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We quantify for the first time marine aerosol properties and their differences in the offshore and remote ocean in the mid-latitude South Asian waters, low-latitude South Asian waters, and equatorial waters of the Western Pacific Ocean, based on shipboard cruise observations conducted by the Western Pacific Ocean Scientific Observation Network in winter 2018, and further investigate the effects of long-range transport of continental aerosols on the marine environment. During the overall observation period, the average number concentration of particle matter which aerodynamic diameters<2.5 μm (PM 2.5N) was 35.1 ± 87.4 cm−3 and the mass concentration (PM 2.5M) was 12.3 ± 9.1 μg/m3. The PM 2.5N and PM 2.5M during the continental air mass transport period were 7.2 and 1.3 times higher than those during the non-transport period (109.2 ± 169.3 cm−3, 15.9 ± 14.9 μg/m3), respectively. Excluding transport period, the average PM 2.5N and PM 2.5M are reduced by 120% and 7%. Coarse mode particle number concentration (PM 2.5–10N) and mass concentration (PM 2.5–10M) are not significantly influenced by continental air masses (only a reduction of 7% and 2%). The variation of marine aerosol concentrations in different latitudes zones is greatly influenced by continental aerosol transport. The offshore PM 2.5M /PM 10M was 30%, 21%, and 22% in the mid-latitude sea of South Asia, a low-latitude sea of South Asia, and the equatorial sea, respectively. In comparison, in the remote ocean, the distribution ratio of PM 2.5M /PM 10M tended to be steady (22%–23%), and the background characteristics of marine aerosols were clearly represented. The aerosol concentration decreases with the increase of wind speed during the transport period, and the wind speed reflects the scavenging effect on aerosol. In the non-transport period, the wind speed at the sea surface promotes the generation of marine aerosols, and the impact in wind speed is strongest in the PM 2.5 –PM 5 particle size range. Schematic diagram of the distribution of aerosols in the western Pacific Ocean at different latitudes and offshore distance. The pie chart represents the particle size composition of different latitudinal zones. [Display omitted] • Large differences in aerosol distribution between near-land and open-ocean area. • Continental transport lead to increase in the proportion of fine particles. • Continental transport affects near-land aerosol distribution to varying degrees. • Different wind speed responses to transport and non-transport period. [ABSTRACT FROM AUTHOR]

Observation and analysis of the optical properties of atmospheric aerosols in a South Asian tropical rain forest showed that the annual mean aerosol optical depth (AOD) and aerosol Ångström exponent (α) at 500 nm were 0.47 ± 0.30 (± value represents the standard deviation) and 1.35 ± 0.32, respectively, from 2012 to 2014, similar with that of Amazon region. Aerosol optical properties in this region varied significantly between the dry and wet seasons. The mean AOD and α were 0.50 ± 0.32 and 1.41 ± 0.28, respectively, in the dry season and 0.41 ± 0.20 and 1.13 ± 0.41 in the wet season. Because of the combustion of the rich biomass in the dry season, fine modal smoke aerosols increased, which led to a higher AOD and smaller aerosol control mode than in the wet season. The average atmospheric humidity in the wet season was 85.50%, higher than the 79.67% during the dry season. In the very damp conditions of the wet season, the aerosol control mode was relatively larger, while AOD appeared to be lower because of the effect of aerosol hygroscopic growth and wet deposition. The trajectories were similar both in dry and wet, but with different effects on the aerosol concentration. The highest AOD values 0.66 ± 0.34 (in dry) and 0.45 ± 0.21 (in wet) both occurred in continental air masses, while smaller (0.38–0.48 in dry and 0.30–0.35 in wet) in oceanic air masses. The range of AOD values during the wet season was relatively narrow (0.30–0.45), but the dry season range was wider (0.38–0.66). For the Ångström exponent, the range in the wet season (0.74–1.34) was much greater than that in the dry season (1.33–1.54). [ABSTRACT FROM AUTHOR]