Summertime observations of HONO, HCHO, and O3 at the summit of Whiteface Mountain, New York

Abstract

HONO, HCHO, and O3 concentrations were measured at the summit of Whiteface Mountain, New York, during the summer of 1999. Concentrations were in the range of ≤5–400 pptv with a median of 27 pptv and a mean of 46 pptv for HONO, in the range of ≤30–6170 pptv with a median of 1260 pptv and a mean of 1340 pptv for HCHO, and in the range of 20–105 ppbv with a median of 51 ppbv and a mean of 49 ppbv for O3. The daily HOx productions from the photolysis of O3, HONO, and HCHO were 6.1, 2.8, and 1.9 ppbv d−1, respectively, contributing 57, 26, and 17% to the overall daily radical budget from these precursors. Significant diurnal variation of average HONO concentrations was observed, with a late morning maximum and a late afternoon/early evening minimum. HNO3 photolysis on the mountain slope surfaces is proposed as the major daytime source for HONO, sustaining the majority of the observed daytime HONO concentrations against its photolytic loss. The daytime HONO/NOx ratio was also found to be high, ∼0.33 between 0800 and 1600 LT, suggesting that the transport of HONO from the ground surface to the observation height should be short, perhaps limited to the top portion of the mountain slope. The late afternoon HONO concentrations (∼27 pptv) may be considered as typical daytime HONO concentration in the aloft air mass not influenced by surface processes. While particulate nitrate photolysis may contribute significantly as a HONO source, more HONO formation/transport mechanisms are still required to account for the majority of the observed HONO concentration. HCHO concentrations exhibited only a small diurnal variation, with daytime concentrations slightly higher than nighttime concentration. HCHO was mainly transported to the site rather than produced locally from the biogenic photooxidation. O3 concentrations exhibited a diurnal variation that was similar to but more pronounced than HCHO, suggesting that the daytime in situ O3 photochemical production was small compared to its loss through photolysis and dry deposition.