Port of Long Beach Monitoring Program Effects of Southern California Wildfires

Transcription

1 Port of Long Beach Monitoring Program Effects of Southern California Wildfires Four graphs are provided covering the period October 18-25, 2007, that illustrate the effects of the Southern California wildfires in the area around the Port of Long Beach.. 1. All PM Concentrations 2. PM 10 Concentrations 3. PM 2.5 Concentrations 4. PM 10 /PM 2.5 Ratio As you can see from the first graph, we started to see very elevated levels of PM 10 at both sites on Sunday October 21 st around 11:00AM. It s interesting that we don t see a similar rise in PM 2.5 concentrations, because PM 2.5 emissions from wildfires are generally much higher than PM 10 emissions. A very similar result in the region has been seen at other stations operated by air regulatory agencies; it may be because the heavier PM 10 particles reach the ground quickly while PM 2.5 particles remain suspended at elevated heights for longer periods. Downed power lines ignited the Malibu Canyon Fire at approximately 5AM on October 21 st located just north of Santa Monica. The sudden jump in PM 10 concentrations approximately 6 hours later at the stations were likely caused by the airborne particulate matter associated with this fire, as light northerly winds associated with the Santa Ana conditions transported these emissions south towards the Long Beach area. Analysis of the wind data at the stations on October 21 st confirms this theory, as we find fairly light NNE winds through the day, which would transport these emissions in a southerly direction down the LA coastline. The highest PM 10 concentrations occur during the nighttime hours of October 21 st, which would be expected due to: 1. The subsequent drop in wind speeds (less mechanical turbulence and dispersion of the particulate matter) 2. A lower nocturnal mixing layer (which traps the particulate matter closer to the ground) 3. The location of the monitoring stations along the coastline. (The physical location of the monitoring stations becomes important, because the nocturnal Santa Ana winds will continue to blow strongly in the mountainous areas of northern Los Angeles. However, we observe fairly light wind speeds out along the coastline during the nighttime hours). With this confluence of factors, the 24-hour average PM 10 concentrations during the worst 24-hr period are several times higher than the federal standard of 150 µg/m 3. We start to see a decline from these very elevated PM 10 concentrations during the evening hours on October 22 nd. This seems to coincide with a wind shift at the monitoring

2 stations from a NNE direction to a more easterly direction. Without the northerly component to the wind direction, the monitors do not seem to pick up the highly concentrated particulate emissions from the Malibu Canyon Fire, rather they seem to be measuring the generally elevated PM 10 concentrations in the greater LA basin. This would explain the still elevated PM 2.5 and PM 10 concentrations observed at both stations from October 23 rd through October 25 th, but not the extreme concentrations observed on October 21 st and 22 nd, where 1-hour averages ranged from µg/m 3. Again, this would generally coincide with the synoptic or regional-scale pattern of a Santa Ana wind event. During the early stages of a Santa Ana, the synoptic pattern generates a more north-northeast wind direction for the LA basin. However as this high pressure cell moved east, the isobars produced a more east-northeast wind pattern. These east-northeast winds would take the emissions from the Malibu Canyon Fire (due north of Long Beach) off the coastline, while the monitoring stations would be picking up the diluted (but still elevated) PM 10 emissions from the other fires around the LA basin. Given the amount of particulates that have pooled offshore, we will likely continue to see elevated particulate matter concentrations for a couple days as light onshore winds transport these offshore particulates back into the LA basin over the weekend. It will likely require a decent low pressure cell or the passage of a strong cold front to have the particulate matter concentrations return to a more seasonal range (~ µg/m 3 ). Lastly, checking the fourth graph - PM 10 /PM 2.5 Ratio you will notice that the normal PM 10 /PM 2.5 ratio is somewhere in the neighborhood of 2. This ratio is a decent surrogate to determine the anthropogenic origin of observed particulate matter concentrations. However on October 21 st, the PM 10 /PM 2.5 ratio is observed to increase to the range, which indicates a much higher component of PM 10 concentrations compared to PM 2.5. The exact cause of this high ratio isn t clear; it may be because the heavier PM 10 particles emitted by the wildfires reach the ground more quickly than the lighter PM 2.5 particles. It is clear, however, that the particulate monitors at the Port of Long Beach readily captured the effect of the wildfires on the local air quality.

3 25.0 Ratio PM 10 /PM 2.5 Concentrations SB - PM10/PM2.5 GP - PM10/PM

4 12 PM 2.5 Concentrations SB - PM2.5 GP - PM2.5 FED Standard - PM

5 100 PM 10 Concentrations SB - PM10 GP - PM10 FED Standard - PM10 CA Standard - PM

6 100 Particulate Matter Concentrations SB - PM10 SB - PM2.5 GP - PM10 GP - PM2.5 FED Standard - PM10 CA Standard - PM