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A long-lasting haze episode which included a strong wind pollution day (SWPD) and stable pollution days (SPD) occurred in Lin’an from January 21 to 26, 2015. On Jan. 21, the Lin’an atmospheric background station experienced a north wind at ground level, which resulted in short-term northern input–based pollution. During January 23 to 26, stable atmospheric conditions led to a local accumulation of pollutants. The average number concentration, surface area concentration, volume concentration, and concentration of PM2.5 were 14,183 ± 7121 cm−3, 576 ± 154 μm2 cm−3, 1009 ± 151 μm3 cm−3, and 145 ± 41 μg/m3 in SWPD; 14,497 ± 7418 cm−3, 1033 ± 241 μm2 cm−3, 1435 ± 540 μm3 cm−3, and 205 ± 44 μg/m3 in SPD. The extinction coefficients calculated in SWPD and SPD were 523.7 ± 281.7 Mm−1 and 918.0 ± 416.9 Mm−1. The number concentration spectra in SWPD were a bimodal distribution, which exhibited peak values at 38 nm and 88 nm (11,422 cm−3 nm−1 and 11,043 cm−3 nm−1). The number concentration spectra in SPD were a unimodal distribution with a peak at 96 nm (15,375 cm−3nm−1). Surface concentration spectra in SWPD and SPD were three-peak distributions with maximum concentration peaks at 230 nm and 575 nm (808 μm2 cm−3 nm−1 and 1087 μm2 cm−3 nm−1), respectively. The volume concentration spectra of SWPD had a four-peak distribution with peaks at 38 nm, 210 nm, 575 nm, and 1.3 μm. Appropriate relative humidity (RH) and particles in the 0.5–1-μm range were the main reasons for lower visibility for SPD. The contributions of ammonium nitrate (NH4NO3), ammonium sulfate ((NH4)2SO4), and organic matter (OM) to the extinction coefficients were 30.3%, 30.1%, and 19.7%, respectively, for SWPD; and 32.1%, 26.3%, and 23.4%, respectively, for SPD. The particles in SWPD came from the long-range transport and conversion of gas to particles, and SO42− and NO3− for SPD were mainly generated by heterogeneous reactions and homogeneous reactions, respectively. It was also determined that the enhancement of atmospheric oxidation resulted in greater secondary organic carbon (SOC) generation, and a reduction in RH led to a decrease in the main water-soluble ions.

H9N2 is the most prevalent low pathogenic avian influenza virus (LPAIV) in domestic poultry in the world. Two distinct H9N2 poultry lineages, G1-like (A/quail/Hong Kong/G1/97) and Y280-like (A/Duck/Hong Kong/Y280/1997) viruses, are usually associated with binding affinity for both α 2,3 and α 2,6 sialic acid receptors (avian and human receptors), raising concern whether these viruses possess pandemic potential. To explore the impact of mouse adaptation on the transmissibility of a Y280-like virus A/Chicken/Hubei/214/2017(H9N2) (abbreviated as WT), we performed serial lung-to-lung passages of the WT virus in mice. The mouse-adapted variant (MA) exhibited enhanced pathogenicity and advantaged transmissibility after passaging in mice. Sequence analysis of the complete genomes of the MA virus revealed a total of 16 amino acid substitutions. These mutations distributed across 7 segments including PB2, PB1, PA, NP, HA, NA and NS1 genes. Furthermore, we generated a panel of recombinant or mutant H9N2 viruses using reverse genetics technology and confirmed that the PB2 gene governing the increased pathogenicity and transmissibility. The combinations of 340 K and 588 V in PB2 were important in determining the altered features. Our findings elucidate the specific mutations in PB2 contribute to the phenotype differences and emphasize the importance of monitoring the identified amino acid substitutions due to their potential threat to human health.