| 1. |
- Abera, Asmamaw, et al.
(författare)
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Air pollution measurements and land-use regression in urban sub-saharan Africa using low-cost sensors—possibilities and pitfalls
- 2020
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Ingår i: Atmosphere. - : MDPI AG. - 2073-4433. ; 11:12
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Tidskriftsartikel (refereegranskat)abstract
- Air pollution is recognized as the most important environmental factor that adversely affects human and societal wellbeing. Due to rapid urbanization, air pollution levels are increasing in the Sub-Saharan region, but there is a shortage of air pollution monitoring. Hence, exposure data to use as a base for exposure modelling and health effect assessments is also lacking. In this study, low-cost sensors were used to assess PM2.5 (particulate matter) levels in the city of Adama, Ethiopia. The measurements were conducted during two separate 1-week periods. The measurements were used to develop a land-use regression (LUR) model. The developed LUR model explained 33.4% of the variance in the concentrations of PM2.5. Two predictor variables were included in the final model, of which both were related to emissions from traffic sources. Some concern regarding influential observations remained in the final model. Long-term PM2.5 and wind direction data were obtained from the city’s meteorological station, which should be used to validate the representativeness of our sensor measurements. The PM2.5 long-term data were however not reliable. Means of obtaining good reference data combined with longer sensor measurements would be a good way forward to develop a stronger LUR model which, together with improved knowledge, can be applied towards improving the quality of health. A health impact assessment, based on the mean level of PM2.5 (23 µg/m3), presented the attributable burden of disease and showed the importance of addressing causes of these high ambient levels in the area.
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| 2. |
- Abera, Asmamaw, et al.
(författare)
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Measurements of nox and development of land use regression models in an east-African city
- 2021
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Ingår i: Atmosphere. - : MDPI AG. - 2073-4433. ; 12:4
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Tidskriftsartikel (refereegranskat)abstract
- Air pollution causes premature mortality and morbidity globally, but these adverse health effects occur over proportionately in low-and middle-income countries. Lack of both air pollution data and knowledge of its spatial distribution in African countries have been suggested to lead to an underestimation of health effects from air pollution. This study aims to measure nitrogen oxides (NOx), as well as nitrogen dioxide (NO2), to develop Land Use Regression (LUR) models in the city of Adama, Ethiopia. NOx and NO2 was measured at over 40 sites during six days in both the wet and dry seasons. Throughout the city, measured mean levels of NOx and NO2 were 29.0 µg/m3 and 13.1 µg/m3, respectively. The developed LUR models explained 68% of the NOx variances and 75% of the NO2. Both models included similar geographical predictor variables (related to roads, industries, and transportation administration areas) as those included in prior LUR models. The models were validated by using leave-one-out cross-validation and tested for spatial autocorrelation and multicollinearity. The performance of the models was good, and they are feasible to use to predict variance in annual average NOx and NO2 concentrations. The models developed will be used in future epidemiological and health impact assessment studies. Such studies may potentially support mitigation action and improve public health.
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| 3. |
- Aldabash, Midyan, et al.
(författare)
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Validation of MODIS C6.1 and MERRA-2 AOD Using AERONET Observations : A Comparative Study over Turkey
- 2020
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Ingår i: Atmosphere. - : MDPI AG. - 2073-4433. ; 11:9
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Tidskriftsartikel (refereegranskat)abstract
- This study validated MODIS (Moderate Resolution Imaging Spectroradiometer) of the National Aeronautics and Space Agency, USA, Aqua and Terra Collection 6.1, and MERRA-2 (Modern-ERA Retrospective Analysis for Research and Application) Version 2 of aerosol optical depth (AOD) at 550 nm against AERONET (Aerosol Robotic Network) ground-based sunphotometer observations over Turkey. AERONET AOD data were collected from three sites during the period between 2013 and 2017. Regression analysis showed that overall, seasonally and daily statistics of MODIS are better than MERRA-2 by the mean of coefficient of determination (R-2), mean absolute error (MAE), and relative root mean square deviation (RMSDrel). MODIS combined Terra/Aqua AOD and MERRA-2 AOD corresponding to morning and noon hours resulted in better results than individual sub datasets. A clear annual cycle in AOD was detected by the three platforms. However, overall, MODIS and MERRA-2 tend to overestimate and underestimate AOD, respectively, in comparison with AERONET. MODIS showed higher efficiency in detecting extreme events than MERRA-2. There was no clear relation found between the accuracy in MODIS/MERRA-2 AOD and surface relative humidity (RH).
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| 4. |
- Armanuos, Asaad M., et al.
(författare)
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Cross Assessment of Twenty-One Different Methods for Missing Precipitation Data Estimation
- 2020
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Ingår i: Atmosphere. - Switzerland : MDPI. - 2073-4433. ; 11:4, s. 1-35
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Tidskriftsartikel (refereegranskat)abstract
- The results of metrological, hydrological, and environmental data analyses are mainlydependent on the reliable estimation of missing data. In this study, 21 classical methods were evaluated to determine the best method for infilling the missing precipitation data in Ethiopia. The monthly data collected from 15 different stations over 34 years from 1980 to 2013 were considered. Homogeneity and trend tests were performed to check the data. The results of the different methods were compared using the mean absolute error (MAE), root-mean-square error (RMSE), coefficient of efficiency (CE), similarity index (S-index), skill score (SS), and Pearson correlation coefficient (rPearson). The results of this paper confirmed that the normal ratio (NR), multiple linear regression (MLR), inverse distance weighting (IDW), correlation coefficient weighting (CCW), and arithmetic average (AA) methods are the most reliable methods of those studied. The NR method provides the most accurate estimations with rPearson of 0.945, mean absolute error of 22.90 mm, RMSE of 33.695 mm, similarity index of 0.999, CE index of 0.998, and skill score of 0.998. When comparing the observed results and the estimated results from the NR, MLR, IDW, CCW, and AA methods, the MAE and RMSE were found to be low, and high values of CE, S-index, SS, and rPearson were achieved. On the other hand, using the closet station (CS), UK traditional, linear regression (LR), expectation maximization (EM), and multiple imputations (MI) methods gave the lowest accuracy, with MAE and RMSE values varying from 30.424 to 47.641 mm and from 49.564 to 58.765 mm, respectively. The results of this study suggest that the recommended methods are applicable for different types of climatic data in Ethiopia and arid regions in other countries around the world.
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| 5. |
- Cooray, Vernon, 1952-, et al.
(författare)
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Could Macroscopic Dark Matter (Macros) Give Rise to Mini-Lightning Flashes out of a Blue Sky without Clouds?
- 2021
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Ingår i: Atmosphere. - : MDPI. - 2073-4433. ; 12:9
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Tidskriftsartikel (refereegranskat)abstract
- A recent study pointed out that macroscopic dark matter (macros) traversing through the Earth's atmosphere can give rise to hot and ionized channels similar to those associated with lightning leaders. The authors of the study investigated the possibility that when such channels created by macros pass through a thundercloud, lightning leaders may be locked in by these ionized channels, creating lightning discharges with perfectly straight channels. They suggested the possibility of detecting such channels as a means of detecting the passage of macros through the atmosphere. In this paper, we show that such macros crossing the atmosphere under fair weather conditions could also give rise to mini-lightning flashes with current amplitudes in the order of a few hundred Amperes. These mini-lightning flashes would generate a thunder signature similar to or stronger than those of long laboratory sparks and they could also be detected by optical means. As in the case of thunderstorm-assisted macro lightning, these mini-lightning flashes are also associated with straight channels. Moreover, since the frequency of mini-lightning flashes is about thirty times greater than the macro-generated lightning flashes assisted by thunderstorms, they could be used as a means to look for the paths of macroscopic dark matter crossing the atmosphere.
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| 6. |
- Cooray, Vernon, 1952-, et al.
(författare)
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Ionization Waves Enhance the Production of X-rays during Streamer Collisions
- 2021
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Ingår i: Atmosphere. - : MDPI. - 2073-4433. ; 12:9
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Tidskriftsartikel (refereegranskat)abstract
- Experimental data show that in laboratory sparks, X-rays are produced in time synchronization with the meeting of streamers of opposite polarity just before the final breakdown of the discharge gap. It has been suggested that the electric field enhancement created during the collision of streamers could provide the necessary conditions for electron acceleration, even though some of the theoretical studies show that the duration of the electric field is not long enough to do so. The experimental data on laboratory discharges show that. when streamers of opposite polarity meet each other, a potential or ionization wave that renders the streamer channels conducting is initiated. This paper shows that these ionization waves that convert the discharge channels from weakly conducting to highly conducting are associated with electric fields large enough to accelerate electrons to relativistic energies.
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| 7. |
- Cooray, Vernon, 1952-, et al.
(författare)
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Latitude and Topographical Dependence of Lightning Return Stroke Peak Current in Natural and Tower-Initiated Negative Ground Flashes
- 2020
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Ingår i: Atmosphere. - : MDPI. - 2073-4433. ; 11:6
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Tidskriftsartikel (refereegranskat)abstract
- Experimental data show that the peak currents of first and subsequent lightning return strokes in negative ground flashes increase with decreasing latitude. In this paper, the reason for this dependence of peak return stroke current on latitude is explained using the fact that the height of the charge centers increases with decreasing latitude. Results show that in tropical regions where the height to the negative charge center is about 8 km, the median values of the first and the subsequent return stroke peak currents are about 42 kA and 15 kA, respectively. If the height to the charge center is larger than 8 km, the peak currents will also become larger. For example, if the location of the charge center is increased to about 9 km, the median values of the first and subsequent return stroke peak currents will increase to about 45 kA and 16 kA respectively. The same reasoning shows that, even in the same geographical region, the peak return stroke current may decrease as the elevation of the ground where the lightning strikes take place increases. The results also indicate that the peak subsequent return stroke current in tower-initiated negative lightning flashes decreases as the height of the tower increases. These theoretical predictions are in general agreement with the available experimental data.
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| 8. |
- Cooray, Vernon, 1952-, et al.
(författare)
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On the Apparent Non-Uniqueness of the Electromagnetic Field Components of Return Strokes Revisited
- 2021
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Ingår i: Atmosphere. - : MDPI. - 2073-4433. ; 12:10
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Tidskriftsartikel (refereegranskat)abstract
- Recent research work shows that there are four procedures that can be used to calculate the electromagnetic fields from a current source. These different procedures, even though producing the same total field, give rise to field components that differ from one procedure to another. This has led to the understanding that the various field terms that constitute the total field cannot be uniquely determined. In this paper, it is shown that all four field expressions can be reduced to a single field expression, and the various field terms arising from acceleration, uniformly moving, and stationary charges can be uniquely determined. The differences in the field terms arising from different techniques are caused by the different ways of summing up the contribution to the total electric field coming from the accelerating, moving, and stationary charges.
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| 9. |
- Cooray, Vernon, 1952-, et al.
(författare)
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Polarity Asymmetry in Lightning Return Stroke Speed Caused by the Momentum Associated with Radiation
- 2021
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Ingår i: Atmosphere. - : MDPI. - 2073-4433. ; 12:12
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Tidskriftsartikel (refereegranskat)abstract
- In positive lightning return strokes, the net momentum transported by the radiation field has the same direction as the momentum associated with electrons, whereas the momentum associated with electrons is in opposite direction to the momentum of radiation in negative return strokes. It is shown here that this polarity asymmetry could limit the maximum speed of positive return strokes with respect to the negative return strokes.
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| 10. |
- Cooray, Vernon, 1952-, et al.
(författare)
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The Energy, Momentum, and Peak Power Radiated by Negative Lightning Return Strokes
- 2020
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Ingår i: Atmosphere. - : MDPI AG. - 2073-4433. ; 11:12
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Tidskriftsartikel (refereegranskat)abstract
- Electromagnetic radiation fields generated by return strokes transport both energy and momentum from the return stroke to outer space. The momentum transported by the radiation field has only a vertical or z component due to azimuthal symmetry (cylindrical symmetry) associated with a vertical return stroke. In this paper, the energy, momentum, and peak power radiated by return strokes as a function of the return stroke current, return stroke speed, and the zero-crossing time of the radiation fields are studied. The results obtained by numerical simulations for the energy, vertical momentum, and the peak power radiated by lightning return strokes (all parameters normalized by dividing them by the square of the radiation field peak at 100 km) are the following: A typical first return stroke generating a radiation field having a 50 μs zero-crossing time will dissipate field normalized energy of about (1.7–2.5) × 103 J/(V/m)2 and field-normalized vertical momentum of approximately (2.3–3.1) × 10−6 Kg m/s/(V/m)2. A radiation field with a zero-crossing time of 70 μs will dissipate about (2.6–3.4) × 103 J/(V/m)2 in field-normalized energy and (3.2–4.3) × 10−6 Kg m/s/(V/m)2 in field-normalized vertical momentum. The results show that, for a given peak radiation field, the radiated energy and momentum increase with increasing zero-crossing time of the radiation field. The normalized peak power generated by a first return stroke radiation field is about 1.2 × 108 W/(V/m)2 and the peak power is generated within about 5–6 μs from the beginning of the return stroke. Conversely, a typical subsequent return stroke generating a radiation field having a 40 μs zero-crossing time will dissipate field-normalized energy of about (6–9) × 102 J/(V/m)2 and field-normalized vertical momentum of approximately (7.5–11) × 10−7 Kg m/s/(V/m)2. The field-normalized peak power generated by a subsequent return stroke radiation field is about 1.26 × 108 W/(V/m)2 and the peak power is generated within about 0.7–0.8 μs from the beginning of the return stroke. In addition to these parameters, the possible upper bounds for the energy and momentum radiated by return strokes are also presented.
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