ALBERT

Description: Chemical composition, microphysical, and optical properties of atmospheric aerosol deep inland in the Negev Desert of Israel are found to be influenced by daily occurrences of sea breeze flow from the Mediterranean Sea. Abrupt increases in aerosol volume concentration and shifts of size distributions towards larger sizes, which are associated with increase in wind speed and atmospheric water content, were systematically recorded during the summertime at a distance of at least 80 km from the coast. Chemical imaging of aerosol samples showed an increased contribution of highly hygroscopic particles during the intrusion of the sea breeze. Besides a significant fraction of marine aerosols, the amount of internally mixed marine and mineral dust particles was also increased during the sea breeze period. The number fraction of marine and internally mixed particles during the sea breeze reached up to 88 % in the PM1–2. 5 and up to 62 % in the PM2. 5–10 size range. Additionally, numerous particles with residuals of liquid coating were observed by SEM/EDX analysis. Ca-rich dust particles that had reacted with anthropogenic nitrates were evidenced by Raman microspectroscopy. The resulting hygroscopic particles can deliquesce at very low relative humidity. Our observations suggest that aerosol hygroscopic growth in the Negev Desert is induced by the daily sea breeze arrival. The varying aerosol microphysical and optical characteristics perturb the solar and thermal infrared radiations. The changes in aerosol properties induced by the sea breeze, relative to the background situation, doubled the shortwave radiative cooling at the surface (from −10 to −20.5 W m−2) and increased by almost 3 times the warming of the atmosphere (from 5 to 14 W m−2), as evaluated for a case study. Given the important value of observed liquid coating of particles, we also examined the possible influence of the particle homogeneity assumption on the retrieval of aerosol microphysical characteristics. The tests suggest that sensitivity to the coating appears if backward scattering and polarimetric measurements are available for the inversion algorithm. This may have an important implication for retrievals of aerosol microphysical properties in remote sensing applications.

Description: Iron is a major component of atmospheric aerosols, influencing the light absorption ability of mineral dust, and an important micronutrient that affects oceanic biogeochemistry. The regional distribution of the iron concentration in dust is important for climate studies; however, this is difficult to obtain since it requires in-situ aerosol sampling or simulation of complex natural processes. Simultaneous studies of aerosol chemical composition and radiometric measurements of aerosol optical properties, which were performed in the Negev desert of Israel continuously for about eight years, suggest a potential for deriving a relationship between chemical composition and light absorption properties, in particular the spectral single-scattering albedo. The two main data sets of the present study were obtained by a sun/sky radiometer and a stacked filter unit sampler that collects particles in coarse and fine size fractions. Analysis of chemical and optical data showed the presence of mixed dust and pollution aerosol in the study area, although their sources appear to be different. Spectral SSA showed an evident response to increased concentrations of iron, black carbon equivalent matter, and their mixing state. An empirical relationship that relates the spectral SSA, the percentage of iron in total particulate mass, and the pollution components was derived. Results calculated using this relationship were compared with measurements from dust episodes in several locations around the globe. The comparison reveals that dust over the eastern Mediterranean and Saudi Arabia contains less iron than that over Asia and the Sahara desert.

Description: Iron is a major component of atmospheric aerosols, influencing the light absorption ability of mineral dust, and an important micronutrient that affects oceanic biogeochemistry. The regional distribution of the iron concentration in dust is important for climate studies; however, this is difficult to obtain since it requires in-situ aerosol sampling or simulation of complex natural processes. Simultaneous studies of aerosol chemical composition and radiometric measurements of aerosol optical properties, which were performed in the Negev desert of Israel continuously for about eight years, suggest a potential for deriving a relationship between chemical composition and light absorption properties, in particular the spectral single-scattering albedo. The two main data sets of the present study were obtained by a sun/sky radiometer and a stacked filter unit sampler that collects particles in coarse and fine size fractions. Analysis of chemical and optical data showed the presence of mixed dust and pollution aerosol in the study area, although their sources appear to be different. Spectral SSA showed an evident response to increased concentrations of iron, black carbon equivalent matter, and their mixing state. A relationship that relates the spectral SSA, the percentage of iron in total particulate mass, and the pollution components was derived. Results calculated, using this relationship, were compared with measurements from dust episodes in several locations around the globe. The comparison showed reasonable agreement between the calculated and the observed iron concentrations, and supported the validity of the suggested approach for the estimation of iron concentrations in mineral dust.

Description: Biological soil crusts (BSC) contribute significantly to the soil surface cover in many dryland ecosystems. A mixed type of BSC, which consists of cyanobacteria, mosses and cyanolichens, constitutes more than 60% of ground cover in the semiarid grass-shrub steppe at Sayeret Shaked in the northern Negev Desert, Israel. This study aimed at parameterizing the carbon sink capacity of well-developed BSC in undisturbed steppe systems. Mobile enclosures on permanent soil borne collars were used to investigate BSC-related CO2 fluxes in situ and with natural moisture supply during 10 two-day field campaigns within seven months from fall 2001 to summer 2002. Highest BSC-related CO2 deposition between −11.31 and −17.56 mmol m−2 per 15 h was found with BSC activated from rain and dew during the peak of the winter rain season. Net CO2 deposition by BSC was calculated to compensate 120%, −26%, and less than 3% of the concurrent soil CO2 efflux from November–January, February–May and November–May, respectively. Thus, BSC effectively compensated soil CO2 effluxes when CO2 uptake by vascular vegetation was probably at its low point. Nighttime respiratory emission reduced daily BSC-related CO2 deposition within the period November–January by 11–123% and on average by 27%. The analysis of CO2 fluxes and water inputs from the various sources showed that the bulk of BSC-related CO2 deposition occurs during periods with frequent rain events and subsequent condensation from water accumulated in the upper soil layers. Significant BSC activity on days without detectable atmospheric water supply emphasized the importance of high soil moisture contents as additional water source for soil-dwelling BSC, whereas activity upon dew formation at low soil water contents was not of major importance for BSC-related CO2 deposition. However, dew may still be important in attaining a pre-activated status during the transition from a long "summer" anabiosis towards the first winter rain.

Description: Biological soil crusts (BSC) constitute a spatially prominent part of the photosynthesizing vegetation in many dryland ecosystems. This study assesses the annual net carbon deposition related to BSC growth in a Mediterranean shrubland for the years 2001–2003 using a model developed to account for the nature of hydration in the poikilohydric life trait of the BSC. Data for BSC-related net CO2 fluxes were obtained from in-situ measurements at the International Long-term Ecological Research site Sayeret Shaked (ILTER-SSK) in the northern Negev Desert, Israel. The BSC was smooth to rugose, up to 15 mm thick and consisted mainly of mosses, cyanobacteria and cyano-lichens. In order to obtain annual estimates, BSC-related CO2 fluxes were correlated with climate records provided by the meteorological station of the Terrestrial Ecosystem Monitoring Site network (TEMS) adjacent to SSK. The annual carbon deposition related to BSC growth was assessed from (1) an overall mean of net CO2 flux multiplied with annual activity periods of BSC based on precipitation records, and (2) from a simple precipitation-driven activity model (PdAM). This model combines an algorithm, previously developed to model gas exchange processes in vascular plants, with an empirical module that switches the algorithm on as soon as water is available to maintain activity of poikilohydric BSC. Based on a constant BSC area index of 0.6 m2 m−2 at ILTER-SSK, the final model suggests a large inter-annual variability in BSC-related net carbon deposition ranging from 7 to 51 kg ha−1 yr−1.

Description: Chemical composition, microphysical and optical properties of atmospheric aerosol deep inland in the Negev Desert of Israel were found be influenced by daily occurrences of sea breeze flow from the Mediterranean Sea. Abrupt increases in aerosol volume concentration and shifts of size distributions towards larger sizes, which are associated with increase in wind speed and atmospheric water content, were systematically recorded during the summertime at a distance of at least 80 km from the coast. Chemical imaging of aerosol samples confirmed an increased contribution of highly hygroscopic particles during the intrusion of the sea breeze. Besides a significant fraction of marine aerosols, the amount of internally mixed marine and mineral dust particles was also increased during the sea breeze period. The number fraction of marine and internally mixed particles during the sea breeze reached up to 88 % in the PM1-2.5 and up to 62 % in the PM2.5-10 size range. Additionally, numerous particles with residuals of liquid coating were observed by SEM/EDX analysis. Ca-rich dust particles that had reacted with anthropogenic nitrates were evidenced by Raman microspectroscopy. The resulting hygroscopic particles can deliquesce at very low relative humidity. Our observations suggest that aerosol hygroscopic growth in the Negev Desert is triggered by the daily sea breeze arrival. The varying aerosol microphysical and optical characteristics perturb the solar and thermal infrared radiation. The changes in aerosol properties induced by the sea breeze, relative to the background situation, doubled the shortwave radiative cooling at the surface (from −10 to −20.5 Wm−2) and increased by almost three times the warming of the atmosphere (from 5 to 14 Wm−2), as evaluated for a case study. Given the important value of observed liquid coating of particles, we also examined the possible influence of the particle homogeneity assumption on the retrieval of aerosol microphysical characteristics. The tests suggest that sensitivity to the coating appears if backward scattering and polarimetric measurements are available for the inversion algorithm. This may have an important implication for retrievals of aerosol microphysical properties in remote sensing applications.

Description: Biological soil crusts (BSC) contribute significantly to the soil surface cover in many dryland ecosystems. A mixed type of BSC, which consists of cyanobacteria, mosses and cyanolichens, constitutes more than 60% of ground cover in the semiarid grass-shrub steppe at Sayeret Shaked in the northern Negev Desert, Israel. This study aimed at parameterizing the carbon sink capacity of well-developed BSC in undisturbed steppe systems. Mobile enclosures on permanent soil borne collars were used to investigate BSC-related CO2 fluxes in situ and with natural moisture supply during 10 two-day field campaigns within seven months from fall 2001 to summer 2002. Highest BSC-related CO2 deposition between –11.31 and –17.56 mmol m−2 per 15 h was found with BSC activated from rain and dew during the peak of the winter rain season. Net CO2 deposition by BSC was calculated to compensate 120%, –26%, and less than 3% of the concurrent soil CO2 efflux from November–January, February–May and November–May, respectively. Thus, BSC effectively compensated soil CO2 effluxes when CO2 uptake by vascular vegetation was probably at its low point. Nighttime respiratory emission reduced daily BSC-related CO2 deposition within the period November–January by 11–123% and on average by 27%. The analysis of CO2 fluxes and water inputs from the various sources showed that the bulk of BSC-related CO2 deposition occurs during periods with frequent rain events and subsequent condensation from water accumulated in the upper soil layers. Significant BSC activity on days without detectable atmospheric water supply emphasized the importance of high soil moisture contents as additional water source for soil-dwelling BSC, whereas activity upon dew formation at low soil water contents was not of major importance for BSC-related CO2 deposition. However, dew may still be important in attaining a pre-activated status during the transition from a long "summer" anabiosis towards the first winter rain.