EVERETT — When the dog days of summer set in, there’s one thing residents in north Everett and Marysville have learned to anticipate: the stink.

The smell, they say, originates from Cedar Grove Composting on Smith Island.

Neighbors have complained for years about the smell, and filed their objections with the Puget Sound Clean Air Agency.

The company has long insisted that it has been successful in containing any noxious odors. It uses a state-of-the-art odor monitoring system called OdoWatch to verify that it is keeping its emissions under control.

Nevertheless, in the first half of the year, the agency received 141 complaints concerning the Smith Island site, spokeswoman Joanne Todd said, although that number comes with an asterisk because it does not reflect 141 separate incidents. Some people call in many more complaints than others, she said.

In July, as summer weather began to heat up, the agency recorded an additional 99* complaints about the smell.

One of those complaints prompted a written warning to Cedar Grove, but none of the odor complaints so far has led to a fine or other sanction, she said.

“One of the difficulties in regulating this and inspecting this is you have to have someone who calls in, and you have to send an inspector who can trace it from that place to the source,” Todd said.

Sometimes the wind shifts or the odor dissipates before the source can be confirmed, she said.

The one written warning did not rise to the level of a so-called Notice of Violation because the complainant wouldn’t identify himself or herself, Todd said.

Cedar Grove said that the smell cited in the warning didn’t come from the composting site.

“We are in communication with PSCAA and our data indicates that we are not likely the source,” the company’s director of community development, Karen Dawson, wrote in an emailed response to questions.

Dawson said that the data from OdoWatch, when coupled with the weather at the time cited in the warning, indicated that the smell came from elsewhere.

The Cedar Grove composting facility shares Smith Island with Pacific Topsoils Inc., from which the smell of mulch is often prominent along Highway 529. In addition, Everett’s and Marysville’s sewage treatment plants are both nearby, and all of these are located in the Snohomish River estuary, which brings its own unique bouquet to the mix.

Cedar Grove runs large composting operations in Everett and Maple Valley in King County, and the Seattle-based company has received numerous complaints over the years.

The company opened the Smith Island facility in 2004, and in 2013 it processed 132,445 tons of food and organic waste there.

In addition to filing complaints, neighbors of the site have taken to the legal system to try to address the smell.

One lawsuit, filed in 2013, names dozens of plaintiffs and is scheduled for trial June 1, 2015. Another suit seeks class-action status for anyone living within four miles of Cedar Grove’s Smith Island facility. Other lawsuits target the company’s Maple Valley location, as well.

Meanwhile, a lawsuit filed by Cedar Grove against the city of Marysville resulted in a $143,000 fine against the city for failure to disclose public records, specifically emails maintained by a consulting firm the city employed to develop public relations strategies to fight the smell.

The city has appealed that decision and wouldn’t comment on its ongoing issues with the company.

The company was cited for four odor violations in June 2013, and in 2011 it was fined $169,000 for violations going back several years.

The fine was later reduced to $119,000, in recognition of the company’s efforts to control odor, and that money went toward funding an odor control study through the agency.

The study last year used a combination of odor-detecting sensors — dubbed “e-noses” — and volunteers to track and trace odors.

The results of the study, however, were inconclusive, with volunteers reporting more frequent and stronger odors than the e-noses were able to substantiate. The study also tracked odors from the two municipal sewage treatment plants, and sometimes their odors were found to be stronger than those from Cedar Grove.

That study reflects the difficulty with which inspectors with the Puget Sound Clean Air Agency investigate and identify noxious odors.

“It’s a challenge because you can get a complaint at 9 in the morning and if we don’t have an inspector out there — it could be a while before someone gets out there — the winds could have come through and moved it, or it’s just stopped,” Todd said.

Odor plumes tend to move around the atmosphere like currents in a river, she said, and mix with other smells from other sources.

Inspectors with the agency are trained to distinguish different odors, Todd said, but an average person might not be able to.

“Also, what is an obnoxious smell for some people might not be an obnoxious smell for others,” she added.

One sunny Friday morning, with the mercury at 72 degrees and heading toward 80, a sweet-and-sour aroma was detectable in Ebey Waterfront Park, where Bill and Carmen Britz were putting their boat into the river.

“But I don’t mind it, it’s compost. I’m a gardener,” Carmen Britz said. Her husband said he hadn’t even noticed the smell until it was pointed out to him.

A slight breeze then picked up, and the odor disappeared.

A lot depends on weather.

One day, with hot weather and stagnant air, the smell might be detectable from north Everett into south Marysville. On another sunny day last week, the odor was absent and was even not detectable by a reporter outside the gates of Cedar Grove’s Smith Island facility, within sight of the compost mounds.

“In that area, the winds are quite a contributing factor as to how the odors come and go,” Todd said.

And odor detection is not the primary function of the agency’s inspectors, who are also responsible for pollution monitoring, permit and site inspections, and investigating other businesses that deal with atmospheric pollutants, such as gas stations, dry cleaners or sites where asbestos removal is under way.

That leads to the agency taking a bit more of an opportunistic approach to monitoring odors.

“We watch the complaints coming in, and if we’ve got an inspector out there, we’ll send them over,” Todd said.

Chris Winters: 425-374-4165; cwinters@heraldnet.com. Twitter: @Chris_At_Herald.

* Correction, Aug. 8, 2014: This article originally used an incorrect figure for the number of complaints against Cedar Grove’s composting operations in July. The agency initially provided a higher, incorrect number.