Direct N2O5 reactivity measurements at a polluted coastal site
Other literature type
Riedel, T. P.
Bertram, T. H.
Ryder, O. S.
Day, D. A.
Russell, L. M.
Gaston, C. J.
Prather, K. A.
Thornton, J. A.
Direct measurements of N<sub>2</sub>O<sub>5</sub> reactivity on ambient aerosol
particles were made during September 2009 at the Scripps Institution of
Oceanography (SIO) Pier facility located in La Jolla, CA. N<sub>2</sub>O<sub>5</sub>
reactivity measurements were made using a custom flow reactor and the
particle modulation technique alongside measurements of aerosol particle
size distributions and non-refractory composition. The pseudo-first order
rate coefficients derived from the particle modulation technique and the
particle surface area concentrations were used to determine the population
average N<sub>2</sub>O<sub>5</sub> reaction probability, <i> γ</i>(N<sub>2</sub>O<sub>5</sub>),
approximately every 50 min. Insufficient environmental controls within
the instrumentation trailer led us to restrict our analysis primarily to
nighttime measurements. Within this subset of data, <i> γ</i>(N<sub>2</sub>O<sub>5</sub>) ranged from <0.001 to 0.029 and showed significant
day-to-day variations. We compare these data to a recent parameterization
that utilizes aerosol composition measurements and an aerosol thermodynamics
model. The parameterization captures several aspects of the measurements
with similar general trends over the time series. However, the
parameterization persistently overestimates the measurements by a factor of
1.5–3 and does not illustrate the same extent of variability. Assuming
chloride is internally mixed across the particle population leads to the
largest overestimates. Removing this assumption only partially reduces the
discrepancies, suggesting that other particle characteristics that can
suppress<i> γ</i>(N<sub>2</sub>O<sub>5</sub>) are important, such as organic coatings
or non-aqueous particles. The largest apparent driver of day-to-day
variability in the measured <i> γ</i>(N<sub>2</sub>O<sub>5</sub>) at this site was the
particle nitrate loading, as inferred from both the measured particle
composition and the parameterizations. The relative change in measured
<i> γ</i>(N<sub>2</sub>O<sub>5</sub>) as a function of particle nitrate loading appears
to be consistent with expectations based on laboratory data, providing
direct support for the atmospheric importance of the so-called "nitrate