"Can a Single Volcano Cool the Earth?"

Yes.
And if it should be in the tropics rather than one of the Russian or Alaskan 'cano's, the effect is magnified. Had the Icelandic eruption mentioned below (and in our 2015 post: Laki: How A Volcano Swallowed Europe) been located at the equator the impact would have been doubled or tripled.

From JSTOR Daily:

Mt Agung, in Bali, Indonesia, has spent the past month or so rumbling and spewing tons of ash into the atmosphere. Thousands of people living nearby have been evacuated, and experts fear a much larger eruption, although the local press claims Bali is now safe from any real threat. The last eruption, in 1963, killed more than 1000 people and displaced thousands of others. It is a dangerous event, but scientists intend to use the eruption to investigate ways to mitigate climate change through geoengineering. It seems far-fetched, but even one volcano can have impacts that affect the planet.

For example, in June of 1783, Iceland’s Laki volcano erupted, spewing tons of dust and aerosols into the air, followed by lava flows. The eruption continued for eight months, and when all was said and done the entire Northern Hemisphere experienced a noted temperature decrease. Some local areas, such as the Eastern United States and Northern Europe, experienced particularly dramatic decreases. There were dire consequences, especially in Iceland.

Over the next few centuries, there were at least five eruptions large enough to affect the temperature over the entire hemisphere. One standout was the Coseguina, Nicaragua eruption in the mid-nineteenth century. The 1963 Mount Agung eruption made a significant impact, as well. More recently, the 1992 eruption of Mt. Pinatubo in the Philippines actually caused global temperatures to drop, bucking the warming trend that was already evident by then. At the same time, local temperature increases were also recorded, demonstrating that while there might be an overall trend the climate impacts might not be geographically universal.

So what happened? All these volcanoes threw an immense amount of dust high into the atmosphere, enough to actually block a measurable percentage of sunlight from reaching Earth....MORE

The (globally) most dangerous eruptions are those rated at 6 - 8 on the Volcanic Explosivity Index (VEI):

VEI

Ejecta volume

Eruption Classification

Description

Plume Height

Frequency of Eruption

Examples

Occurrences in last 10,000 years*

*****

5	> 1 km³	Plinian	Paroxysmal	20–35 km	≥ 10 yrs	Mount Vesuvius, Mount St. Helens (1980)	166
6	> 10 km³	Plinian/Ultra-Plinian	Colossal	> 30 km	≥ 100 yrs	Krakatoa, Indonesia (1883), Mount Pinatubo, Philippines (1991)	51
7	> 100 km³	Ultra-Plinian	Super-colossal	> 40 km	≥ 1,000 yrs	Tambora (1815)	5 (+2 suspected)
8	> 1,000 km³	Supervolcanic	Mega-colossal	> 50 km	≥ 10,000 yrs	Yellowstone (Pleistocene)	0