Abstract
Abstract. 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.
Highlights
Transport of stratospheric air into the troposphere, referred to as stratosphere-to-troposphere transport (STT), contributes to and alters the trace gas composition of the troposphere, and as such STT has been extensively studied for over 50 yr (e.g., Danielsen, 1968; Danielsen and Mohnen, 1977; Lamarque and Hess, 1994; Sprenger et al, 2003; Thompson et al, 2007; Lefohn et al, 2011)
For the STT analysis we focus on reverse domain filling (RDF) O3, large-scale mixing efficiency, and continental planetary boundary layer (PBL) exposure
The difference in the trace gas composition of the stratosphere compared to the troposphere permits the identification of air masses of stratospheric origin found within the free troposphere occurring during STT
Summary
Transport of stratospheric air into the troposphere, referred to as stratosphere-to-troposphere transport (STT), contributes to and alters the trace gas composition of the troposphere, and as such STT has been extensively studied for over 50 yr (e.g., Danielsen, 1968; Danielsen and Mohnen, 1977; Lamarque and Hess, 1994; Sprenger et al, 2003; Thompson et al, 2007; Lefohn et al, 2011). The frequency and magnitude of STT events are important factors in understanding the possible degree to which they affect surface and free troposphere O3 mixing ratios (Lefohn et al, 2011). Tropopause PV values in literature range from 1.6 to 3.5 potential vorticity units (PVU) with 2 PVU used most often (Stohl et al, 2003) Both of these tropopause definitions are routinely used in forecasting and modeling STT events (e.g., Sprenger et al, 2003; Stohl et al, 2003). Evidence of STT at sea-level surface sites has been presented (Chung and Dann, 1985; Langford et al, 2012; Lefohn et al, 2012; Lin et al, 2012), the magnitude of the effects of STT on boundary layer O3 mixing ratios is still under debate (Fiore et al, 2003; Langford et al, 2009; Lefohn et al, 2011; Lin et al, 2012).
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