Ring Diagrams Overview

Ring Diagram Legend

GOOD - Green Hexagon

Measurement is within the interquartile range (IQR), defined as the 25th to 75th percentile or middle half (50%) of the measurements. Examples: sulfate, ammonium, sodium, and potassium.

SATISFACTORY - Blue Trapezoid

Measurement is within the range defined by the median + IQR/1.349. The ratio, IQR/1.349, is the non-parametric estimate of the standard deviation, sometimes called the pseudo-standard deviation. Examples: nitrate, chloride, and calcium.

UNSATISFACTORY - Red Triangle

Measurement is outside the range defined by the median + IQR/1.349. Examples: pH, conductivity, and magnesium.

DETECTION LIMIT - Open Circle

Measurement is below the detection limit of the laboratory’s analytical method. Example: fluoride.

Notes:

  1. An outward pointing trapezoid or triangle indicates the measurement is high. An inward pointing trapezoid or triangle indicates the measurement is low.
  2. A hexagon or trapezoid with cross-hatching (for example: potassium) indicates a measurement that meets the GOOD or SATISFACTORY definition but fails the measurement Data Quality Objective (DQO) listed in Table 1, below. DQOs are defined in the Manual for the GAW Precipitation Chemistry Programme. The Science Advisory Group for Total Atmospheric Deposition met in 2017 and decided to update the DQOs based on Inter-laboratory Comparison Study results. The changes, listed in red, became effective on 1 January 2018 and will be used in Inter-laboratory Comparison Studies after this date.
  3. A straight line without a symbol indicates no measurement was reported. For example: acidity.

Table 1. Precipitation Chemistry - Data Quality Objectives

Measurement DQO (before January 2018) DQO (effective January 2018)
pH < 4.00 + 0.07 units + 0.05 units
pH: 4.00 - 4.99 + 0.07 units + 0.07 units
pH > 5.00 + 0.07 units + 0.10 units
conductivity + 7% + 7%
acidity + 25% + 25%
sulfate + 7% + 5%
nitrate + 7% + 5%
ammonium + 7% + 7%
fluoride none + 20%
chloride + 10% + 10%
sodium + 10% + 10%
potassium + 20% + 20%
calcium + 15% + 15%
magnesium + 10% + 10%