Lifetimes and timescales of tropospheric ozone: Ozone emission experiments

The lifetime of tropospheric O3 is difficult to quantify because we model
O3 as a secondary pollutant, without direct emissions.  For other
reactive greenhouse gases like CH4 and N2O, we readily model lifetimes and
timescales that include chemical feedbacks based on direct
emissions.  Here, we devise a set of artificial experiments with
a chemistry-transport model where O3 is directly emitted into the
atmosphere at a quantified rate.  We create three primary
emission patterns for O3, mimicking secondary production by surface
industrial pollution, that by aviation, and primary injection through
stratosphere-troposphere exchange (STE).  The perturbation
lifetimes for these O3 sources includes chemical feedbacks and varies from
6 to 27 days depending on source location and season.  Previous
studies derived lifetimes around 24 days estimated from the mean
odd-oxygen loss frequency.  The timescales for decay of excess O3
varies from 10–20 days in NH summer to 30–40 days in NH winter. 
For each season, we identify a single O3 chemical mode applying to all
experiments.  Understanding how O3 sources accumulate (the
lifetime) and disperse (decay timescale) provides some insight into how
changes in pollution emissions, climate, and stratospheric O3 depletion
over this century will alter tropospheric O3.  This work
incidentally found two distinct mistakes in how we diagnose tropospheric
O3, but not how we model it.  First, the chemical pattern of an
O3 perturbation or decay mode does not resemble our traditional view of
the odd-oxygen family of species that includes NO2.  Instead, a
positive O3 perturbation is accompanied by a decrease in NO2. 
Second, heretofore we diagnosed the importance of STE flux to tropospheric
O3 with a synthetic ‘tagged’ tracer O3S, which had full stratospheric
chemistry and linear tropospheric loss based on odd-oxygen loss
rates.  These O3S studies predicted that about 40 % of
tropospheric O3 was of stratospheric origin, but our lifetime and decay
experiments show clearly that STE fluxes add about 8 % to tropospheric O3,
providing further evidence that tagged tracers do not work when the tracer
is a major species with chemical feedbacks on its loss rates, as shown for
CH4.