HONO, Particulate Nitrite, and Snow Nitrite at a Midlatitude Urban Site during Wintertime

Nitrous acid (HONO) plays an important role in the oxidative capacity of the atmosphere during wintertime via photolysis to produce hydroxyl radicals (OH). While it is known that HONO is emitted from the Arctic snowpack, sparse observations of HONO in the midlatitude urban wintertime environment have hindered our understanding of cold-season atmospheric chemistry. In this study, measurements of ambient HONO, particulate nitrite (pN(III); N(III) = NO2(aq)^- + HONO_(aq) + H₂ONO(aq)⁺), and snow nitrite (sN(III)) were conducted in Kalamazoo, Michigan during January-February 2018. Elevated levels of HONO and particulate nitrite were observed over snow-covered ground, likely due to emissions of HONO from the snowpack, as well as weak turbulent mixing in the atmospheric boundary layer. The noontime peak in HONO of 87 ± 60 (1σ) parts per trillion (ppt) over snow-covered ground suggests photochemical snowpack HONO production, likely in part through snowpack nitrate photolysis, with only a minor contribution from particulate nitrate photolysis. High concentrations of snow nitrite (0.4 ± 0.3 (1σ) μM) support the hypothesis that the snowpack is a significant source of HONO to the atmosphere. On average, the OH production rate from HONO photolysis, in the near-surface atmosphere (≈ 2 m above ground), was calculated to be about an order of magnitude higher than that from O₃ photolysis over snow-covered ground. Future studies are needed to quantify HONO emissions from the midlatitude urban snowpack, given expected HONO production due to high concentrations of snow nitrate and nitrite from anthropogenic particulate nitrate and nitrite deposition.