Dirty snow and ozone
The first thing we do when we get snow samples back to the lab is melt them and filter out the absorbing impurities. Absorbing impurities are insoluble particles such as soot and dust. They appear black or brown to the eye because they are absorbing sunlight. Since chemical reactions in the snow require sunlight to happen, the more absorbing impurities that exist in the snow, the less snow chemistry can contribute to the recycling of nitrogen oxide pollutants.
The image below shows our filter apparatus that is used to filter out the absorbing impurities.
The image below shows a collection of filters after snow samples have been filtered through. Some filters are darker than others because they have a higher concentration of absorbing impurities. We will take these filters back to the lab in Seattle, where we will measure the amount of sunlight they absorb using an instrument called a spectrophotometer. This information will be fed into our snow chemistry model.
The top layer of snow is the dirtiest, getting cleaner as Maria digs deeper in the snow. It hasn't snowed for quite awhile, so dust and soot are depositing to the surface of the snow over time. Dust in the snow can not only limit snow chemistry by absorbing sunlight, but also by reducing the acidity of the snow. Snow chemistry is also acid catalyzed.
This dirty snow may explain why ozone concentrations have not gotten as high as they did in last year's field campaign. Last year, surface ozone concentrations got up to over 150 ppb. This is compared to the EPA standard of 75 ppb. This year, peak ozone has gotten over 80 ppb a few times, but most of the time it has been below the EPA limit (though still unusually high for a rural location in winter). What is the difference between this year and last year? One important difference is that last year they got lots of fresh snowfall. We haven't had any snowfall yet since the start of the campaign, but there is a chance of snow tomorrow night through the end of the week. Snowfall will allow us to test our hypothesis that fresh snow can lead to high ozone concentrations by promoting the recycling of pollutants. We are keeping our fingers crossed for new snow!