Temporal variability, sources, and sinks of C₁-C₅ alkyl nitrates in Coastal New England

Abstract

Abstract. Seven C1-C5 alkyl nitrates were measured both on the mainland and off the coast of New Hampshire using gas chromatographic techniques. Five separate data sets will be presented to characterize the seasonal and diurnal trends and the major sources and loss processes of these compounds. In situ measurements were conducted at the University of New Hampshire (UNH) Atmospheric Observing Station at Thompson Farm (TF) located in southeast NH during winter (January–February) and summer (June–August) 2002 and summer (July–August) 2004. The median (±standard deviation) total alkyl nitrate mixing ratio (ΣRONO2) was 25 (±7) in winter and 16 (±14) pptv in summer. Furthermore, daily canister samples collected at midday and later analyzed in the laboratory from January 2004–February 2008 gave median ΣRONO2 of 23 (±8) in winter and 14 (±10) pptv in summer. Alkyl nitrate mixing ratios increased throughout the morning and were highest in the afternoon reflecting mixing of remnant boundary layer air toward the surface and photochemical production during the day. During summers 2002 and 2004, MeONO2 decreased overnight and reached minimum hourly average mixing ratios in the early morning (05:00–07:00 LT). Comparison with wind speed and trace gas (i.e., hydrocarbons, ozone, carbon monoxide, total reactive nitrogen) trends suggested that dry deposition contributed to the early morning MeONO2 minimum which is a previously unaccounted for removal mechanism. The mean dry deposition rate and velocity of MeONO2 was estimated to be −0.5 nmol m−2 hr−1 and 0.13 cm s−1, respectively. Results from ambient air and surface seawater measurements made onboard the NOAA R/V Ronald H. Brown in the Gulf of Maine during the 2002 New England Air Quality Study and from ambient canister samples collected throughout the Great Bay estuary in August 2003 are also presented. Comparisons between the alkyl nitrate trends with anthropogenic and marine source fingerprints and tracers suggest that a marine source of alkyl nitrates is not significant in coastal New England. Given the apparent prominence of a secondary source, comparisons between observed and predicted alkyl nitrate/parent hydrocarbon ratios were made which demonstrated that background mixing ratios have a continuous and prevalent influence on the alkyl nitrate distribution.