The dependence of new particle formation rates on the interaction between cluster growth, evaporation, and condensation sink

New particle formation (NPF) is one of the major contributors to atmospheric aerosol number concentrations. The initial step of NPF includes the formation and growth of small clusters, their evaporation and loss to pre-existing particles (characterized by the condensation sink, CS). In the polluted atmospheric boundary layer, the high environmental CS suppresses NPF and it can work synergistically with evaporation to further reduce the NPF rates. In this study, to quantitatively include CS into NPF analysis, we make simplifications to the cluster balance equations and develop approximate equations for the NPF rates in the presence of pre-existing particles, which are applicable to nucleation mechanisms that can be represented by a nonbranched nucleation pathway. The developed equations show that the proportion of clusters that finally lead to new particle formation is given by the cluster-specific ratio of growth rate/CS | evaporation rate | growth rate. As a result, the cumulative product of this ratio for all clusters in the nucleation pathway determines the NPF rates. By comparing with benchmark cluster dynamics simulations of sulfuric acid-dimethylamine and sulfuric acid-ammonia nucleation systems, the developed equations were confirmed to give good estimates of the NPF rates and approximately capture the dependency of NPF rates on CS and nucleating vapor concentrations. The CS dependency predicted by the developed equations shows larger deviations from the simulations when the cluster evaporation rates are high, i.e., when the underlying assumptions of the equations are not satisfied. The equations were also found to be in good agreement with atmospheric NPF rates measured in long-term field observations in urban Beijing.Peer reviewe

Genetic characterization of the hemagglutinin genes of wild-type measles virus circulating in china, 1993-2009.

BACKGROUND: China experienced several large measles outbreaks in the past two decades, and a series of enhanced control measures were implemented to achieve the goal of measles elimination. Molecular epidemiologic surveillance of wild-type measles viruses (MeV) provides valuable information about the viral transmission patterns. Since 1993, virologic surveillnace has confirmed that a single endemic genotype H1 viruses have been predominantly circulating in China. A component of molecular surveillance is to monitor the genetic characteristics of the hemagglutinin (H) gene of MeV, the major target for virus neutralizing antibodies. PRINCIPAL FINDINGS: Analysis of the sequences of the complete H gene from 56 representative wild-type MeV strains circulating in China during 1993-2009 showed that the H gene sequences were clustered into 2 groups, cluster 1 and cluster 2. Cluster1 strains were the most frequently detected cluster and had a widespread distribution in China after 2000. The predicted amino acid sequences of the H protein were relatively conserved at most of the functionally significant amino acid positions. However, most of the genotype H1 cluster1 viruses had an amino acid substitution (Ser240Asn), which removed a predicted N-linked glycosylation site. In addition, the substitution of Pro397Leu in the hemagglutinin noose epitope (HNE) was identified in 23 of 56 strains. The evolutionary rate of the H gene of the genotype H1 viruses was estimated to be approximately 0.76×10(-3) substitutions per site per year, and the ratio of dN to dS (dN/dS) was <1 indicating the absence of selective pressure. CONCLUSIONS: Although H genes of the genotype H1 strains were conserved and not subjected to selective pressure, several amino acid substitutions were observed in functionally important positions. Therefore the antigenic and genetic properties of H genes of wild-type MeVs should be monitored as part of routine molecular surveillance for measles in China

Phylogenetic relationship based on the complete H gene sequences.

<p>Neighbour-joining tree was reconstructed with full-length H gene sequences from 56 genotype H1 wild-type measles isolates from mainland China, the WHO reference strains of each genotype and Chinese vaccine strains. The sequences of the circulating strains in 1993–1994 and in 2000–2009 are indicated by symbol “▴” and “•”, respectively, each color of symbol “•” represents the annually circulating strains. The genotype H2 reference strain below cluster 2 which is also marked by a triangle was identified in 1994, China. The branches for the different lineages are marked by various colors. The WHO standard name of MeVs and GenBank accession numbers of all the sequences are available in the figure. Numbers at nodes represent the percentage of 1,000 bootstrap replicates (values <70 are not shown). Bar, 0.005 nucleotide substitutions per site.</p

Primers used for amplification and sequencing of the entire H gene.

a<p>: s, sense orientation; as, antisense orientation.</p>b<p>: the primers' nucleotide locations and range on the basis of the Measles prototype strain: Edmonston complete genome (GenBank ID: AF266288).</p>c<p>: Length of the PCR product or use of the primer for sequencing (Seq).</p

Reported measles cases and incidence in China, 1991–2009.

<p>The number above the column represents the number of representative measles strains selected for the complete H gene sequence analysis. Blue bars indicate the number of reported measles cases and yellow solid diamonds indicate the incidence (/100,000 population) of each year, the Arabic numerals above the x-axis indicates the number of deaths. X-axis denotes year, y-axis on left denotes reported number of cases and y-axis on right denotes the incidence per 100,000 population.</p

Alignment of the predicted amino acid sequences of the partial H gene.

<p>The variation of amino acid Ser240Asn (highlighted in blue) leads to the absence of a glycosylation site; the exchange of Pro397Leu (highlighted in pink) that results in loss of recognition of two monoclonal antibodies directed against HNE; the amino acid residues of putative binding sites for CD46, SLAM and seven cysteine residues were highlighted in red, yellow and purple, respectively.</p