A Global-scale Dust Equation Based on Measured Data

Abstract:

A robust method to estimate mineral dust mass in ambient particulate matter (PM) is essential as the dust fraction cannot be directly measured but is needed to understand the dust impacts on human health and environment. In this study, a global-scale dust equation is proposed that builds on the Interagency Monitoring of Protected Visual Environments (IMPROVE) network’s “soil” formula. We incorporate K, Mg, and Na using the mineral-to-aluminum (MAL) mass ratio of (K₂O+MgO+Na₂O)/Al₂O₃ and investigate its variation across major desert regions using measured data (including ambient PM and soil data) from literature and monitoring networks. We find that the MAL ratio in most regions is consistent with the MAL ratio (0.63) in the average upper continental crust, and use this to develop a general dust equation, 3.08Al+2.14Si+1.40Ca+1.36Fe+1.67Ti. To evaluate the new equation, we use measured total-mineral-mass (summing oxides of measured mineral elements) and residual-mass (subtracting carbonaceous material, ions, sea salt and particle-bound water from total PM) approaches for obtaining the reference dust mass. For the desert dust in source regions, the resulting estimates from the new equation are consistent with the “total mineral mass” with a small normalized mean bias (NMB) of -0.4~2%. For the desert dust transported to non-source regions such as Sahara dust over southern Europe and Gobi dust over Japan, the new equation still shows good performance (NMB within ±5%). When applied to major types of anthropogenic dust (agricultural soil, paved road, and unpaved road), in general, the new equation has lower bias than the IMPROVE equation. Additionally, we examine the performance of the new equation for PM_2.5 with high dust content from the U.S. IMPROVE network (mostly rural) and Chemical Speciation Network (mostly urban). The new equation estimates the dust mass well (NMB within ±5%) at most monitoring sites, but some southeastern sites require a lower MAL ratio in summer because of the Sahara dust impacts. Thus, the global-scale dust equation developed in this study is a promising approach for characterizing mineral dust of different types from various regions.