Abstract
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.
Highlights
Atmospheric nucleation involving sulfuric acid, water, and ammonia is believed to have a large impact on the properties of clouds and their effects on the radiation balance of the atmosphere (Dunne et al, 2016; Coffman and Hegg, 1995)
Despite repeated flushing of the gasdelivery lines, room air and perhaps dust may get introduced into the photolytic flow reactor (PFR) during a changeover; degassing of the exit of the dewar’s valve and the regulator port that are exposed to the atmosphere for weeks while at the supplier (AirGas) may play a role
A series of experiments were conducted where particles generated in a bulk flow reactor (BFR; Zollner et al, 2012; Glasoe et al, 2015) were introduced into the PFR where they could be grown with photochemical H2SO4
Summary
Atmospheric nucleation involving sulfuric acid, water, and ammonia is believed to have a large impact on the properties of clouds and their effects on the radiation balance of the atmosphere (Dunne et al, 2016; Coffman and Hegg, 1995). Over the decades there is a wide divergence in nucleation rates measured in laboratory experiments, in the binary system – see Fig. 6 in Zollner et al (2012) and Fig. 9 in Hanson et al (2019) While many of these discrepancies may be attributable to contamination or other experimental conditions, this leaves nucleation rates for atmospheric conditions somewhat uncertain. We report more measurements in the putative binary system to gain further information on the effects of potential contaminants in our photolytic flow reactor (PFR) (Hanson et al, 2019). Kürten (2019) and Hanson et al (2019) used cluster models to derive thermodynamic information from the CLOUD and the photolytic flow reactor (PFR) results, respectively. We present the effects of relative humidity (RH) on the nucleation rate in the ternary ammonia–sulfuric acid–water system
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