Tropospheric ozone seasonal and long-term variability as seen by lidar and surface measurements at the JPL-Table Mountain Facility, California

Abstract

A combined surface and tropospheric ozone climatology and interannual variability study was performed for the first time using co-located ozone photometer measurements (2013–2015) and tropospheric ozone differential absorption lidar measurements (2000–2015) at the Jet Propulsion Laboratory Table Mountain Facility (TMF; elev. 2285 m), in California. The surface time series were investigated both in terms of seasonal and diurnal variability. The observed surface ozone is typical of high-elevation remote sites, with small amplitude of the seasonal and diurnal cycles, and high ozone values, compared to neighboring lower altitude stations representative of urban boundary layer conditions. The ozone mixing ratio ranges from 45 ppbv in the winter morning hours to 65 ppbv in the spring and summer afternoon hours. At the time of the lidar measurements (early night), the seasonal cycle observed at the surface is similar to that observed by lidar between 3.5 and 9 km. Above 9 km, the local tropopause height variation with time and season impacts significantly the ozone lidar observations. The frequent tropopause folds found in the vicinity of TMF (27 % of the time, mostly in winter and spring) produce a dual-peak vertical structure in ozone within the fold layer, characterized by higher-than-average values in the bottom half of the fold (12–14 km), and lower-than-averaged values in the top half of the fold (14–18 km). This structure is consistent with the expected origin of the air parcels within the fold, i.e., mid-latitude stratospheric air folding down below the upper tropospheric sub-tropical air. The influence of the tropopause folds extends down to 5 km, increasing the ozone content in the troposphere. No significant signature of interannual variability could be observed on the 2000–2015 de-seasonalized lidar time series, with only a statistically non-significant positive anomaly during the years 2003–2007. Our trend analysis reveals however an overall statistically significant positive trend of 0.3 ppbv year−1 (0.6 %) in the free troposphere (7–10 km) for the period 2000–2015. A classification of the air parcels sampled by lidar was made at 1 km intervals between 5 and 14 km altitude, using 12-day backward trajectories (HYSPLIT, Hybrid Single Particle Lagrangian Integrated Trajectory Model). Our classification revealed the influence of the Pacific Ocean, with air parcels of low ozone content (43–60 ppbv below 9 km), and significant influence of the stratosphere leading to ozone values of 57–83 ppbv down to 8–9 km. In summer, enhanced ozone values (76 ppbv at 9 km) were found in air parcels originating from Central America, probably due to the enhanced thunderstorm activity during the North American Monsoon. Influence from Asia was observed throughout the year, with more frequent episodes during spring, associated with ozone values from 53 to 63 ppbv at 9 km.