Variability of SO=4, Total Sulfate, NO−3, and Total Nitrate Scavenging Ratios for the Frontal Boundary Study

Abstract

Abstract Precipitation and air concentration data collected during a U.S. Department of Energy–sponsored Frontal Boundary Study (FBS) were used to calculate scavenging ratios. The precipitation data were collected on a 100 km × 100 km surface grid containing 36 sampling sites, while the air concentration data were collected by aircraft. Radar and rawinsonde data were used to position the aircraft into the air mass feeding the precipitation. The coordination of the aircraft data with surface precipitation data allows the calculation of scavenging ratios. During the study, three out of seven events sampled allowed the calculation of scavenging ratios for SO=4, total sulfate, NO−3, and total nitrate. The 36 precipitation samplers allowed calculation of multiple scavenging ratios to represent an event. From these scavenging ratios, both a geometric mean and a geometric standard deviation of the scavenging ratio were calculated for each event. The geometric mean (geometric standard deviation) scavenging ratio for total sulfate is 6 × 104 (1.1) for 10 October 1989, 10 × 104 (1.5) for 16–17 October 1989, and 4 × 104 (1.3) for 31 October 1989. For SO=4 the scavenging ratios are 10 × 105 (1.1) for 10 October 1989, 4 × 105 (1.5) for 16–17 October 1989, and 3 × 105 (1.3) for 31 October 1989. The scavenging ratio for NO−3 is 3 × 106 (1.2) for 10 October 1989, 20 × 106 (1.4) for 16–17 October 1989, and 0.4 × 106 (1.5) for 31 October 1989. The scavenging ratio for total nitrate is 2 × 106 (1.2) for 10 October 1989, 2 × 106 (1.4) for 16–17 October 1989, and 0.2 × 106 (1.5) for 31 October 1989. The most important finding is the small variation of the numbers within the events as reflected in the geometric standard deviations. These values ranged from 1.1 to 1.5. Based on these results, a single scavenging ratio can be used on a 100 km × 100 km area with a minimum of error. Two other results were found when comparing these scavenging ratios to total precipitation in the sampler. The comparison revealed that the variation in the scavenging ratio increased with decreasing total precipitation. The increase was up to a factor of 2 for less than 5 mm when compared with greater than 5 mm of total precipitation.