Wind, sand, and dust storms will become more prevalent as the dry season approaches. One phenomenon that occurs during dust storms is saltation. Saltation is a method of particle transport driven by a fluid—in this case, wind.
Finer particulates (<0.1mm) are generally suspended in the moving fluid, are carried into the atmosphere, and can be brought back down to the surface via precipitation. These particulates can travel over hundreds of miles. Coarser particulates (0.5 – 1mm) exhibit “soil creep,” where they roll, slide, or are pushed along with the wind, rarely coming off the surface.
Mid-range particulates—those that fall in the 0.1 – 0.5mm range—are subject to saltation. With wind speeds that hit a critical value, these particulates are carried by the wind and exhibit a lifting and falling motion, similar to bouncing. Once the particle has gained enough acceleration, it can either (1) collide with particles on the ground and dislodge them into motion at wind velocities lower than those required to move them by wind alone. This process is called reptation. Or (2) through the process of attrition, break into smaller particles upon impact and continue the loop. Saltation occurs within 30cm of the surface and accounts for 50 – 90% of the total movement of soil by wind.
Regardless of the mechanism, these Aeolian processes create ripples and dunes in the landscape. Sand accumulates on the windward side and slides down the leeward face. Both ripples and dunes are comprised of an asymmetrical wave with longer, gradual upwind slopes and shorter, steeper downwind slopes.
Sand and dust storms generally occur in arid and semi-arid regions but are also prevalent in agricultural areas and coastal zones. Notable areas in California that are subject to these storms are the Salton Sea, Mono Lake, the Central Valley area, as well as coastal dunes. These environments exhibit either sparse or nonexistent vegetation cover, plenty of fine sediment such as clay, silt, or sand, and relatively strong winds. The winds mobilize the sand and dust, resulting in major impacts on the environment, economy, and human health.
We can assess the emissions of particulates due to saltation in a few ways.
Sand Flux Measurements
Saltating particles travel relatively small distances during wind events, so measurements of horizontal sand flux can indicate the amount of wind erosion occurring. One method for conducting sand flux measurements requires two instruments: a Cox Sand Catcher (CSC)— essentially a particle collection bin—and a Sensit—a real-time particle impact sensor. Placing the inlet of the Sand Catcher and the sensor of the Sensit at the same height, generally 15cm above the surface, assures accuracy. These two instruments are collocated at sites 100 – 1000m apart, and placing additional CSC units in the field without collocated Sensits provides better spatial resolution.
Sample tubes within the Sand Catchers are collected monthly, or more frequently during windy periods, and then delivered to the gravimetric lab for weighing. Sensits use a piezo electrical crystal to continuously detect and measure saltation activity as particle count and kinetic energy. Saltation flux is a mass measurement of particles that pass horizontally through a vertical plane; this is measured in units of mass/area, as opposed to PM concentration, measured in units of mass/volume.
Once this is determined, PM can be estimated using dispersion modeling programs such as AERMOD or CALPUFF.
Particle Counters
The SANTRI (Standalone AeoliaN Transport Real-time Instrument) is a particle counter that operates on the principle of interruptions through a wavelength of light. It counts particles as they pass through an infrared beam, detecting up to seven (7) different grain-size ranges.
SANTRIs require a solar panel and a water-resistant electrical enclosure containing a 12V lead-acid battery to provide power. The enclosure mounts on a freely rotating pole with a wind vane attached, allowing it to align with the wind direction.
One problem with the SANTRI as an optical sensor is it can overestimate particle transport when particles moving largely outside the detection beam are partially registered and counted. Alternatively, finer particles may cause insufficient attenuation of the beam signal and, in turn, are underestimated.
Particulate Matter Monitoring
Ambient particulate matter monitors such as Tapered Element Oscillating Microbalances (TEOMs) and Beta Attenuation Monitors (BAMs) log hourly concentrations of PM10 or PM2.5.
TEOMs are gravimetric instruments that pull ambient air through a filter at a constant flow rate, simultaneously weigh the filter, and calculate the real-time mass concentration.
DustTraks Monitors are real-time, near reference instruments that can measure PM1, PM2.5, Respirable, PM10, and PM Total at the same time. It uses a light-scattering photometer. Due to their low power consumption, they can deploy easily in remote areas and help develop a correlation between Saltation and PM concentration in specific areas according to the soil type.
Studies show a strong positive correlation between saltation flux and downwind PM concentrations. Measuring saltation can allow us to estimate particulate matter lofted into the air we breathe; however, the inverse isn’t always true.
Depending on the local conditions, there are different means to reduce the occurrence or compounding effects of saltation. Soil properties, topography, and meteorology are driving factors. Common methods are:
Nature can create beautiful landscapes of seemingly endless rolling waves of sand, but it can also be a major source of economic and human loss. As the climate continues to experience extremes, water scarcity and drought will further exasperate this problem in the western United States, among other regions with similar climate patterns. Lakes have been and will continue to dry and recede, exposing playa, and consequently, a major source of minerals and hyper saline particles subject to saltation.
Measurements of these natural phenomena can help us understand and plan for coexisting with nature in areas prone to wind events and storms. Measurement studies involving installing systems to quantify, assess, and control particulate matter in various locations surrounding a source—while costly and time-consuming as measurements take place on a years-long basis—are highly informative and help us adapt or create environmental solutions.
About the Author: Ms. Eva Luu is a project manager for the SCS Engineers’ specialty Tracer Environmental practice. She has over five years of experience in ambient air monitoring, ranging from black carbon in the LA basin, and formaldehyde at the highest peak in Germany, to odor mitigation in different applications nationwide. Ms. Luu conducts routine operation and inspection of air monitoring stations as well as the calibration and operation of pollutant analyzers, meteorological sensors, chart recorders, data acquisition systems, computers, and their associated software and peripherals. She also compiles monthly air monitoring reports and submits validated, processed data for air monitoring stations to the California Air Resource Board and the Santa Barbara County APCD. Ms. Luu has conducted various odor assessment projects across the nation, ranging from alternative agriculture farms to landfills. You may reach Eva at or LinkedIn if you want more information.
If you are interested in a career where your work matters to human health and the environment as Eva’s does — we’d enjoy hearing from you!
