Stratosphere-to-troposphere transport (STT) results in air masses of stratospheric origin intruding into the free troposphere. Once in the free troposphere, ozone (O₃)-rich stratospheric air can be transported and mixed with tropospheric air masses, contributing to the tropospheric O₃ budget. Evidence of STT can be identified based on the differences in the trace gas composition of the two regions. Because O₃ is present in such large quantities in the stratosphere compared to the troposphere, it is frequently used as a tracer for STT events. This work reports on airborne in situ measurements of O₃ and other trace gases during two STT events observed over California, USA. The first, on 14 May 2012, was associated with a cutoff low, and the second, on 5 June 2012, occurred during a post-trough, building ridge event. In each STT event, airborne measurements identified high O₃ within the stratospheric intrusion, which were observed as low as 3 km above sea level. During both events the stratospheric air mass was characterized by elevated O₃ mixing ratios and reduced carbon dioxide (CO₂) and water vapor. The reproducible observation of reduced CO₂ within the stratospheric air mass supports the use of non-conventional tracers as an additional method for detecting STT. A detailed meteorological analysis of each STT event is presented, and observations are interpreted with the Realtime Air Quality Modeling System (RAQMS). The implications of the two STT events are discussed in terms of the impact on the total tropospheric O₃ budget and the impact on air quality and policy-making.

The NASA Ames Research Center operates a new research platform for atmospheric studies: an instrumented Alpha Jet. The present complement of instruments allows for the determination of carbon dioxide, ozone, water vapor, and methane concentrations as well as measurements of three-dimensional wind speeds, temperature, and pressure. Planned future instrumentation includes an Air-Core sampler and an instrument to measure formaldehyde. We give examples of measurements that have been made, including measurements carried out during a downward spiral over an expected methane source. An attractive property of this airborne system is its ability to respond rapidly to unexpected atmospheric events such as large forest fires or severe air quality events.

Increasing free tropospheric ozone (O₃), combined with the high elevation and often deep boundary layers at western U.S. surface stations, poses challenges in attaining the more stringent 70 ppb O₃National Ambient Air Quality Standard. As such, use of observational data to identify sources and mechanisms that contribute to surface O₃is increasingly important. This work analyzes surface and vertical O₃observations over California and Nevada from 1995 to 2015. Over this period, the number of high O₃events (95th percentile) at the U.S. Environmental Protection Agency Clean Air Status and Trends Network (CASTNET) sites has decreased during summer, as a result of decreasing U.S. emissions. In contrast, an increase in springtime 5th percentile O₃indicates a general increase of baseline O₃. During 2012 there was a peak in exceedances and in the average spring‐summer O₃mixing ratios at CASTNET sites. Goddard Earth Observing System‐Chem results show that the surface O₃attributable to transport from the upper troposphere and stratosphere was increased in 2013 compared to 2012, highlighting the importance of measurements aloft. Vertical O₃measurements from aircraft, ozonesondes, and lidar show distinct seasonal trends, with a high percentage of elevated O₃laminae (O₃ > 70 ppb, 3–8 km) during spring and summer. Analysis of the timing of high O₃surface events and correlation between surface and vertical O₃data is used to discuss varying sources of western U.S. surface O₃.

Ground based in-situ measurements of carbon dioxide (CO₂) and methane (CH₄) at the dry lakebed at Railroad Valley (RRV) playa, Nevada, USA (38°30.234′ N, 115°41.604′ W, elevation 1437 m) were conducted over a five day period from 20–25 June 2010. The playa is a flat, desert site with virtually no vegetation, an overall size of 15 km × 15 km and is approximately 110 km south-west of the nearest city, Ely (elevation 1962 m, inhabitants 4000). The measurements were taken in support of the vicarious calibration experiment to validate column-averaged dry air mole fractions of CO₂ and CH₄ (X_CO2 and X_CH4) retrieved from the Greenhouse Gases Observing Satellite (GOSAT) which was launched in January 2009. This work reports on ground-based in-situ measurements of CO₂ and CH₄ from RRV playa and describes comparisons made between in-situ data and X_CO2 and X_CH4 from GOSAT.