Estimating the lifetime of boundary layer NO_x for different polluted regions using GOME data

Nitrogen oxides (NO+NO_2=NO_x) are important trace gases with impact on health, rain acidity and especially ozone production and OH concentration in the troposphere. NO_2 is detectable from satellite platforms using differential optical absorption spectroscopy (DOAS). Tropospheric column densities can be retrieved by estimating and subtracting the stratospheric fraction.

A major goal of NO_2 retrievals from satellites is to estimate actual NO_x emissions. Therefore, knowledge of the lifetime τ of NO_x is essential. τ is highly variable due to its dependency on factors like OH concentration, humidity, actinic flux or wind transport. Typical values in the troposphere range from hours to days.

In this study we demonstrate the potential of GOME NO_2 data to estimate mean lifetimes of boundary layer NO_x for different regions and seasons. For this task, we investigate the weekly cycle of NO_2 in industrialized regions. Anthropogenic activities are reduced during the weekend. Hence, we find significant reduction (up to 50%) of tropospheric NO_2 on Sunday compared to working days. Furthermore, the weekend minimum of emissions also affects the Monday levels of NO_2. Thus the specific shape of the weekly cycle allows us to deduce τ. With this method, for instance for Germany, we obtain the mean lifetime of boundary layer NO_x to be 6 hours in summer and about 24 hours in winter.