Abstract
Abstract. Hydroxyl radicals play a central role in the troposphere as they control the lifetime of many trace gases. Measurement of OH reactivity (OH loss rate) is important to better constrain the OH budget and also to evaluate the completeness of measured VOC budget. Total atmospheric OH reactivity was measured for the first time in an European Megacity: Paris and its surrounding areas with 12 million inhabitants, during the MEGAPOLI winter campaign 2010. The method deployed was the Comparative Reactivity Method (CRM). The measured dataset contains both measured and calculated OH reactivity from CO, NOx and VOCs measured via PTR-MS, GC-FID and GC-MS instruments. The reactivities observed in Paris covered a range from 10 s−1 to 130 s−1, indicating a large loading of chemical reactants. The present study showed that, when clean marine air masses influenced Paris, the purely local OH reactivity (20 s−1) is well explained by the measured species. Nevertheless, when there is a continental import of air masses, high levels of OH reactivity were obtained (120–130 s−1) and the missing OH reactivity measured in this case jumped to 75%. Using covariations of the missing OH reactivity to secondary inorganic species in fine aerosols, we suggest that the missing OH reactants were most likely highly oxidized compounds issued from photochemically processed air masses of anthropogenic origin.
Highlights
The hydroxyl radical (OH) represents the most important oxidant in the troposphere
In order to understand the OH reactivity results, a short description of the air masses impacting the site and a brief discussion on the pollution levels registered in Paris during the MEGAPOLI winter campaign are presented as follows
OH reactivity measurements using the Comparative Reactivity Method (CRM) method were realized in Paris between 23 January and until 13 February, during the MEGAPOLI winter campaign 2010
Summary
The hydroxyl radical (OH) represents the most important oxidant in the troposphere. It governs the atmospheric lifetime of most reactive trace species of anthropogenic and biogenic origin and contributes to the self-cleansing capacity of the atmosphere.Budget of OH radical is still associated with large uncertainty. The hydroxyl radical (OH) represents the most important oxidant in the troposphere It governs the atmospheric lifetime of most reactive trace species of anthropogenic and biogenic origin and contributes to the self-cleansing capacity of the atmosphere. In most cases the overall sink term of OH radicals is estimated by the sum of the products of concentration and the rate coefficient of all individually measured species reactive to the OH radical. This method has proved to be rather limited due to the large number of “OH reactive” volatile organic compounds (VOCs) in urban air (Lewis et al, 2000; Xu et al, 2003). Recent studies showed that there are unknown reactive species that contribute to the total OH reactivity
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