Narragansett Bay, the largest estuary in the northeastern U.S., is a heavily urbanized watershed impacted by deposition and runoff. Nutrient budgets and local policy rely on deposition data from a 1990 study that did not include any direct observations of dry deposition of gaseous ammonia (NH3) and particulate ammonium (NH4+) due to uncertainties in their flux direction and measurement difficulty. Recent precipitation measurements show that wet deposition of NH4+ to the Bay has increased by a factor of 6 over the past three decades, leading to a 2.5-fold increase in wet nitrogen (N) deposition. The documented increase in wet deposition of NH4+ concurrent with managed nutrient reductions in urbanized estuaries has potentially increased the impacts of atmospheric N deposition, including the dry deposition of NH3 and NH4+. However, a scarcity of measurements hinders our interpretation of this important N source. For the first time over Narragansett Bay, and to our knowledge over open water, dry (particulate and gas phase) deposition of total NHx (NHx = NH3 + NH4+) and bidirectional NH3 flux was quantified using a relaxed eddy accumulation sampling technique. We find that total N entering the Bay from the atmosphere has doubled since 1990. Dry deposition of NHx comprises 9.6% of total N deposition. During the fall season, the dominant flux direction for NH3 is upward, which also has implications for urban air quality. We estimate that NH3 emitted from the Bay to the atmosphere makes up to 10% of the local NH3 emission budget.

Biomass burning is a primary emission source for a host of gas- and aerosol-phase compounds, which can damage environmental and human health. During the FIREX-AQ campaign in July and August of 2019, we measured reactive nitrogen species (NOx, NO2, HONO, HNO3 and p-NO3-), in wildfire plumes aboard NASA Langley’s Mobile Aerosol Characterization Laboratory (MACH-2). Nitrous acid (HONO) and nitric acid (HNO3) mixing ratios were measured at nominal 5-minute resolution using a dual mist chamber-ion chromatograph from five separate areas of fire in the western US and are the primary focus of this paper. Average HONO mixing ratios were significantly higher in young daytime smoke compared to young nighttime smoke, while no statistical differences were observed between young versus aged smoke during the day or night. In the largest fire sampled during the day, UV-A irradiation was highly correlated (R2 = 0.91) with HONO to nitrogen dioxide (NO2) ratios indicating that photo-enhanced heterogeneous NO2 to HONO conversion, likely facilitated by ground surfaces (e.g. soil, foliage, and dust), more than compensated for rapid photolytic loss of HONO.

Concentrations and the stable isotopic composition of bulk aerosol nitrate (NO3-) were quantified from two fall GEOTRACES cruises: 1) Alaska–Tahiti (GP15; n=22) and 2) Peru–Tahiti (GP16; n=17) to explore the hypothesis that marine emissions influence aerosol NO3- in the Equatorial Pacific. The δ15N-NO3- ranged from -14.5–0.5‰, with lowest values far from the coast and primarily reflecting a shift in sources. The δ18O- and Δ17O-NO3- were both high (65.2–85.4‰ and 21.4–30.7‰, respectively) and decreased away from continental regions, reflecting a shift in the oxidants that influence the formation of NO3-. Transport modeling and co-occurrence of low δ15N, δ18O and Δ17O provided evidence for an important influence of alkyl nitrates (RONO2) on aerosol NO3- formation. Based on the Δ17O, we quantified that the contribution of RONO2 to aerosol NO3- can be as high as 48% (range 15–48%). We were also able to estimate an average δ15N-RONO2 of -22.9 ± 19.1‰.