Total VOC reactivity in the planetary boundary layer 2. A new indicator for determining the sensitivity of the ozone production to VOC and NOx

A new indicator is proposed for determining if tropospheric ozone productio n in a specific area is limited by VOC or NOx. The indicator Theta = tau (V OC)(OH)/tau (NOx)(OH) describes the ratio of the lifetimes of OH against th e losses by reacting with VOC and NOx. Whereas tau (NOx)(OH) can be obtaine d by conventional measurements, the new pump and probe OH approach which is described in part one of this publication makes it now possible to obtain also tau (VOC)(OH). Indicator values above a threshold value of 0.2 +/- 50% are representative of NOx-saturated conditions where an increase of NOx em issions causes lower ozone production. For values below 0.01 the ozone prod uction is very insensitive to changes of VOC emissions. The robustness of t his indicator against several parameters such as temperature, humidity, pho tolysis, and initial ozone concentrations is tested in a box model and comp ared to the robustness of other earlier proposed indicators. In contrast to earlier proposed indicators, this new one is not based on photochemically produced long-lived species but describes the instantaneous regime of an ai r parcel. Three-dimensional simulation shows that this indicator is quite s uccessful in estimating the impact of increased or reduced emissions on the ozone concentrations for each location in the modeling area. This will mak e it a very helpful tool for developing ozone abatement strategies.