[en] With the rapid development of precision measurements relying on atom absorption such as atomic inertial sensors and optical magnetometry, advanced lasers are urgently required especially ultralow-noise lasers corresponding to the atom absorption line. For the D2 line of rubidium atoms, an ultralow-intensity-noise 780 nm single-frequency fiber-based laser with an output power of 340 mW is demonstrated. By eliminating polarization sensitivity in the noise suppression process based on the gain saturation of a semiconductor optical amplifier, a relative intensity noise of −151 dB Hz⁻¹ in the frequency range from 0.1 to 50 MHz is achieved, which approaches the shot noise limit. (author)

[en] Highlights: • Real part refractive index (RRI) of different atmospheric aerosols are measured. • RRI varies significantly between 1.36 and 1.78 for different atmospheres. • Aerosol optical and radiative properties vary significantly due to the variation RRI. The real part of the refractive index (RRI) of ambient aerosol, which is widely used in remote sensing and atmospheric models, is one of the key factors determining its particles' optical properties. The characteristics of ambient aerosol RRI in China have not yet been well studied owing to a lack of observations. For the first time, the properties of aerosol RRI were studied based on field measurements in China at four sites with different atmospheres. The results revealed that the measured ambient aerosol RRI varied significantly between 1.36 and 1.78, increasing with the mass ratio of organic components. The scattering coefficient and direct radiative effects of the aerosols were estimated to increase by factors of 2 and 3, respectively, when RRI increased from 1.36 to 1.78. Our results indicate that variation in ambient aerosol RRI should be considered in aerosol and climate models to achieve an accurate estimation of aerosol's radiative impacts.

[en] A continuously spacing-tunable multi-wavelength single-frequency fiber laser (MW-SFFL) at 1.06 μm is investigated based on cascaded four-wave mixing (CFWM) in a high-nonlinearity photonic crystal fiber (HN-PCF). By using two separate wavelength-tunable continuous-wave (CW) SFFLs for pumping the HN-PCF, the MW-SFFL with up to ten stable wavelengths is generated under a total pump power of 890 mW. Furthermore, when the wavelength and the power of pump lasers are appropriately manipulated, the wavelength spaces of the MW-SFFL can be tuned freely within 166 pm and the numbers of wavelengths can be controlled to 4, 6, 8, and 10, respectively. To our knowledge, it is demonstrated for the first time that the MW-SFFL based on CFWM at 1.0 μm band are directly excited by CW SFFLs. (paper)

[en] Highlights: • The typical evolution patterns of PM_2.5 in each season of Beijing were depicted. • A novel method was developed to elucidate the driving species of PM_2.5 pollution. • Organic matters were the driving species in the winter and nitrate contributed more than sulfate during severe hazes. • Aqueous pathway was dominant in the winter while multiple pathways coexisted in the other seasons. • The variation of fine particle pH during pollution episodes was revealed. In recent years, air pollution has become a major concern in China, especially in the capital city of Beijing. Haze events occur in Beijing over all four seasons, exhibiting distinct characteristics. In this study, the typical evolution patterns of atmospheric particulate matter with a diameter of less than 2.5 μm (PM_2.5) in each season were illustrated by episode-based analysis. In addition, a novel method was developed to elucidate the driving species of pollution, which is the largest contributor to the incremental PM_2.5 (ΔPM_2.5), not PM_2.5. This method revealed a temporal variation of the driving species throughout the year: nitrate-driven spring, sulfate-driven summer, nitrate-driven early fall, and organic matters (OM)-driven late fall and winter. These results suggested that primary organic particles or volatile organic compounds emissions were dominant in the heating season due to residential heating, while NOx and SO₂ emissions dominated in the other seasons. Besides, nitrate formation seemed more significant than sulfate formation during severe pollution episodes. It was also found that the pollution formation mechanism in the winter showed some unique features in comparison with the other seasons: aqueous reactions were more important in the winter, while multiple pathways coexisted in the other seasons. Furthermore, this study confirmed that the PM_2.5 in Beijing was moderately acidic despite a fully neutralized system. In addition, the acidity variation during pollution episodes displayed different patterns between seasons and was driven by both the variation of aerosol water and chemical compositions. These results provide a new perspective to understand the characteristics and mechanisms of aerosol pollution in Beijing. However, more accurate measurements are necessary for effective air pollution control that depends on the seasonal variation of fine particle formation in Beijing and the surrounding areas.

[en] Highlights: • Nitrate accounted for the largest mass fraction in PM_2.5 during pollution episodes. • Nitrate contributed mostly to visibility impairment during the haze days. • Visibility degraded significantly with increasing nitrate fraction in PM_2.5. The annual mean PM_2.5 mass concentration has decreased because of the stringent emission controls implemented in Beijing, China in recent years, whereas the nitrate $\left({\mathrm{NO}}_3^{–}\right)$ mass fraction in PM_2.5 increases gradually. Low-visibility events occur frequently even though PM_2.5 pollution has been mitigated significantly, with the daily mean PM_2.5 mass concentration mostly less than 75 μg/m³. In this study, the non-linear relationship was analyzed between atmospheric visibility and PM_2.5 based on chemical composition from a two-year field observation. Our results showed that ${\mathrm{NO}}_3^{–}$ became the main constituent of PM_2.5, especially during the haze pollution episodes. A localized parameterization scheme was proposed between the atmospheric extinction coefficient (${\mathrm{\sigma }}_{\mathrm{ext}}$) and major chemical constituents of PM_2.5 by multiple linear regression (MLR). The contribution of ${\mathrm{NO}}_3^{–}$ to ${\mathrm{\sigma }}_{\mathrm{ext}}$ increased with increasing air pollution, and ${\mathrm{NO}}_3^{–}$ became the most important contributor for PM_2.5 above 75 μg/m³. The visibility decreased with increasing ${\mathrm{NO}}_3^{–}$ mass fraction for the same PM_2.5 mass concentration when PM_2.5 was above 20 μg/m³. The hygroscopicity of PM_2.5 increased with increasing mass fraction of hygroscopic ${\mathrm{NO}}_3^{–}$. These results stressed the importance of reducing particulate ${\mathrm{NO}}_3^{–}$ and its precursors (for instance, NH₃) through effective emission control measures as well as the tightening of PM_2.5 standards to further improve air quality and visibility in Beijing.