Impact of NOx in SOA and organonitrates production

<p>The concentration of nitrogen oxides (NOx) and their reservoir species, the organonitrates (ON), impacts on the secondary organic aerosol (SOA) production. To estimate the effect of different NOx levels on SOA, we carried out a series of laboratory experiments at the Harvard Environmental Chamber (HEC) investigating the production and partitioning of total organonitrates from &#945;-pinene photo-oxidation in a NOx range varying between 1 ppb and 24 ppb. We measured not only the aerosol mass concentration by using a Scanning Mobility Particle Sizer (SMPS) and composition by an on-line aerosol mass spectrometry (AMS), but also the gas phase and particle-phase organonitrates (gON and pON, respectively) by a thermal dissociation laser-induced fluorescence (TDLIF). In our experimental conditions, we found the presence of crossover point of 6 ppb of NOx between clean and polluted conditions that affect the SOA production: in fact, the SOA yield for 1 to 6 ppb NO<em>_x</em> increased, and for >6 ppb NO<em>_x</em> steadily dropped. The ON partitioning ratio (pON/(pON+gON)) has been estimated, identifying that also this ratio is strongly affected by the NOx concentrations; in fact, it decreased from 0.27 to 0.13 as the NO<em>_x</em> increased from <1 to 24 ppb.&#160;</p>