Secondary particles formed via new particle formation (NPF)
dominate cloud condensation nuclei (CCN) abundance in most parts of the
troposphere and are important for aerosol indirect radiative forcing (IRF).
Laboratory measurements have shown that certain organic compounds can
significantly enhance the binary nucleation of sulfuric acid and H
Atmospheric particles, by acting as cloud condensation nuclei (CCN), modify cloud properties and precipitation, thus indirectly affecting the hydrological cycle and the climate. Aerosol indirect radiative forcing (IRF) remains a major uncertainty in assessing climate change (IPCC, 2013). Secondary particles formed via nucleation dominate particle number concentrations in many parts of the troposphere (Spracklen et al., 2008; Pierce and Adams, 2009; Yu and Luo, 2009), and global simulations indicate that nucleation schemes and parameterizations have a strong effect on the aerosol IRF estimations (Wang and Penner, 2009; Kazil et al., 2010; Yu et al., 2012). Different nucleation schemes, with nucleation rates depending on different variables, predict significantly different spatial patterns and seasonal variations in nucleation rates and CCN concentrations (Yu et al., 2010, 2015). Therefore, it is important to understand the mechanisms of new particle formation (NPF) and the key parameters controlling the contribution of NPF to CCN formation under a wide range of varying atmospheric conditions and to validate their representation in regional and global climate models.
A number of laboratory chamber studies indicate that certain organic species
can significantly enhance NPF (e.g., Zhang et al., 2004; Riccobono et al.,
2014). This finding may have important implications for the interactions of
anthropogenic and biogenic emissions and the associated climate forcing. In
this regard, it is necessary to assess the ability of organics-enhanced
nucleation to explain the nucleation phenomena observed in the atmosphere and to
determine the contribution of organics to atmospheric NPF and climate
implications. In several laboratory studies, an empirical parameterization of
the formation rate as a function of the concentrations of sulfuric acid and
low-volatility highly oxidized organics has been derived (Metzger et al.,
2010; Riccobono et al., 2014). One of the most important limitations of these
empirical parameterizations is that they were derived from the chamber
measurements carried out under a limited range of well-controlled conditions,
and reliably extrapolating these data to a wide range of atmospheric
conditions thus remains a major issue. It should also be noted that empirical
activation and kinetic nucleation formulas (
In a recent study comparing particle size distributions measured in nine
forest areas in North America (NA) with those predicted by a global size-resolved
(sectional) aerosol model, Yu et al. (2015) showed that the
H
Based on the CLOUD chamber study of nucleation processes involving sulfuric acid and organic compounds of relatively low volatility from the oxidation of pinanediol, Riccobono et al. (2014) derived the following organics-mediated nucleation parameterization (Nucl-Org):
The Nucl-Org parameterization given in Eq. (1), derived from laboratory
chamber studies at
Based on the classical homogeneous nucleation theory, the rate of nucleation
(
The changes in enthalpy (
One challenge here is to obtain the enthalpy change (
The equilibrium geometry of the most stable isomers of the heteromolecular
trimer composed of (C
Figure 1 presents the equilibrium geometry of the most stable isomers of the
heteromolecular trimer composed of (C
The calculated temperature dependence correction factor for Nucl-Org
parameterization (
The horizontal distributions of monthly mean nucleation rates (
It should be noted that
This work represents a major global modeling attempt in studying the effect
of temperature on organics-mediated nucleation in the atmosphere. This study
is built upon the work reported in Yu et al. (2015), and we use the
same global model (GEOS-Chem) and configurations as those described in Yu et
al. (2015). GEOS-Chem is a global 3-D model of atmospheric composition driven
by assimilated meteorological observations from the Goddard Earth Observing
System (GEOS) of the NASA Global Modeling and Assimilation Office (GMAO)
(e.g., Bey et al., 2001). More detailed information about GEOS-Chem and
updates can be found at the model website (
The main difference between the present study and the previous results reported
by Yu et al. (2015) is that the present study employs the
Figure 3 shows the effect of the
A
The dependence of organics-mediated nucleation rates (left axis). CN10
and CCN0.4 (right axis) averaged in the boundary layer (0–1 km) over the
whole globe for July 2006 on the
The horizontal distributions of monthly mean CN10 in the boundary
layer (0–1 km above surface) in July 2006 based on two organics-mediated
nucleation schemes:
As we have pointed out earlier, the previous comparisons of the simulated and
observed particle size distributions measured in nine forest areas in North
America (NA) (Yu et al., 2015) showed that
The effect of
The particle size distributions (PSDs) observed
To further illustrate the difference and improvement for the cases with and
without the
Figure 8 shows the ratios of the CCN concentration in the lower troposphere
(0–3 km) based on Nucl-Org to the CCN concentration based on Nucl-OrgT. The
CCN concentrations are calculated at a water supersaturation ratio of
0.2 % (CCN0.2) from the simulated PSDs. As a result of higher nucleation
rates, CCN0.2 based on Nucl-Org are about 10–20 % higher than those
based on Nucl-OrgT in July over most parts of the Northern Hemisphere (Fig. 6a),
with the largest difference up to 30–70 % reached over parts of NA,
Europe, and Asia. It is noteworthy that the present model simulation only
considers the Nucl-Org parameterization. While this enables us to show the effect of the
The ratios of the concentration of CCN (at a water supersaturation ratio of 0.2 %) in the lower troposphere (0–3 km) based on the Nucl-Org scheme to those based on the Nucl-OrgT scheme.
Simple empirical nucleation parameterizations, which were derived from
laboratory or field measurements under limited conditions and do not consider
any temperature dependence of nucleation rates, have been widely used in
global aerosol modeling and aerosol indirect radiative forcing studies. Based
on the classical nucleation theory, temperature should be one of the key
parameters controlling nucleation rates, unless the nucleation is barrierless. A
recent study indicates (Yu et al., 2015) that the empirical parameterization
of H
The study highlights the importance of including the temperature dependence
of nucleation rates in the global modeling of NPF and aerosol indirect radiative
forcing. In a recent study, Dune et al. (2016) also showed a substantial
impact of the temperature dependence on the contribution of organic
nucleation to overall nucleation. The temperature dependence factor derived
in this study can be applied to study the temperature effect on
organics-mediated nucleation in the global atmosphere and improve the
agreement of the simulated particle number concentrations with the observations.
Although it may be subject to uncertainties due to the possible difference
between the molecules involved in the nucleation and the proxy molecule, the
temperature-dependent
The work described in this paper is based on GEOS-Chem
version 8-03-02. The code, written in FORTRAN-90, is portable and efficient
on available parallel computing platforms (
The authors declare that they have no conflict of interest.
This study was supported by NASA under grant NNX13AK20G and the NSF under grant 1550816. We would like to acknowledge high-performance computing support from Yellowstone (ark:/85065/d7wd3xhc) provided by NCAR's Computational and Information Systems Laboratory, sponsored by the NSF. The GEOS-Chem model is managed by the Atmospheric Chemistry Modeling Group at Harvard University with support from NASA's Atmospheric Chemistry Modeling and Analysis Program. Edited by: V.-M. Kerminen Reviewed by: three anonymous referees