Coupling Sr–Nd–Hf Isotope Ratios and Elemental Analysis to Accurately Quantify North African Dust Contributions to PM_2.5 in a Complex Urban Atmosphere by Reducing Mineral Dust Collinearity

Abstract

Tracking Saharan−Sahelian dust across the globe is essential to elucidate its effects on Earth's climate, radiation budget, hydrologic cycle, nutrient cycling, and also human health when it seasonally enters populated/industrialized regions of Africa, Europe, and North America. However, the elemental composition of mineral dust arising locally from construction activities and aeolian soil resuspension overlaps with African dust. Therefore, we derived a novel "isotope-resolved chemical mass balance" (IRCMB) method by employing radiogenic strontium, neodymium, and hafnium isotopes to accurately differentiate and quantitatively apportion collinear proximal and synoptic-scale crustal and anthropogenic mineral dust sources. IRCMB was applied to two air masses that transported African dust to Barbados and Texas to track particulate matter (PM) spikes at both locations. During Saharan−Sahelian intrusions, the radiogenic content of urban PM 2.5 increased with respect to 87 Sr/ 86 Sr and 176 Hf/ 177 Hf but decreased in terms of 143 Nd/ 144 Nd, demonstrating the ability of these isotopes to sensitively track African dust intrusions even in complex metropolitan atmospheres. The principal aerosol strontium, neodymium, and hafnium end members were concrete dust and soil, soil and motor vehicles, and motor vehicles and North African dust, respectively. IRCMB separated and quantified local soil and distal crustal dust even when PM 2.5 concentrations were low, opening a promising source apportionment avenue for urbanized/industrialized atmospheres.

Respirable Metals, Bacteria, and Fungi during a Saharan–Sahelian Dust Event in Houston, Texas

“… ,− Sections S6–S13 provide significantly more details on aerosol mass and elemental concentrations, synoptic-scale modeling to detect African dust, and its influences on the atmospheric geochemistry of Houston. In brief, the presence of long-range transported dust during our sampling campaign was indicated by a lower PM 10 /PM 2.5 mass ratio during the peak event (average 3.4 for samples S2–S5) compared to routine days (average 4.7 for samples S1, S6–S9) as previously reported in Houston. , Additionally, the average La/Ce ratio throughout this study was 0.55, which compared favorably with the upper continental crust and Barbados Saharan dust values of 0.50 , further evidencing dominant crustally sourced dust during the entire study. Vanadium-rich industrial oil combustion aerosols, an important source in Houston, were detected by a lower La/V ratio of 0.24 compared with a North African dust value of 0.38. , These qualitative observations were quantitatively validated via chemical mass balance (CMB) modeling (Figure a) that confirmed the dominance of North African dust for samples S2–S5 contributing nearly half their measured total PM 10 mass (average 48 ± 9%, range 34–57%) during the peak episode.…”

“…Vanadium-rich industrial oil combustion aerosols, an important source in Houston, were detected by a lower La/V ratio of 0.24 compared with a North African dust value of 0.38. , These qualitative observations were quantitatively validated via chemical mass balance (CMB) modeling (Figure a) that confirmed the dominance of North African dust for samples S2–S5 contributing nearly half their measured total PM 10 mass (average 48 ± 9%, range 34–57%) during the peak episode. Saharan dust was also detected in all other samples but in smaller amounts, contributing an average of 12 ± 9% (2–25%) consistent with its presence on days before and after the peak intrusion. , Other sources are described in Section S5.…”

“…Saharan dust was also detected in all other samples but in smaller amounts, contributing an average of 12 ± 9% (2−25%) consistent with its presence on days before and after the peak intrusion. 21,22 Other sources are described in Section S5. Figure 1b summarizes simultaneous variations in La, V, and Ni concentrations (labeled with sample numbers) after normalization by upper continental crust (UCC) abundan-ces.…”

“…16,28,30 Consequently, the physical−chemical characteristics of particulate matter (PM) in this region have been studied in detail, but like in most other places, the aerosols' biological composition is relatively unknown. 10,11,[16][17][18]21,22 Windblown dust from North Africa 20,25 adversely impacts human health 2,9,19,[23][24][25][26]31 and disperses microorganisms to distal locations, 2,9,25,26,32−34 which to date has been largely delineated in places relatively proximal to the source including Africa, the Mediterranean Sea, the Middle East, and southern Europe. 2,9,23,24,26,27,35−44 Although few investigations have identified microorganisms associated with African dust in the Caribbean, Middle East, and remote portions of the Atlantic Ocean, 27,32,38,40,41,45−47 none have considered population centers in the North American mainland.…”

“…Ambient aerosols in population centers across the globe are a mixture of local emissions and long-range transported particles, including desert dust. ,,,− Houston, Texas, is no exception with its air quality being dominated by local anthropogenic emissions , and periodically impacted by aeolian dust from North Africa, principally the Sahara-Sahel region. − Greater Houston has a population of more than 7 million (the 5 th largest in the United States) and faces scrutiny about its air quality and environmental justice. ,, Consequently, the physical–chemical characteristics of particulate matter (PM) in this region have been studied in detail, but like in most other places, the aerosols’ biological composition is relatively unknown. ,,− ,, …”