropical forest in southeastern China for two consecutive years. The flux and transfer velocity generally reached the maximum absolute values in the midday, with the largest diurnal maximum of absolute flux values observed in the winter of 2015 and that of transfer velocity in the autumn of 2015. The annual average transfer velocity was -0.79 and -0.38cms-1 in 2015 and 2016, respectively. Although the net downward NOx fluxes predominated for both years, upward flux (net emission) of NOx was observed during spring months, which reflected the possible bi-directional exchange balanced by soil-atmosphere and foliage-atmosphere exchanges. The NOx concentration seemed to be the most important factor controlling the NOx exchange above canopy, and could mainly explain the seasonal variation of N deposition. The linear regression between the NOx flux and concertation was explored, and it was observed that the deposition of NOx was offset by possible underlayer emission of NOx when the ambient NOx concentration below1.7 ppbv and 1.9 ppbv at night and in the day, respectively. The average dry deposition of NOx for the two years was 6.28±0.06kgNha-1 a-1, >40% of which might be uptake by the canopy, estimated by comparing the wet/throughfall deposition measurement of nitrate with the observation of NOx flux. This indicated the importance of stomatal uptake of NOx in nitrogen budget in subtropical forests.

Quantifying atmospheric reactive nitrogen concentrations, dry deposition, and isotope dynamics surrounding a Marcellus Shale well pad

Unconventional natural gas (UNG) extraction activities have become important contributors to regional NOx emissions inventories. Currently, there is a knowledge gap in the amount of total N deposition surrounding well pads undergoing UNG extraction despite the fact that some areas with extensive natural gas extraction activity are already in exceedance of nitrogen critical loads. In this study, we measured the magnitude of total dry N deposition from NO2, HNO3, and NH3 attributable to the development of two UNG wells at a Marcellus Shale well pad study site. This study documents concentrations, deposition fluxes, and isotope values of NO2, HNO3, O3, and NH3 up- and down-wind along a 750-m well pad passive sampling transect across a 16-acre well pad containing two unconventional wells during all phases of development and extraction comprising fifteen distinct sampling periods. An access road transect was also utilized to explore reactive N dynamics in a near-road environment on the well pad where NO2 concentration and isotope dynamics were highly correlated with daily traffic count (r2 = 0.78–0.88, p < 0.01). An onsite chemiluminescent NO2 source apportionment model was compared against δ15N–NO2 and δ18O–NO2 source apportionment models (r2 = 0.57 and 0.82 respectively), demonstrating the possible utility of δ18O–NO2 as a source apportionment tool in near-source environments. In addition, the δ15N–NO2 source apportionment method compared well against a background-subtraction method (slope = 0.82, r2 = 0.88, p < 0.001) during non-wintertime conditions and was used to find N loadings directly attributable to well pad activities. In total, the total N deposition across the transect, attributable to well pad activities, utilizing industry's best management practices, ranged from 0.16 to 0.55 kg N ha−1 yr−1. This magnitude of well pad attributable N deposition is high enough to result in exceedances of nitrogen critical loads in areas with high well count densities and high baseline N deposition.

Atmospheric Dry Deposition of Water-Soluble Nitrogen to the Subarctic Western North Pacific Ocean during Summer

To estimate dry deposition flux of atmospheric water-soluble nitrogen (N), including ammonium (NH4+), nitrate (NO3−), and water-soluble organic nitrogen (WSON), aerosol samples were collected over the subarctic western North Pacific Ocean in the summer of 2016 aboard the Korean icebreaker IBR/V Araon. During the cruise, concentrations of NH4+, NO3−, and WSON in bulk (fine + coarse) aerosols ranged from 0.768 to 25.3, 0.199 to 5.94, and 0.116 to 14.7 nmol m−3, respectively. Contributions of NH4+, NO3−, and WSON to total water-soluble N represented ~74%, ~17%, and ~9%, respectively. Water-soluble N concentrations showed a strong gradient from the East Asian continent to the subarctic western North Pacific Ocean, indicating that water-soluble N species were mainly derived from anthropogenic or terrestrial sources. During sea fog events, coarse mode NO3− was likely to be scavenged more efficiently by fog droplets than fine mode NO3−; besides, WSON was detected only in fine mode, suggesting that there may have been a significant influence of sea fog on WSON, such as the photochemical conversion of WSON into inorganic N. Mean dry deposition flux for water-soluble total N (6.3 ± 9.4 µmol m−2 d−1) over the subarctic western North Pacific Ocean was estimated to support a minimum carbon uptake of 42 ± 62 µmol C m−2d−1 by using the Redfield C/N ratio of 6.625.

Atmospheric Nitrogen Dioxide, Nitric Acid, Nitrate Nitrogen Concentrations, and Wet and Dry Deposition Rates in a Double Rice Region in Subtropical China

Nitrogen dioxide (NO2) and nitric acid (HNO3) are nitrogen-containing acidic gases in the atmosphere, and they are important precursors of nitrate in aerosol and rainwater. The emission intensity of atmospheric nitrogen oxides is high in the subtropical region of China, but the concentrations and deposition rates of atmospheric nitrogen dioxide, nitric acid, particulate nitrate-nitrogen (NO3--Np), and rainwater nitrate-nitrogen (NO3--Nr) in a double rice region in subtropical China are still unclear,. In this study, the atmosphere concentrations of NO2-N, HNO3-N, NO3--Np in PM10, and NO3--Nr and related meteorological parameters were simultaneously monitored in a typical double rice region within a subtropical hilly region of China, with the aim of determining the characteristics and influencing factors of NO2-N, HNO3-N, NO3--Np, and NO3--Nr concentrations and quantifying the wet and dry deposition rates. The results showed that the annual mean concentrations of NO2-N, HNO3-N, NO3--Np, and NO3--Nr were 4.2 μg·m-3, 0.7 μg·m-3, 4.0 μg·m-3, and 1.0 mg·L-1, respectively, and the deposition rates were 1.5, 3.2, 2.3, and 6.1 kg·hm-2, respectively. The NO2-N concentrations were negatively correlated with air temperatures, and the HNO3-N concentrations were negatively correlated with wind speeds. TheNO3--Np concentrations were negatively correlated with air temperatures, positively correlated with NO2-N concentrations, but not significantly correlated with HNO3-N concentrations, thus indicating that NO2-N concentrations were an important limiting factor forNO3--Np pollution in this study area. The NO3--Nr concentrations were negatively correlated with rainfall, as well as the concentrations of HNO3-N and NO3--Np. The annual total dry and wet depositions of the atmospheric NO2-N, HNO3-N, NO3--Np, and NO3--Nr were 13.0 kg·hm-2, which indicates that these compounds are important sources of nitrogen in paddy fields and may have significant impacts on paddy fields and surrounding ecosystems.

Enhancement of dry deposition of PM2.5 nitrate in a cool-temperate forest

Fine particulate matter (PM2.5) is one of most concerning air pollutants in East Asia. In order to further understand the dry deposition process of PM2.5, measurements were carried out in a cool-temperate forest located in a remote area of northern Japan. These measurements took into account the improvement of simulations by chemical transport models in this region. We measured the vertical profiles of fine (PM2.5), coarse aerosol components, HNO3, and other relevant gases using two denuder/filter-pack sampling systems together with three 4-stage filter-pack sampling systems from July 22, 2017 to August 07, 2017. The vertical profile measurements clearly showed the difference in dry deposition process between fine NO3− and fine SO42−. Concentration gradients of fine NO3− from over to under the canopy were significantly higher than those of fine SO42−. Since most of fine NO3− existed as NH4NO3, it was considered that these high gradients of NO3− were associated with the process of the conversion between NH4NO3 and HNO3/NH3. The concentration gradients of fine NO3− increased with the increase in daytime air temperature from over to under the canopy. The simulations of the thermodynamic equilibrium of NH4NO3 at canopy surface temperatures using ISORROPIA II showed that the equilibrium shift of NH4NO3 into the gas phase during the daytime hot canopy can decrease fine NO3− by approximately 9% when compared to normal atmospheric conditions. The volatilized HNO3 was most probably immediately removed by the canopy as the HNO3 concentrations near the canopy were extremely low. Therefore, the equilibrium shift enhanced the dry deposition of nitrogen within fine NO3−.

Nitrogen dry deposition to Lake Superior and Lake Michigan

Abstract Nitrate (NO3−) levels in Lake Superior have increased from historic levels of about 5 μM to its current concentration of about 25 μM. The atmosphere makes a substantial contribution to the nitrogen budgets for Lake Superior and Lake Michigan. This study provides a more well-defined estimate of nitrogen dry deposition rates derived from the measurement of over-water concentrations, and in situ meteorological measurements, which were input into the Resistance Model. We obtained a nitrogen dry deposition rate of [(3.41 ± 2.26) × 107 kg N/yr; (5.90 ± 3.91) kg N/ha/yr] over Lake Michigan, and [(1.54 ± 1.06) × 107 kg N/yr; (1.87 ± 1.27) kg N/ha/yr] over Lake Superior. Nitric acid (HNO3), which originates from the combustion of fossil fuels, contributes 84% of the total nitrogen dry deposition to Lake Michigan; and 66% to Lake Superior. Ammonia (NH3), which originates from agricultural activities and gasoline combustion, is the second highest contributor of nitrogen dry deposition to both lakes: contributing 13% to Lake Michigan and 32% to Lake Superior. The nitrogen dry deposition is approximately 68% of the nitrogen wet deposition over Lake Superior, and approximately 80% of wet deposition over Lake Michigan. The over-water dry deposition velocity of HNO3 and NH3 were also evaluated. We obtained morning deposition velocities of 0.099 cm/s for NH3 and 0.095 cm/s for HNO3; and afternoon values of 0.137 cm/s for NH3 and 0.132 cm/s for HNO3. Another key finding is that the atmospheric concentrations of nitrogen compounds near Lake Michigan and Lake Superior have decreased since 2003.

Atmospheric dry nitrogen deposition and its relationship with local land use in a high nitrogen deposition region

Atmospheric dry nitrogen (N) deposition is a key process in the N cycle of the earth, and the land-use type might profoundly affect N deposition. However, little is known about the relationship between local land-use types and dry N deposition in an urban-rural region. Therefore, this study observed atmospheric dry N deposition in continuous 24 months (from January 2015 to December 2016) and then applied space influence area analysis to explore the multi-scale relationship between land use types and dry N deposition. The results showed that the key land use areas in the range of 4 km of monitoring point were closely related to dry N deposition flux. The spatial and temporal variation of atmospheric dry N deposition was explained by "source" and "sink" land use types. For “source” land use type, agro-facility area contributed to dry NH4⁺-N deposition, while dry NO3−-N and dry organic N deposition was attributed to urban and high way areas. On the other hand, agricultural land with suitable and limited fertilizer application, country road without hardening and forests were considered as the “sink” for N deposition. These “sink” land use types should be considered actively in urban and rural planning.

Dry and Wet Deposition of Atmospheric Nitrogen in Small Catchments

Atmospheric nitrogen deposition is one of the important pathways for the transmission of terrestrial pollutants and nutrients to aquatic ecosystems. A considerable amount of nitrogen substances is delivered into the surface water environment via atmospheric deposition in the catchments, which are greatly affected by human activities, and have serious negative effects on the nutrient level and aquatic habitat environment. The Shixia Catchment north of the Miyun Reservoir was selected as study area. Samples of particulate dry and wet deposition were collected and tested. Subsequently, the variation of the atmospheric nitrogen deposition flux and factors impacting it were determined; ultimately, the contribution of atmospheric nitrogen deposition to the nitrogen budget of the catchment was estimated. The major results are as follows:① The total nitrogen deposition flux of particulate dry and wet deposition is characterized by a significant temporary variation. Regarding the total nitrogen, the ammonia nitrogen and wet deposition flux are the highest in summer and dissolved organic nitrogen is the highest in spring. However, the flux of nitrate nitrogen shows no notable seasonal fluctuation. The general trend of the particulate dry deposition flux is supreme in winter, followed by that in autumn and spring. In detail, the total nitrogen and ammonia nitrogen deposition are the largest in winter, while the seasonal variance of nitrate nitrogen appears to be less apparent. The dry deposition flux of dissolved organic nitrogen is the greatest in autumn. ② The total flux of nitrogen deposition is 43.14 kg·hm-2 in the Shixia Catchment; the wet and dry deposition flux account for 39.85% and 60.15%, respectively. ③ Rainfall and wind speed conditions are important factors impacting the atmospheric nitrogen deposition. The rainfall and rainfall intensity are negatively correlated with the wet nitrogen deposition concentration. With respect to the particulate dry deposition, the average wind speed during the monitoring period is important to ammonia nitrogen. ④ The contribution of atmospheric nitrogen deposition to the total nitrogen input in this catchment is approximately 15.09%, second only to livestock and poultry farming and rural life. The results of this study provide information that is useful for the nitrogen management in the catchment.

An important mechanism of regional O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; transport for summer smog over the Yangtze River Delta in eastern China

Abstract. Severe ozone (O3) pollution episodes plague a few regions in eastern China at certain times of the year, e.g., the Yangtze River Delta (YRD). However, the formation mechanisms, including meteorological factors, contributing to these severe pollution events remain elusive. A severe summer smog stretched over the YRD region from 22 to 25 August 2016. This event displayed hourly surface O3 concentrations that exceeded 300 µg m−3 on 25 August in Nanjing, an urban area in the western YRD. The weather pattern during this period was characterized by near-surface prevailing easterly winds and continuous high air temperatures. The formation mechanism responsible for this O3 pollution episode over the YRD region, particularly the extreme values over the western YRD, was investigated using observation data and by running simulations with the Weather Research and Forecasting model with Chemistry (WRF-Chem). The results showed that the extremely high surface O3 concentration in the western YRD area on 25 August was largely due to regional O3 transport in the nocturnal residual layer (RL) and the diurnal change in the atmospheric boundary layer. On 24 August, high O3 levels, with peak values of 220 µg m−3, occurred in the daytime mixing layer over the eastern YRD region. During nighttime from 24 to 25 August, a shallow stable boundary layer formed near the surface which decoupled the RL above it from the surface. Ozone in the decoupled RL remained quite constant, which resulted in an O3-rich “reservoir” forming in this layer. This reservoir persisted due to the absence of O3 consumption from nitrogen oxide (NO) titration or dry deposition during nighttime. The prevailing easterly winds in the lower troposphere governed the regional transport of this O3-rich air mass in the nocturnal RL from the eastern to the western YRD. As the regional O3 transport reached the RL over the western YRD, O3 concentrations in the RL accumulated and rose to 200 µg m−3 over the western Nanjing site during the sunrise hours on 25 August. The development of the daytime convective boundary layer after sunrise resulted in the disappearance of the RL, as the vertical mixing in the convective boundary layer uniformly redistributed O3 from the upper levels via the entrainment of O3-rich RL air down to the O3-poor air at the ground. This net downward transport flux reached up to 35 µg m−3 h−1, and contributed a considerable surface O3 accumulation, resulting in severe daytime O3 pollution during the summer smog event on 25 August in the western YRD region. The mechanism of regional O3 transport through the nocturnal RL revealed in this study has great implications regarding understanding O3 pollution and air quality change.