Relative Importance Of Dry Deposition Research Articles

Large-eddy simulation of the optimal street-tree layout for pedestrian-level aerosol particle concentrations – A case study from a city-boulevard

Abstract Street vegetation has been found to have both positive and negative impacts on pedestrian-level air quality, but the net effect has remained unclear. In this study, the effect of street trees on aerosol mass (PM10 and PM2.5) and number in a boulevard-type street canyon with high traffic volumes in Helsinki is examined using the large-eddy simulation model PALM. Including a detailed aerosol module and a canopy module to comprise permeable trees, PALM allows to examine the effect of street trees in depth. The main aim is to understand the relative importance of dry deposition and the aerodynamic impact of street trees on the different aerosol measures at pedestrian-level and to find a suitable street-tree layout that would minimise the pedestrian-level aerosol particle concentrations over the boulevard pavements. The layout scenarios were decided together with urban planners who needed science-based knowledge to support the building of new neighbourhoods with boulevard-type streets in Helsinki. Two wind conditions with wind being parallel and perpendicular to the boulevard under neutral atmospheric stratification are examined. Adding street trees to the boulevard increases aerosol particle concentrations on the pavements up to 123%, 72% and 53% for PM10, PM2.5 and total number, respectively. This shows decreased ventilation to be more important for local aerosol particle concentrations than dry deposition on vegetation. This particularly for PM10 and PM2.5 whereas for aerosol number, dominated by small particles, the importance of dry deposition increases. Therefore the studied aerosol measure is important when the effect of vegetation on pedestrian-level air quality is quantified. Crown volume fraction in the street space is one of the main determining factors for elevated mass concentrations on the pavements. The lowest pedestrian-level mass concentrations are seen with three rows of trees of variable height, whereas the lowest number concentrations with four rows of uniform trees. The tree-height variation allows stronger vertical turbulent transport with parallel wind and largest volumetric flow rates with perpendicular wind. Introducing low (height The results show how street trees in a boulevard-type street canyon lead to decreased pedestrian-level air quality with the effect being particularly strong for larger aerosol particles. However, with careful planning of the street vegetation, significant reductions in pedestrian-level aerosol particle concentrations can be obtained.

Assessment of the Evolution of Nitrate Deposition Using Remote Sensing Data Over the Yangtze River Delta, China

Along with the increasing concentration of nitrogenous pollutants emitted from the combustion and fertilizers, atmospheric nitrogen (N) deposition has become a great concern due to its significant ecological effect, especially in severe N emission regions such as the Yangtze River Delta (YRD) in east China. The spatial and temporal nitrate deposition fluxes were conducted using satellite data in YRD from 1996 to 2011. Our study reveals significant spatial variations of nitrate deposition in YRD region. In general, the fluxes of total (dry plus wet) nitrate deposition in YRD were up to 22.03 kg·N·ha-1·yr-1 with large loading received in winter. Most high fluxes appeared over urban (37.72 kg·N·ha-1·yr-1) and cropland (30.29 kg·N·ha-1·yr-1) areas. During the study period (1996–2011), a significant increasing trend of nitrate deposition was clearly observed in YRD with an annual rate of 1.33 kg·N·ha-1·yr-1. The spatial patterns of estimated nitrate deposition also showed that there were much higher fluxes and annual increasing trend in the middle region of YRD, i.e., the metropolitan areas contained Shanghai–Nanjing–Hangzhou cities, than in other areas. Our results also reveal that dry nitrate deposition contributed more than 50% of the total nitrate deposition over all provinces and land covers except coastal sea (14.27%), which indicates the relative importance of dry deposition to the total nitrate deposition in the YRD region. Therefore, it is necessary to consider both dry and wet deposition when evaluating the influences of nitrate deposition on environment and ecosystem health.

Atmospheric ammonia measurements at low concentration sites in the northeastern USA: implications for total nitrogen deposition and comparison with CMAQ estimates

We evaluated the relative importance of dry deposition of ammonia (NH3) gas at several headwater areas of the Susquehanna River, the largest single source of nitrogen pollution to Chesapeake Bay, including three that are remote from major sources of NH3 emissions (CTH, ARN, and KEF) and one (HFD) that is near a major agricultural source. We also examined the importance of nitrogen dioxide (NO2) deposition at one of these sites. Over the past decade, increasing evidence has suggested that NH3 deposition, in particular, may be an important contributor to total nitrogen deposition and to downstream nitrogen pollution. We used Ogawa passive samplers to measure NH3 concentrations over several years (2006–2011) for CTH, and primarily in 2008 and 2009 for the other sites. NO2 was measured at CTH mainly in 2007. Chamber calibration studies for NH3 and NO2, and field comparisons with annular denuders for NH3, validated the use of these passive samplers over a range of temperatures and humidity observed in the field, if attention is given to field and laboratory blank issues. The annual mean NH3 concentrations for the forested sites were 0.41 ± 0.03, 0.41 ± 0.06 and 0.25 ± 0.08 µg NH3/m3 for CTH, ARN and KEF, respectively. NO2 passive sampler mean annual concentration was 3.19 ± 0.42 µg NO2/m3 at CTH. Direct comparison of our measured values with the widely used Community Multiscale Air Quality (CMAQ) model (v4.7.1) show reasonably good agreement. However, the model-based estimates tend to be lower than our measured average NH3 concentration, by 8 % at our best studied site where we measured moderately low concentration, and up to 60 % at our site with the lowest concentrations and lowest sampling frequency. CMAQ NO2 concentration estimates were substantially higher than our measured values. Along a transect of sites near a source of NH3 emissions from animal agriculture, we found NH3 concentrations to be far higher than predicted for this area by the CMAQ model. This is not surprising, since the CMAQ model integrates over a relatively wide area. The higher NH3 concentrations we measured were generally within 1 km of the agricultural source. Such locally high atmospheric concentrations leading to locally high deposition may be ecologically significant. Analysis of such issues requires more locally scaled estimates than can be provided from the 12 km grid scale estimates of CMAQ used in this study. We estimated deposition of NH3 and NO2 using our concentration data and modified (concentration-weighted) deposition velocities derived from the CMAQ model. We estimate dry gaseous NH3 deposition as 2.0 ± 0.3 (CTH), 2.2 ± 0.4 (ARN) and 1.4 ± 0.7 kg N/ha-year (KEF). NO2 deposition at CTH is estimated to be 0.16 kg N/ha-year. NO2 deposition is a very small component of total nitrogen deposition at this site. On the other hand, NH3 deposition is either the largest or the second largest form of dry deposition at our sites, depending on how total N deposition is estimated. Based on total deposition best estimates of 9.2 kg N/ha for CTH and 8.6 kg N/ha for KEF, NH3 contributes between 16 and 22 % of total nitrogen deposition. Such deposition has normally not been measured through traditional national monitoring programs, yet is significant as a source of nitrogen pollution to areas such as the highly nitrogen-sensitive Chesapeake Bay ecosystem.

Estimation of dry deposition and canopy exchange in Chinese fir plantations

Based on the data from three years of chemical monitoring for incident precipitation, throughfall and stemflow, the net throughfall flux (NTF) was analyzed to evaluate the relative importance of dry deposition and canopy exchange in two Chinese fir ( Cunninghamia lanceolata) plantations with different distances from the pollution sources in the suburbs of Nanping, China. NTF analysis showed consistent canopy effects on rainfall chemistry, with H +, K +, Na +, Ca 2+, Mg 2+ and SO 4 2− added to rainfall by foliage, whereas NH 4 + and NO 3 − ions were absorbed from precipitation. Regression analysis of single-event data showed that canopy exchange (leaching and uptake) was a dominant process in controlling NTF for most solutes. However, dry deposition contributed significantly to NTF for Na + and Ca 2+ at the two sites, and for SO 4 2− and NO 3 − at XQF site close to emission sources. Increased precipitation acidity caused increased leaching of Ca 2+. NH 4 + concentration in precipitation had a significant positive effect on the retention of it in the canopy, suggesting that uptake of NH 4 + by canopy is an active process. The uptake of nitrogen by canopy is strongly controlled by the stand ages, with annual uptake of 2.97 kg ha −1 at FFC site where the stand is in the fast-growing stage (16-year old), but only 0.56 kg ha −1 in the mature stand (41-year old) at XQF.

Analysis of regional budgets of sulfur species modeled for the COSAM exercise

The COSAM intercomparison exercise (comparison of large-scale sulfur models) was organized to compare and evaluate the performance of global sulfur cycle models. Eleven models participated, and from these models the simulated surface concentrations, vertical profiles and budget terms were submitted. This study focuses on simulated budget terms for the sources and sinks of SO2 and sulfate in three polluted regions in the Northern Hemisphere, i.e., eastern North America, Europe, and Southeast Asia. Qualitatively, features of the sulfur cycle are modeled quite consistently between models, such as the relative importance of dry deposition as a removal mechanism for SO2, the importance of aqueous phase oxidation over gas phase oxidation for SO2, and the importance of wet over dry deposition for removal of sulfate aerosol. Quantitatively, however, models may show large differences, especially for cloud-related processes, i.e., aqueous phase oxidation of SO2 and sulfate wet deposition. In some cases a specific behavior can be related to the treatment of oxidants for aqueous phase SO2 oxidation, or the vertical resolution applied in models. Generally, however, the differences between models appear to be related to simulated cloud (micro-)physics and distributions, whereas differences in vertical transport efficiencies related to convection play an additional rôle. The estimated sulfur column burdens, lifetimes and export budgets vary between models by about a factor of 2 or 3. It can be expected that uncertainties in related effects which are derived from global sulfur model calculations, such as direct and indirect climate forcing estimates by sulfate aerosol, are at least of similar magnitude.

Description and evaluation of a model of deposition velocities for routine estimates of dry deposition over North America. Part II: review of past measurements and model results

Numerical sensitivity tests and four months of complete model runs have been conducted for the Routine Deposition Model (RDM). The influence of individual model inputs on dry deposition velocity as a function of land-use category (LUC) and pollutant (SO 2, O 3, SO 2− 4 and HNO 3) were examined over a realistic range of values for solar radiation, stability and wind speed. Spatial and temporal variations in RDM deposition velocity ( V d) during June – September 1996 time period generated using meteorological input from a mesoscale model run at 35 km resolution over north-eastern North America were also examined. Comparison of RDM V d values to a variety of measurements of dry deposition velocities of SO 2, O 3, SO 2− 4 and NHO 3 that have been reported in the literature demonstrated that RDM produces realistic results. Over northeastern NA RDM monthly averaged dry deposition velocities for SO 2 vary from 0.2 to 3.0 cm s −1 with the highest deposition velocities over water surfaces. For O 3, the monthly averaged dry deposition velocities are from 0.05 to 1.0 cm s −1 with the lowest values over water surfaces and the highest over forested areas. For HNO 3, the monthly averaged dry deposition velocities have the range of 0.5 to 6 cm s −1, with the highest values for forested areas. For SO 2− 4, they range from 0.05–1.5 cm s −1, with the lowest values over water and the highest over forest. The monthly averaged dry deposition velocities for SO 2 and O 3 are higher in the growing season compared to the fall, but this behaviour is not apparent for HNO 3 and sulphate. In the daytime, the hourly averaged dry deposition velocities for SO 2, O 3, SO 2− 4 and HNO 3 are higher than that in the nighttime over most of the vegetated area. The diurnal variation is most evident for surfaces with large values for leaf area index (LAI), such as forests. Based on the results presented in this paper, it is concluded that RDM V d values can be combined with measured air concentrations over hourly, daily or weekly periods to determine dry deposition amounts and with wet deposition measurements to provide seasonal estimates of total deposition and estimates of the relative importance of dry deposition.

Temporal and spatial variations of airborne Mg, Cl, Na, Ca and K in rural areas of Norway

The Norwegian air chemistry monitoring network includes measurements of gaseous and particulate compounds in air at 12 rural sites. The sampling method is designed to determine concentration levels of sulphur and nitrogen compounds and uses a three-stage filter pack sampler. Concentration levels of base-cations and sea salts may also be determined from the chemical analysis of the filter extracts. In this study, concentration levels of water soluble Mg, Cl, Na, Ca and K in air have been investigated in terms of temporal, seasonal and spatial trends during the period 1986–1996. The results have been combined with precipitation chemistry data to estimate total deposition, and to evaluate the relative importance of dry deposition. While Na, Cl and Mg are of marine origin, which is clearly reflected in concentration ratios and spatial gradients, Ca and K are of a mixed origin. Air trajectory analyses indicate that air masses originating in Eastern Europe result in significantly higher airborne concentrations of non-sea salt Ca and K, than air from other sectors. Major sources of airborne Ca and K in Norway are expected to be of anthropogenic origin from combustion plants and industrial processes in Eastern Europe, while aeolian dust from agricultural areas within Europe or from Sahara is of limited importance. Concentration levels of non-sea salt base-cations in air were 20–30% lower during the 1990s, compared to 1986–1989. Precipitation chemistry data available since the early 1980s indicate even larger reductions, and in the order of 50%. Total deposition of non-sea salt base-cations during 1993–1996 was generally below 10 mmol(c) m −2 year −1, whereas in coastal areas with large precipitation amounts, inputs exceeding 15 mmol(c) m −2 year −1 occurred. Deposition of base-cations may thus counteract up to 25% of the strong acid anion input, but more typically in the order of 5–10%. Dry deposition of non-sea salt base-cations of southern Norway is comparatively small, typically contributing 10–30% to the total deposition in the inland areas, and relatively less in other regions.

Dry deposition of reactive nitrogen compounds: A review of leaf, canopy and non-foliar measurements

Abstract This review summarizes dry deposition data for several reactive nitrogen compounds (gases and particles) for both foliar and non-foliar sites of deposition, and differentiates these data with respect to the techniques by which they were obtained. Brief summaries of the atmospheric behavior of reactive nitrogen gases, the processes governing their deposition, and a discussion of the relative importance of dry deposition to total landscape N deposition are also included. Estimates of dry deposition velocities for these species vary significantly depending on the method used and vegetation studied. Leaf-level conductances to the principal reactive nitrogen gases are typically 4–20 times lower than canopy deposition velocity measurements, but this difference can be partially reconciled if canopy leaf area is accounted for when scaling from the leaf to canopy level. Field data inficate that dry deposition of N compounds to plant surfaces is comparable to wet deposition, contributing from 20 to 70% of total atmospheric nitrogen inputs.