P Ox

The net Ox production derived from observation of A[Ox] is equal to the gross Ox production minus the gross photochemical loss, deposition and dispersion of Ox.

is the ozone production efficiency. It is the number of O3 molecules produced per molecule of NOx consumed photochemically and a key factor in understanding the photochemical formation of O3 (Liu et al., 1987; Trainer et al., 2000). Typical values of £ over the US are in the range of 1-20mol/mol (Jacob, 1999).

The most straightforward way to evaluate the net production rate of O3 is to use a model. However, modeling results usually contain large uncertainties that exist in the emission inventories, meteorology parameters, photochemical processes, etc. which are essential components of all models. In the following, we use the data from field measurements to derive the ozone production efficiency and P(Ox). This is an observation based method/model (OBM) which can reduce the uncertainties by using observations to bypass some of the processes, including emission inventories, photochemical processes and meteorology parameters.

In the daytime, the major removal process of NOx is the oxidation of NO2 by OH that produces HNO3. HNO3 is readily scavenged by gas-to-particle conversions, washout or dry deposition. A minor sink of NOx is the formation of peroxyacetylnitrate (PAN). However, PAN can undergo rapid thermal decomposition and regenerate NOx at the prevailing temperature during the experiments. Nighttime reactions of NO3 and N2 O5 are significant but neglected because the region studied in this work has substantial fresh emissions in the morning and we are mainly dealing with the production of O3 a few hours after the emissions of O3 precursors. Therefore we consider the reaction of NO2 with OH as the only removal process for NOx and assume that the removal of NOx is pseudo-first-order, as shown below. In this case, following a Lagrangian trajectory, we have

where k is the reaction rate constant for NOx with OH. The reaction rate constant for NO2 with OH is 1.04 • 10-11 cm3 s-1 at 25°C and one atmosphere pressure (Sander et al., 2002). Since NO2 is part of the NOx, the value of k should be scaled down by the ratio NO2/NOx. The average of the NO2/NOx ratio is about 0.6, and thus k for NOx is prescribed at 6.0 • 10-12 cm3 s-1.

The ratio of two hydrocarbons which have the same emission sources but different reactivities with OH can be used as a measure of photochemical oxidation by OH (Calvert, 1976; Singh et al., 1977; Robert et al., 1984; Parrish et al., 1992; Mckeen and Liu, 1993; Kramp et al., 1997). We choose a pair of aromatic hydrocarbons, ethylbenzene and m,p-xylene, for this purpose (Chang et al., 2007). Again following the Lagrangian trajectory, we have

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