Tropospheric Ozone Studies
A differential absorption lidar (DIAL) for tropospheric ozone profile measurements has been operational at TMF since November 1999. This system has measured high resolution ozone profiles from ~4 to 20 km altitude on more than 2000 nights since 1999. The use of an additional channel from the stratospheric ozone lidar system allows extending the upper limit of the profile to altitudes near 25 km, contributing high-quality profiles to NDACC up to the Lower Stratosphere. In 2012, the system joined the new US-based network TOLNet (Tropospheric Ozone Lidar Network). In 2018, the system was augmented with automation and with 2 very-near-range channels allowing the measurement of ozone down to about 100-m above ground, and for extended periods of time (e.g., several consecutive days) without operator intervention.
Vertical and seasonal structure of the nighttime ozone number density
A clear annual cycle peaking above 50% at altitudes around 15 km is characterized by a maximum in late winter/spring and a minimum in late summer/fall. A secondary maximum is observed in early winter. The annual cycle in the upper troposphere reverses around 23 km to display a typical maximum during summer in the mid-stratosphere, as already observed by our co-located stratospheric ozone lidar.
The springtime maximum between 15 and 20 km is associated with enhanced ozone in the frequent polar vortex filaments passing over TMF when the spring vortex is either disturbed, or breaks down. The springtime maximum between 10 and 15 km, is also connected to meridional circulation, as the mid- and high latitude tropopause surface is displaced southward of the TMF location, leaving layers of enhanced ozone aloft. A weak signature of stratospheric intrusion is also observed in the mid-troposphere in late spring.
Figure 1 (TMO, top-left) and Figure 2 (TMF, top-right):
The annual cycle is clearly characterized by a persistent ozone minimum near 15 km altitude. However strong internannual variability can be observed. Day-to-day inspection of the ozone profiles also show a large day-to-day variability.
Figure 3 (TMO, bottom-left) and Figure 4 (TMF, bottom-right):
The 5-year climatology shows a very well pronounced annual cycle around 15 km characterized by a maximum in late winter/spring, and a minimum in late summer/fall. The peak-to-peak variation exceeds 100%, which is not surprising considering the small amounts of ozone at this altitude. A secondary maximum is observed in December around 17-22 km, but the present statistics are not robust enough yet to clearly identify its origin. Another ozone maximum is observed in late spring around and below 10 km. This is likely a consequence of the occasional stratospheric intrusions observed during this season. Despite the fair data time-overlap between the stratospheric ozone measurements (no measurements in 2000) and the tropospheric ozone measurements, there is an impressive agreement between the climatologies obtained from each lidar. In particular, the “double minimum” in summer and fall around 15 km is remarkably well captured by both lidars.
Monitoring tropospheric ozone variability for several hours from the ground to the stratopshere
Since the very-near-range channels (referred to as "AQ" channels) were added in 2012, the JPL lidar group performed several long streaks of continuous measurements (up to 17 hours non-stop). During one of these extended measurement periods, specifically on April 9, 2013, the lidar was able to capture the variaiton in ozone during a stratospheric intrusion event. Figure 5 below summarizes these measurements, including the record-close range of the lidar measurent at 95-m above ground.
Stratospheric intrusions and their relationship with the subtropical jet
PV-ozone (respectively ozone-water vapor) correlations (respectively anti-correlations) are remarkably well captured by the JPL tropopsheric oozne and water vapor lidars at small vertical and horizontal scales, and short time scales. The lidars measure systematically dry, ozone-rich air originating from the high-laitude lowermost stratosphere on the poleward side of the subtropical jet, and mosit, ozone-poor originating on the equatorward side of the jet.
The example of 27 February 2004 was chosen for its outstanding double-layer features. Each layer of enhanced ozone is representative of a well identified feature throughout the 4½ years of measurements. To illustrate the major role of meridional transport, the latitudinal cross-section of Modified Ertel's PV (Lait PV, [Lait, 1991]) is plotted in Figure 6, and the PV maps at 2 isentropic surfaces 350 K and 450 K are plotted in Figure 7.
Figure 6a (PV cross-section below left), Figure 6b (ozone profile below right):
In figure 6a, two layers of higher MPV can be observed to extend south of 35ºN. One layer represents the southern extension of the tropopause surface (near 350-360 K and 35 km). The other layer represents the southward extension of the polar vortex filament around 435-450 K. These two layers are associated to the layers of enhanced ozone observed by lidar at 18-19 km and 13 km (figure 6b). Between the two layers of enhanced MPV, low values of MPV typical of the tropical upper tropospheric reservoir extend north of 35ºN. This corresponds to the layer of poor ozone observed around 15-16 km by lidar.
Figure 7a-b (isentropic PV maps below):
As can be seen in Figure 7a, the upper layer is associated with a polar filament passing over TMF. Filaments like this one are observed either when the vortex is strongly disturbed by planetary waves, or after the vortex breaks down and subsequently “dilutes” at mid and lower latitudes.
The lower layer of enhanced MPV/ozone is associated with the southward displacement of the tropopause surface intersecting the 350 K surface. For reference, the commonly used “2 PVU” dynamical tropopause is located well south of TMF, insuring the lidar site to be located in the bulk of the mid-latitude lower stratosphere (rich in ozone).
Relationship between tropospheric ozone, water vapor and the subtropical jet
The JPL water vapor and ozone lidar measurements, together with the advected potential vorticity results from the high-resolution transport model MIMOSA, allow the identification and study of a deep stratospheric intrusion over TMF. Figure 8 below shows PV maps from the high-resolution advection model MIMOSA, and ozone and water vapor anomalies measured by the JPL lidars on the night of October 19, 2009. The PV maps are shown for 03 UT (left) and 12 UT (right), at 355 K (top) and 330 K (bottom). Open circles and arrows point towards well identified regions of PV/ozone/water vapor correlation. At 355 K (top maps), the tropopause line steadily approaches TMF as the night progresses. This is characterized on the lidar data by increasing ozone mixing ratio throughout the night at around 12 km. At 330 K (bottom maps), a filament of enhanced PV passes over TMF early in the night, then exits the TMF area later in the night. It is clearly associated with an ozone-rich and dry layer near 9-10 km early in the night, splitting in two at 0400 UT, the bottom part propagating downward throughout the night.
Impact of the Los Angeles urban environment and stratospheric intrusions on the ozone budget near the surface at TMF
Figure 9 below shows how a high-elevation site like the JPL-Table Mountain Facility can experience high levels of ozone at -or- near the surface, under quite different meteorogical conditions. On June 10 and 11, hot summer days facilitated the build up of a deep, polluted planetary boundary layer (PBL), causing ozone to peak at values exceeding 100 ppbv in the afternoon and evening, and persisting through the night. A colocated ceilometer provided simultaneous PBL heigth information (black dots), and a colocated photometer provided simultaneous surface ozone measurements (thin horizontal strip at the bottom of the contour plot). After a day break (June 12), ozone concentrations again reach very high values (up to 120 ppbv) in the first 1 km above the surface. This time, high ozone near the surface was brought by two deep stratospheric intrusions (June 13-14), clearly identified by the lidar as ozone increases progressively with time, starting at the tropopause (11 km) and ending at the PBL. This "downward-like" propagation is the Eulerian representation of an ozone-rich tongue of stratospheric air passing over TMF. Interestingly, local meterology associated with the PBL keeps the second ozone-rich tongue (June 14) just above the PBL, with little apparent mixing.
Chouza, Chouza, F., et al. (2021), The impact of Los Angeles Basin pollution and stratospheric intrusions on the surrounding San Gabriel Mountains as seen by surface measurements, lidar, and numerical models, Atmos. Chem. Phys., 21(8), 6129-6153, doi:10.5194/acp-21-6129-2021.
Leblanc, T., McDermid, I. S., and Walsh, T. D.: Ground-based water vapor raman lidar measurements up to the upper troposphere and lower stratosphere for long-term monitoring, Atmos. Meas. Tech., 5, 17-36, 10.5194/amt-5-17-2012, 2012. https://doi.org/10.5194/amt-5-17-2012
McDermid, I. S., Beyerle, G., Haner, D. A., and Leblanc, T.: Redesign and improved performance of the tropospheric ozone lidar at the Jet Propulsion Laboratory Table Mountain Facility, Appl. Opt., 41, 7550-7555, 2002. https://doi.org/10.1364/AO.41.007550