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We demonstrate a periodic cycle of PM episodes in Beijing that is governed by meteorological conditions and characterized by two distinct aerosol formation processes of nucleation and growth, but with a small contribution from primary emissions and regional transport of particles.

Nucleation consistently precedes a polluted period, producing a high number concentration of nano-sized particles under clean conditions.

Our results highlight that regulatory controls of gaseous emissions for volatile organic compounds and nitrogen oxides from local transportation and sulfur dioxide from regional industrial sources represent the key steps to reduce the urban PM level in China.–5).

Aerosols can be directly emitted (primary) or formed through the gas-to-particle conversion process (secondary) in the atmosphere (6, 7).

During the polluted period, the movement of the air mass is less than 50 km⋅d transport. 1 and 2 indicate that the aerosol nucleation and growth processes occur on the regional (several hundred kilometers) to urban (less than 100 km) scales, respectively, and local-scale phenomena, such as primary particle emissions by traffic, exert an insignificant impact on the particle mass growth.).

The shaded colors denote the mass concentrations of the aerosol constituents, i.e., green for organics, blue for nitrate, red for sulfate, yellow for ammonium, purple for chloride, and black for black carbon.).

There is a noticeable absence of new particle formation as the pollution episode develops, indicating the suppression of nucleation by preexisting particles.

During the transition and polluted periods, a continuous growth from the nucleation mode particles is clearly depicted by the evolution in the mean particle size (Fig.

For example, the noticeably decreasing mass concentrations during the morning hours on 26, 27, and 28 September coincide with wind speed increasing by 1.8, 0.5, and 1.8 m⋅s); the timing and size of the elevated aerosols are indicative of primary particle emissions originating from local transportation (24, 25).

Although primary emissions by local traffic are clearly discernible by the transiently elevated number concentrations and may play a role in maintaining the particle number concentrations during the polluted period, these primary particles do not appreciably alter the particle mass growth.

We illustrate the similarity and difference in particulate matter (PM) formation between Beijing and other world regions.

The periodic cycle of PM events in Beijing is regulated by meteorological conditions.

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