Ambient Measurements of the NOx Reservoir Species N2O5 using Cavity Ring-down Spectroscopy

The regulated control of pollutants is essential to maintaining good air quality in urban areas. A major concern is the formation of tropospheric ozone, which can be especially harmful to those with lung conditions and has been linked to the occurrence of asthma. Ozone is formed through reactions of oxidized volatile organic compounds with nitrogen oxides, and the accurate modeling of the process is necessary for smart and effective regulations. Ambient measurements are important to understanding the mechanisms involved in tropospheric chemistry.

This dissertation describes the characterization of a novel instrument for the ambient measurement of dinitrogen pentoxide, N2O5, and the results of several field studies. This is an important intermediate in the major nighttime loss pathway of nitrogen oxides. The understanding of this process requires correct modeling formation, as any nitrogen oxides not removed at night will result in increased ozone formation at sunrise.

Calibration studies have been performed in order to quantify the loss of reactive species within the instrument, and the sampling flow and N2O5 detection have been well characterized. The results of the laboratory measurements are presented.

Results are presented from the SHARP Field Study in Houston, TX in the spring of 2009. N2O5 measurements are compared to measurements of other species, including nitric acid and nitryl chloride, which were performed by other research groups. Mixing ratios exceeding 300 ppt were observed following ozone exceedance days, and a dependence of the concentration on both wind speed and direction was noticed. There was a strong correlation determined between N2O5 with HNO3 and ClNO2 indicating both a fast heterogeneous hydrolysis and N2O5 as the primary source of the species. Observed atmospheric lifetimes for N2O5 were short, ranging from several seconds to several minutes.

We have also investigated the presence of N2O5 in College Station, TX. Low mixing ratios peaking at approximately 20 ppt were observed, with longer atmospheric lifetimes of up to several hours. The role of biogenic emissions in the NO3-N2O5 equilibrium is discussed.