Quantification of chemical and physical processes influencing ozone during long-range transport using a trajectory ensemble

Abstract

Abstract. During long-range transport, many distinct processes -including photochemistry, deposition, emissions and mixing -contribute to the transformation of air mass composition. Partitioning the effects of different processes can be useful when considering the sensitivity of chemical transformation to, for example, a changing environment or anthropogenic influence. However, transformation is not observed directly, since mixing ratios are measured, and models must be used to relate changes to processes. Here, four cases from the ITCT-Lagrangian 2004 experiment are studied. In each case, aircraft intercepted a distinct air mass several times during transport over the North Atlantic, providing a unique dataset and quantifying the net changes in composition from all processes. A new framework is presented to deconstruct the change in O 3 mixing ratio ( O 3 ) into its component processes, which were not measured directly, taking into account the uncertainty in measurements, initial air mass variability and its time evolution.The results show that the net chemical processing ( O 3chem ) over the whole simulation is greater than net physical processing ( O 3phys ) in all cases. This is in part explained by cancellation effects associated with mixing. In contrast, each case is in a regime of either net photochemical destruction (lower tropospheric transport) or production (an upper tropospheric biomass burning case). However, physical processes influence O 3 indirectly through addition or removal of precursor gases, so that changes to physical parameters in a model can have a larger effect on O 3chem than O 3phys . Despite its smaller magnitude, the physical processing distinguishes the lower tropospheric export cases, since the net photochemical O 3 change is −5 ppbv per day in all three cases.Processing is quantified using a Lagrangian photochemical model with a novel method for simulating mixing through an ensemble of trajectories and a background profile that evolves with them. The model is able to simulate the magnitude and variability of the observations (of O 3 , CO, NO y and some hydrocarbons) and is consistent with the time-average OH following air-masses inferred from hydrocarbon measurements alone (by . Therefore, it is a useful new method to simulate air mass evolution and variability, and its sensitivity to process parameters.

Meteorological factors controlling low-level continental pollutant outflow across a coast

“…Also ozone is depleted rapidly in the MBL (below 1 km during this time) as a result a shift from photochemical production to destruction of ozone (as shown in Fig. 9 of (Cain et al, 2012)) associated with the higher humidity and also deposition of species to the ocean surface such as nitric acid and ozone itself .…”

“…However, there were marked episodes of cross-coastal pollutant transport, some of which were observed by research aircraft as the air crossed the Atlantic to the Azores and Europe (Methven et al, 2006;Owen et al, 2006). Chemical processing following air masses for several days during these ICARTT Lagrangian cases has been examined in detail by Real et al (2008); Lee et al (2011);Cain et al (2012); Davis et al (2012).…”

“…Methven et al (2006) also identified a Lagrangian connection between these flights and two flights of the DLR Falcon aircraft flying near Ireland and the UK on 25 and 26 July respectively. They named it the ICARTT Lagrangian Case 3, and it has been used to examine long-range transport and chemical transformation (Real et al, 2008;Cain et al, 2012). The near-range chemical evolution of the air mass has been examined by several authors including Lee et al (2011) and Davis et al (2012).…”

“…Within the MBL the high humidity typically enables faster photochemical processing (via higher OH radical concentrations, e.g., Real et al (2008)) and soluble species also experience deposition to the surface. Cain et al (2012) used a Lagrangian model to quantify the integrated effects of chemical and physical processes on the composition of air masses undergoing longrange transport. For example, while the instantaneous rate of ozone change is dominated by photochemical loss, the ozone concentration is actually most sensitive to physical parameters which control mixing of precursors and deposition of species such as nitric acid to the ocean.…”

Meteorological Influences on Trace Gas Transport along the North Atlantic Coast during ICARTT 2004

“…Methven et al [21] advanced a novel Lagrangian method of tracking observed plumes across the North Atlantic (including the 7/20 case study) through the use of whole air samples from the aircraft intercepts to construct hydrocarbon footprints in conjunction with trajectory model forecasts. Real et al [22] and Cain et al [23] added to this Lagrangian approach, advancing new insights into the processes influencing the plume's chemical transformation and ozone production in particular based upon photochemical model simulations. In addition, these authors advanced that potential errors in their Lagrangian analyses may have derived from uncertainties in the lower tropospheric wind fields used to force the trajectory forecasts and diagnose mixing and transport in their simulations.…”