Determination of Atmospheric Dust Fall: Gravimetric Testing Methods and Procedures

Atmospheric dust fall determination quantifies settleable particulate matter deposited from ambient air using the gravimetric method specified in GB/T 15265 and ASTM D1739. This technical article explains the sampling devices, laboratory procedures, calculation formulas, quality control requirements, and typical applications of dust fall measurement. It serves as a practical reference for environmental monitoring agencies, accredited third-party laboratories, industrial facilities, and air quality management authorities.

Technical Overview

Dust fall, also referred to as settleable particulate matter or atmospheric deposition, consists of solid particles that settle out of the ambient atmosphere onto horizontal surfaces under the influence of gravity, impaction, and precipitation scavenging. The deposited material is dominated by coarse particles larger than 10 micrometers, typically originating from soil and mineral dust, construction and demolition activities, road traffic resuspension, wind erosion of stockpiles, and industrial emissions from processes such as material handling, crushing, and combustion.

Dust fall is a classical and widely used indicator of regional air pollution because it directly reflects the nuisance and soiling impact of coarse particulate pollution on surfaces, vegetation, and property. It complements measurements of suspended particulate matter such as TSP, PM10, and PM2.5, which characterize airborne concentrations rather than deposition fluxes. Results are conventionally reported in tonnes per square kilometer per 30 days, abbreviated as t/(km²·30d), or equivalently in g/(m²·30d).

The principal reference procedures are the gravimetric method described in GB/T 15265, Ambient Air – Determination of Dust Fall – Gravimetric Method, and the dustfall jar method described in ASTM D1739, Standard Test Method for Collection and Measurement of Dustfall (Settleable Particulate Matter). Both standards are based on passive collection of deposited particles in open-mouthed cylinders exposed for approximately one month, followed by quantitative gravimetric analysis in the laboratory.

Testing Samples

Dust Fall Cylinder Samples

The standard sampling device is a dust collection cylinder, commonly fabricated from glass, ceramic, porcelain, or chemically resistant plastic, with a typical internal diameter of 15 cm and a height of 30 cm. A bird guard ring or wire mesh is fitted around the rim to prevent contamination by birds and large debris. Before exposure, the cylinder is cleaned, dried, and charged with 300 to 500 mL of water to wet the deposited dust, together with a small quantity of algistatic and antifreeze additives where required.

Auxiliary and Quality Control Samples

• Field blank cylinders charged with water and additives, handled and transported identically to exposed samples, used for blank correction.
• Parallel duplicate cylinders installed at a defined proportion of monitoring sites, for example ten percent of the network, to assess precision.
• Optional wet-only and dry-only deposition samples collected with automated collectors for source apportionment and research studies.

Testing Items

• Total dust fall quantity, expressed in t/(km²·30d) or g/(m²·30d).
• Water-insoluble dust fall, representing the mineral and coarse combustion fraction retained on the filter.
• Water-soluble dust fall, representing dissolved salts and soluble combustion residues recovered from the filtrate.
• Optional chemical characterization of the deposited matter, including pH, electrical conductivity, water-soluble ions such as sulfate, nitrate, chloride, ammonium, calcium, magnesium, sodium, and potassium, and heavy metals such as lead, cadmium, chromium, arsenic, copper, and zinc.

Testing Methods

Method Principle

Particles deposited into an open collection cylinder over a precisely recorded exposure period are quantitatively recovered from the cylinder, separated into water-insoluble and water-soluble fractions by filtration, dried to constant mass at 105 ± 5 °C, and weighed on an analytical balance with 0.1 mg readability. The measured mass is normalized to the cross-sectional area of the cylinder aperture and to a standard 30-day exposure period, after subtraction of a reagent blank.

Sampling Procedure

• Select an open, unobstructed site representative of the monitoring zone, away from local point sources, tall chimneys, and sheltering structures.
• Install the cylinder on a stable stand at a height of 5 to 15 meters above ground, typically on a building rooftop, with the rim level and elevated clear of the roof parapet and any nearby obstructions.
• Record the installation date and time, cylinder number, aperture dimensions, and the volume and composition of the charging solution.
• Expose the cylinder continuously for 30 ± 2 days; inspect periodically and top up the water after heavy evaporation so that the collecting surface remains wet.
• At the end of the period, replace the cylinder with a freshly prepared one and seal the exposed cylinder with a cover for transport to the laboratory.

Laboratory Analysis

• Rinse the internal walls of the cylinder with a measured volume of water and transfer the entire suspension quantitatively to a clean beaker.
• Remove extraneous objects such as insects, leaves, and twigs with forceps after washing them thoroughly with water, and combine the washings with the sample.
• Filter the suspension through a pre-conditioned, pre-weighed quantitative filter or glass fiber filter to obtain the water-insoluble fraction.
• Dry the filter and retained residue at 105 ± 5 °C to constant mass and weigh to determine the water-insoluble dust mass.
• Evaporate an aliquot of the filtrate in a pre-weighed evaporating dish on a water bath, dry the residue at 105 ± 5 °C to constant mass, and weigh to determine the water-soluble dust mass.
• Process a reagent blank containing the same water volume and additives through the identical procedure for blank correction.

Calculation

Because one gram per square meter is numerically equal to one tonne per square kilometer, the total dust fall is calculated as follows, where W1 is the blank-corrected total mass of collected dust in grams, S is the cylinder aperture area in square meters, and n is the actual number of exposure days:

Q [t/(km²·30d)] = (W1 − W0) × 30 / (S × n)

The water-insoluble and water-soluble fractions are calculated with the same formula using their respective blank-corrected masses, and the two fractions should sum to the total within acceptable rounding. As a worked example, a cylinder with an aperture area of 0.0177 m² exposed for 30 days that collects a blank-corrected dust mass of 0.20 g yields a dust fall value of approximately 11.3 t/(km²·30d).

Quality Control Requirements

• Calibrate and verify the analytical balance regularly with certified reference weights.
• Apply a constant-weight criterion, requiring successive weighings to agree within 0.4 mg.
• Limit the relative deviation between parallel duplicate cylinders, commonly to within 20 percent.
• Use field blanks in every sampling campaign and correct all results for blank contribution.
• Maintain complete chain-of-custody records, including exposure dates, weather events, and any anomalies observed during the collection period.

Applications

• Urban and regional ambient air quality monitoring networks operated by environmental authorities.
• Fugitive dust supervision at construction and demolition sites, where dust fall is a key assessment indicator.
• Compliance monitoring around industrial parks, mines, quarries, ports, and bulk material stockpiles.
• Road dust and transportation corridor evaluation, including uncovered truck routes and unpaved roads.
• Environmental impact assessment, post-project acceptance testing, and pollution source verification.
• Scientific research on atmospheric deposition fluxes of nutrients, salts, and heavy metals to soils, water bodies, and ecosystems.

FAQ

What units are used to report dust fall results?

The standard reporting unit is tonnes per square kilometer per 30 days, written as t/(km²·30d) and sometimes described as tonnes per square kilometer per month. The equivalent unit g/(m²·30d) is also accepted, and older literature may use kg/(hm²·month).

How long must the collection cylinder be exposed?

The standard exposure period is 30 ± 2 days of continuous sampling, which integrates day-to-day variations in emissions and meteorology. The exact number of days is recorded and used to normalize the result to a 30-day basis in the calculation.

How does dust fall differ from TSP, PM10, and PM2.5?

TSP, PM10, and PM2.5 measure the mass concentration of particles suspended in the air volume, typically in micrograms per cubic meter. Dust fall measures the mass flux of particles that deposit onto a surface over time, dominated by coarse particles larger than about 10 micrometers, and is therefore an indicator of soiling and deposition nuisance rather than inhalation exposure.

Why are additives placed in the collection cylinder?

A small amount of copper sulfate solution is added to suppress the growth of algae and microorganisms that would add spurious mass, while ethylene glycol or another freezing-point depressant is added in winter to prevent the water from freezing and cracking the cylinder or ejecting the collected dust.

What is the difference between water-soluble and water-insoluble dust fall?

The water-insoluble fraction consists of mineral dust and coarse combustion particles retained on the filter, whereas the water-soluble fraction consists of dissolved salts, sulfates, nitrates, and other soluble residues recovered from the evaporated filtrate. Reporting both fractions provides useful information about the relative contribution of soil and road dust versus combustion and secondary sources.

What precision can be expected from the method?

With proper siting, parallel cylinders, and blank correction, duplicate measurements typically agree within 20 percent relative deviation. Method performance depends strongly on correct cylinder placement, protection from splashing and contamination, and strict adherence to constant-weight weighing procedures.