Dust-free Days Research Articles

The measurement and parameterization of ice nucleating particles in different backgrounds of China

Investigation of the number concentration of ice nucleating particles (INP) in the deposition nucleation mode during a dust event is reported. The results discussed in this paper are the first continuous INP measurements in Xinjiang, northwest of China, over a period with a strong dust event. The average INP concentration at −20°C and 22% of supersaturation with respect to ice during non-dust days is found around 11 particles per liter, but it reached several hundred per liter in a dust event. A close correlation is also found between the INP number concentration with the number concentration of aerosol particles larger than 0.5μm in diameter measured during a dust event, which means that a higher concentration of larger particles induced higher INP number concentration. Parameterizations were developed based on measurements to represent the variations of INP concentration with temperature, supersaturation, and the number concentration of aerosol particles with size larger than 0.5μm. It should be the first ever, as we have known so far, to measure ice nuclei and aerosol properties simultaneously in a desert area and to contrast INP concentrations in dust and dust-free days, and could advancing our understanding of the effects of dust particles on ice nucleation.

Effect of Dust Storms on the Atmospheric Microbiome in the Eastern Mediterranean.

We evaluated the impact of Saharan dust storms on the local airborne microbiome in a city in the Eastern Mediterranean area. Samples of particles with diameter less than 10 μm were collected during two spring seasons on both dusty and nondusty days. DNA was extracted, and partial 16S rRNA gene amplicons were sequenced using the Illumina platform. Bioinformatic analysis showed the effect of dust events on the diversity of the atmospheric microbiome. The relative abundance of desert soil-associated bacteria increased during dust events, while the relative abundance of anthropogenic-influenced taxa decreased. Quantitative polymerase chain reaction measurements of selected clinically significant antibiotic resistance genes (ARGs) showed that their relative abundance decreased during dust events. The ARG profiles on dust-free days were similar to those in aerosol collected in a poultry house, suggesting a strong agricultural influence on the local ambient profiles. We conclude that dust storms enrich the ambient airborne microbiome with new soil-derived bacteria that disappear as the dust settles, suggesting that the bacteria are transported attached to the dust particles. Dust storms do not seem to be an important vector for transport of probed ARGs.

Impact of desert dust events on short- and long-wave radiation at the surface over south-eastern Italy

Clear-sky short-wave (SW) and long-wave (LW) irradiance measurements at the surface were combined with AErosol RObotic NETwork (AERONET) sun/sky photometer aerosol products to study the desert dust impact on irradiance measurements at a Central Mediterranean site during the year 2012, by comparing measurements performed on dusty and dust-free days. Daily mean values of both the aerosol Angstrom exponent (A), calculated from the aerosol optical depths (AOD) at 440 and 870 nm retrieved from AERONET sun/sky photometer measurements, and the desert dust loading (DL) from the Barcelona Supercomputing Center Dust REgional Atmospheric Model (BSC-DREAM8b) were used to select dusty days. In particular, we have identified as dusty days the ones characterized by A values less than 0.9 and DL values larger than 0.5 g m−2. The desert dust events occurred from March to September during the analyzed year, in which we have identified 30 and 96 clear-sky dusty and dust-free days, respectively. The daytime SW and LW downward fluxes (FDN) on average decreased by 8 % and increased by 3 %, respectively, on the dusty days with respect to the dust-free ones. These flux variations were associated with an average increase of 40 % of the AOD at 440 nm and an average decrease of 39 % of the fine mode fraction (η) at 500 nm. The daily means of SW- and LW-FDN were reasonably correlated with the corresponding AOD and η values on the dusty days, revealing that the increase of the coarse mode particle contribution was mainly responsible for the flux changes. Conversely, both the SW- and the LW-FDN values were not correlated with the corresponding AOD and η values on the dust-free days.

Saharan dust impact on the chemical composition of PM10 and PM1 samples over south-eastern Italy

Angstrom exponents (A) and dust concentrations from the Barcelona Supercomputing Center-Dust REgional Atmospheric Model (BSC-DREAM) were used to infer the impact of long-range transported desert dust particles at the ground level and evaluate their role on the chemical composition of PM1 and PM10 samples. A values were calculated from the scattering coefficients at 450 and 635 nm, retrieved from integrating nephelometer measurements. Nephelometer measurements were performed at a coastal site (Lecce, 40.33° N, 18.11° E) of south-eastern Italy from December 2011 till November 2012. Days characterized by A daily mean values smaller than 0.95 and modelled daily dust concentrations larger than 0.1 μg m−3 at 86 m above the ground level were considered representative of days affected by African dust particles up to the ground level (dusty days). Both criteria have allowed identifying 86 dusty days during the investigated period. The analysis of 24-h simultaneously collected PM10 and PM1 samples revealed that the PM1 mass concentrations increased linearly with PM10 both in dusty and dust-free days, which were identified as the ones characterized by A daily mean values larger than 1.3 and PM1/PM10 ratios larger than 0.35. These results suggested that the PM1 samples were also affected by desert particles on dusty days. In fact, chemical analyses revealed that the Al and Fe mean mass concentrations were larger in dusty day PM1 and PM10 samples. Then, we found that the crustal matter contribution was nearly twice and more than twice larger in dusty PM1 and PM10 samples, respectively, than in corresponding dust-free samples. Mass contributions of organic and elemental carbon, sulfates, and ammonium even if smaller in dusty samples than in dust-free PM1 and PM10 samples revealed the significant role of the anthropogenic pollution also on dusty days.

Aerosol scattering and absorption Angström exponents as indicators of dust and dust-free days over Granada (Spain)

This paper focuses on the assessment of atmospheric aerosol optical properties at the surface and in atmospheric column during both desert dust and dust-free conditions over Granada, South-eastern Iberian Peninsula. Indeed, the spectral dependence of aerosol absorption and scattering properties is analyzed in detail. The analyzed period ranges from June 2008 to December 2010. On dusty days, the mean scattering Angström exponent value obtained in the atmospheric column (SAEcol) (0.5±0.3) was lower than the observed at the surface level (SAEis) (1.3±0.6), indicating higher contribution of coarse particles at high atmospheric level than at ground level during the analyzed dust events. In addition, it is noticed that the absorption Angström exponent in the atmospheric column (AAEcol) with mean value of 1.5±0.2 and at the surface (AAEis) with mean value of 1.3±0.2 obtained during dusty situations are indicative of mixture of desert dust and black carbon particles as dominant absorbers both in the atmospheric column and at the surface during dust intrusions over Granada. On the other hand, a non-parametric test (Kolmogorov–Smirnov) revealed that no significant statistical difference was found for AAEis between desert dust and free-dust conditions. This result may be due to the important contribution of urban absorbing aerosol (e.g. Black carbon) at ground level in the study location. Therefore, these parameters (AAEcol and AAEis) are not very useful to detect desert dust events without the use of other information (e.g., aerosol size) over urban area like Granada. A dust extreme event was analyzed in order to retrieve optical parameters during situation dominated by desert dust. The values of SAEcol and SAEis obtained during this extreme event were in agreement with the values showed above for the period 2008–2010, although the differences between dust-free and dust conditions are more noticeable in this special event.

Synoptic-scale dust transport events in the southern Himalaya

Abstract The variability of long-range dust transport events observed in the southern Himalaya and its relation with source areas have been studied thanks to five years’ continuous measurements which were carried out at the “Nepal Climate Observatory-Pyramid” (NCO-P, 27°57′N, 86°48′E), the highest Northern Hemisphere GAW-WMO global station sited at 5079 m a.s.l. in the high Khumbu valley (Nepal) on the southern Himalaya. During the period March 2006–February 2011, the analyses of the aerosol particle concentrations and LAGRANTO three-dimensional backward trajectories indicated the occurrence of 275 days affected by synoptic-scale dust transport, which account for 22.2% of the investigated period. The frequency of dust transport days (DTDs) showed a clear seasonal cycle, with the highest seasonal value observed during pre-monsoon season (33.5% of the pre-monsoon’s days are DTDs). Large enhancements in coarse aerosol number concentration N 1–10 (average: +689%) and mass PM 1–10 (average: +1086%) were observed during the dust transport events as compared to the days without dust (dust-free days, DFDs). In addition, the single scattering albedo (SSA) also showed higher values, ranging from 0.87 to 0.90, during DTDs with respect to DFDs (0.80–0.87). The predominant source of mineral dust reaching the measurement site was identified in the arid regions of the north-western Indian subcontinent (Thar desert), which accounted for 41.6% of the trajectories points associated with DTDs. Seasonal analysis also indicated that the winter season was significantly influenced by far western desert regions, such as North Africa and the Arabic Peninsula.

Dust Aerosol Characteristics and Shortwave Radiative Impact at a Gobi Desert of Northwest China during the Spring of 2012

The Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) project initiated an intensive field experiment on dust aerosols in Dunhuang from April 1 to June 12, 2012. Using sky radiometer measurements and conducting model simulations, we investigated the dust aerosol characteristics and its short-wave radiative impact on the regional climate. The daily averaged optical features of the aerosols markedly varied throughout the study period. High aerosol loading and predominantly coarse particulates were observed in the spring of 2012 ascribed to the influence of prevalent dust storm. The single scattering albedo at 500 nm (SSA(500)) varied from 0.91 to 0.97 on dusty days and from 0.86 to 0.91 on dust-free days, indicating that the dust aerosols sourced from northwest China were not strongly absorbing. Surface radiation quantities estimated by the radiative transfer model excellently agreed with ground-based and satellite observations, with correlation coefficients exceeding 0.990 and mean differences ranging from -3.9 to 17.0 W m(-2). The daily mean aerosol shortwave direct radiative forcing (ARF) values were largely negative at the surface (-79.4 to -3.2 W m(-2)) and moderately positive in the atmosphere (2.2-25.1 W m(-2)), indicating strong cooling at the surface and moderate atmospheric warming. The monthly averaged ARFEs (ARFs per unit aerosol optical depth at 500 nm (AOD(500))) at the surface were (-73.9 +/- 11.6) W m(-2), (-67.4 +/- 8.3) W m(-2), and (-74.4 +/- 5.4) W m(-2) in April, May, and June, respectively (overall average of (-70.8 +/- 7.9) W m(-2)), comparable to previously obtained values in East Asia and India domains. The relations between the diurnal ARFs at the surface and top of the atmosphere (TOA) and the AOD(500) indicate that aerosol composition remained relatively stable at Dunhuang during the spring of 2012. The ARF at the TOA was positive for SSA(500) less than 0.85 or when the imaginary part at 500 nm exceeded 0.015.

Saharan dust and the association between particulate matter and daily hospitalisations in Rome, Italy

IntroductionOutbreaks of Saharan dust have been shown to exacerbate the effect of particulate matter (PM) on mortality. Their role on PM–morbidity association is less clear. This study aims to evaluate...

Saharan Dust and Associations between Particulate Matter and Daily Mortality in Rome, Italy

Background: Outbreaks of Saharan-Sahel dust over Euro-Mediterranean areas frequently induce exceedances of the Europen Union's 24-hr standard of 50 μg/m3 for particulate matter (PM) with aerodynamic diameter ≤ than 10 μm (PM10).Objectives: We evaluated the effect of Saharan dust on the association between different PM fractions and daily mortality in Rome, Italy.Methods: In a study of 80,423 adult residents who died in Rome between 2001 and 2004, we performed a time-series analysis to explore the effects of PM2.5, PM2.5–10, and PM10 on natural, cardiac, cerebrovascular, and respiratory mortality. We defined Saharan dust days by combining light detection and ranging (LIDAR) observations and analyses from operational models. We tested a Saharan dust–PM interaction term to evaluate the hypothesis that the effects of PM, especially coarse PM (PM2.5–10), on mortality would be enhanced on dust days.Results: Interquartile range increases in PM2.5–10 (10.8 μg/m3) and PM10 (19.8 μg/m3) were associated with increased mortality due to natural, cardiac, cerebrovascular, and respiratory causes, with estimated effects ranging from 2.64% to 12.65% [95% confidence interval (CI), 1.18–25.42%] for the association between PM2.5–10 and respiratory mortality (0- to 5-day lag). Associations of PM2.5–10 with cardiac mortality were stronger on Saharan dust days (9.73%; 95% CI, 4.25–15.49%) than on dust-free days (0.86%; 95% CI, –2.47% to 4.31%; p = 0.005). Saharan dust days also modified associations between PM10 and cardiac mortality (9.55% increase; 95% CI, 3.81–15.61%; vs. dust-free days: 2.09%; 95% CI, –0.76% to 5.02%; p = 0.02).Conclusions: We found evidence of effects of PM2.5–10 and PM10 on natural and cause-specific mortality, with stronger estimated effects on cardiac mortality during Saharan dust outbreaks. Toxicological and biological effects of particles from desert sources need to be further investigated and taken into account in air quality standards.

Saharan Dust and Associations between Particulate Matter and Daily Mortality in Rome, Italy

Outbreaks of Saharan-Sahel dust over Euro-Mediterranean areas frequently induce exceedances of the Europen Union's 24-hr standard of 50 μg/m3 for particulate matter (PM) with aerodynamic diameter ≤ than 10 μm (PM10). We evaluated the effect of Saharan dust on the association between different PM fractions and daily mortality in Rome, Italy. In a study of 80,423 adult residents who died in Rome between 2001 and 2004, we performed a time-series analysis to explore the effects of PM2.5, PM2.5-10, and PM10 on natural, cardiac, cerebrovascular, and respiratory mortality. We defined Saharan dust days by combining light detection and ranging (LIDAR) observations and analyses from operational models. We tested a Saharan dust-PM interaction term to evaluate the hypothesis that the effects of PM, especially coarse PM (PM2.5-10), on mortality would be enhanced on dust days. Interquartile range increases in PM2.5-10 (10.8 μg/m3) and PM10 (19.8 μg/m3) were associated with increased mortality due to natural, cardiac, cerebrovascular, and respiratory causes, with estimated effects ranging from 2.64% to 12.65% [95% confidence interval (CI), 1.18-25.42%] for the association between PM2.5-10 and respiratory mortality (0- to 5-day lag). Associations of PM2.5-10 with cardiac mortality were stronger on Saharan dust days (9.73%; 95% CI, 4.25-15.49%) than on dust-free days (0.86%; 95% CI, -2.47% to 4.31%; p = 0.005). Saharan dust days also modified associations between PM10 and cardiac mortality (9.55% increase; 95% CI, 3.81-15.61%; vs. dust-free days: 2.09%; 95% CI, -0.76% to 5.02%; p = 0.02). We found evidence of effects of PM2.5-10 and PM10 on natural and cause-specific mortality, with stronger estimated effects on cardiac mortality during Saharan dust outbreaks. Toxicological and biological effects of particles from desert sources need to be further investigated and taken into account in air quality standards.

Micro pulse lidar observations of mineral dust layer in the lower troposphere over the southwest coast of Peninsular India during the Asian summer monsoon season

A large aerosol plume with optical depth exceeding 0.7 engulfs most parts of the Arabian Sea during the Asian summer monsoon season. Based on Micro Pulse Lidar observations during the June–September period of 2008 and 2009, the present study depicts, for the first time, the existence of an elevated dust layer occurring very frequently in the altitude band of 1–3.5 km over the west coast of peninsular India with relatively large values of linear depolarization ratio ( δ L ). Large values of δ L indicate the dominance of significantly non-spherical aerosols. The aerosol optical depth of this layer (0.2) is comparable to that of the entire atmospheric column during dust-free days. Back-trajectory analysis clearly shows the advection of airmass from the arid regions of Arabia and the west Arabian Sea, through the altitude region centered around 3 km. This is in contrast to the airmass below 1 km originating from the pristine Indian Ocean region which contains relatively spherical aerosols of marine origin with δ L generally <0.05.

Effect of Saharan Dust on the Association Between Particulate Matter and Daily Mortality in Rome, Italy

ISEE-0541 Background and Objectives: Outbreaks of Saharan-Sahel dust over the Euro-Mediterranean areas frequently induce exceedances of the 50 μg/m3-EU 24h standard. Public concern about the health adverse effects of these natural events on mortality has increased. This study evaluated the effect modification of Saharan dust on the association between different PM sizes (fine, coarse and PM10) and total and cause-specific mortality. Materials and Methods: 80,423 residents aged 35+ who died in Rome between 2001–2004 were selected. A time series analysis was performed to explore the effects of different (PM2.5, PM2.5–10, PM10) on natural, cardiac, cerebrovascular, and respiratory daily mortality. We then added a Saharan dust-PM interaction term to test the hypothesis that the effect of PM, especially coarse and PM10, on mortality would be higher in dust affected days. Results: Coarse particles (PM2.5–10, IQR: 10.8 μg/m3) and PM10 (IQR: 19.8 μg/m3) were associated with natural (lag 0–2, 2.96%, 95%CI = 1.23, 4.72 and 3.00%, 95%CI 1.51, 4.51), cardiac (lag 0, 3.72%, 95%CI = 0.78, 6.73 and 3.62%, 95%CI = 1.11, 6.18), cerebrovascular (5.41%, 95%CI = 1.74, 9.22 and 3.01, 95%CI = −0.84, 7.02) and respiratory mortality (lag 0–5, 12.65%, 95%CI = 1.18, 25.42 and 4.57%, 95%CI = −2.46, 12.09). Statistically significant effect modification (P < 0.01) was seen for cardiac mortality with stronger PM2.5–10 and PM10 effects in Saharan dust days compared to dust free days (9.73% vs 0.86% and 9.44% vs 1.83%). An effect modification was also suggested for PM2.5–10 and respiratory mortality. The results were stable after adjustment for ozone. Conclusions: Saharan dust outbreaks should not be considered as harmless events. Even if the toxicological capacity of particles coming from desert sources needs to be further investigated, anthropogenic contribution to coarse PM should be controlled and limited particularly in areas frequently advected by desert dust episodes.