Abstract. Nucleation rates involving sulfuric acid and water
measured in a photolytic flow reactor have decreased considerably over a
time period of several years. Results show that the system – flow reactor,
gas supplies and lines, flow meters, valves, H2SO4 photo-oxidant
sources – has reached a baseline stability that yields nucleation
information such as cluster free energies. The baseline nucleation rate is
punctuated by temporary bursts that in many instances are linked to cylinder
changes, delineating this source of potential contaminants. Diagnostics were
performed to better understand the system, including growth studies to assess
H2SO4 levels, chemiluminescent NO and NOx detection to
assess the HONO source, and deployment of a second particle detector to
assess the nanoparticle detection system. The growth of seed particles shows
trends consistent with the sizes of nucleated particles and provides an
anchor for calculated H2SO4 concentrations. The chemiluminescent
detector revealed that small amounts of NO are present in the HONO source,
∼ 10 % of HONO. The second condensation-type particle
counter indicates that the nanoparticle mobility sizing system has a bias at
low sulfuric acid levels. The measured and modeled nucleation rates
represent upper limits to nucleation in the binary homogeneous system,
H2SO4-H2O, as contaminants might act to enhance nucleation
rates and ion-mediated nucleation may contribute. Nonetheless, the
experimental nucleation rates, which have decreased by an order of magnitude
or larger since our first publication, extrapolate to some of the lowest rates
reported in experiments with photolytic H2SO4. Results from
experiments with varying water content and with ammonia addition are also
presented and have also decreased by an order of magnitude from our previous
work; revised energetics of clusters in this three-component system are
derived which differ from our previous energetics mainly in the five-acid and
larger clusters.