Photolysis of nitrate (NO) produces reactive nitrogen and oxygen species via three different channels, forming: (1) nitrogen dioxide (NO) and hydroxyl radical (OH), (2) nitrite (NO) and oxygen atom (O(P)), and (3) peroxynitrite (ONOO). These photoproducts are important oxidants and reactants in surface waters, atmospheric drops, and snowpacks. While the efficiency of the first channel, to form NO, is well documented, a large range of values have been reported for the second channel, nitrite, above 300 nm. In part, this disagreement reflects secondary chemistry that can produce or destroy nitrite. In this study, we examine factors that influence nitrite production and find that pH, nitrate concentration, and the presence of an OH scavenger can be important. We measure an average nitrite quantum yield (Φ(NO)) of (1.1 ± 0.2)% (313 nm, 50 μM nitrate, pH ≥ 5), which is at the upper end of past measurements and an order of magnitude above the smallest-and most commonly cited-value reported for this channel. Nitrite production is often considered a very minor channel in nitrate photolysis, but our results indicate it is as important as the NO channel. In contrast, at 313 nm we observe no formation of peroxynitrite, corresponding to Φ(ONOO) < 0.26%.

We evaluate a regional-scale simulation with the WRF-Chem model for the VAMOS (Variability of the American Monsoon Systems) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx), which sampled the Southeast Pacific's persistent stratocumulus deck. Evaluation of VOCALS-REx ship-based and three aircraft observations focuses on analyzing how aerosol loading affects marine boundary layer (MBL) dynamics and cloud microphysics. We compare local time series and campaign-averaged longitudinal gradients, and highlight differences in model simulations with (W) and without (NW) wet deposition processes. The higher aerosol loadings in the NW case produce considerable changes in MBL dynamics and cloud microphysics, in accordance with the established conceptual model of aerosol indirect effects. These include increase in cloud albedo, increase in MBL and cloud heights, drizzle suppression, increase in liquid water content, and increase in cloud lifetime. Moreover, better statistical representation of aerosol mass and number concentration improves model fidelity in reproducing observed spatial and temporal variability in cloud properties, including top and base height, droplet concentration, water content, rain rate, optical depth (COD) and liquid water path (LWP). Together, these help to quantify confidence in WRF-Chem's modeled aerosol-cloud interactions, especially in the activation parameterization, while identifying structural and parametric uncertainties including: irreversibility in rain wet removal; overestimation of marine DMS and sea salt emissions, and accelerated aqueous sulfate conversion. Our findings suggest that WRF-Chem simulates marine cloud-aerosol interactions at a level sufficient for applications in forecasting weather and air quality and studying aerosol climate forcing, and may do so with the reliability required for policy analysis

Abstract. Several field studies have proposed that the volatilization of NH3 from evaporating dew is responsible for an early morning pulse of ammonia frequently observed in the atmospheric boundary layer. Laboratory studies conducted on synthetic dew showed that the fraction of ammonium (NH4+) released as gas-phase ammonia (NH3) during evaporation is dependent on the relative abundances of anions and cations in the dew. Hence, the fraction of NH3 released during dew evaporation (Frac(NH3)) can be predicted given dew composition and pH. Twelve separate ambient dew samples were collected at a remote high-elevation grassland site in Colorado from 28 May to 11 August 2015. Average [NH4+] and pH were 26 µM and 5.2 respectively and were on the lower end of dew [NH4+] and pH observations reported in the literature. Ambient dew mass (in g m−2) was monitored with a dewmeter, which continuously measured the mass of a tray containing artificial turf representative of the grass canopy to track the accumulation and evaporation of dew. Simultaneous measurements of ambient NH3 indicated that a morning increase in NH3 was coincident in time with dew evaporation and that either a plateau or decrease in NH3 occurred once the dew had completely evaporated. This morning increase in NH3 was never observed on mornings without surface wetness (neither dew nor rain, representing one-quarter of mornings during the study period). Dew composition was used to determine an average Frac(NH3) of 0.94, suggesting that nearly all NH4+ is released back to the boundary layer as NH3 during evaporation at this site. An average NH3 emission of 6.2 ng m−2 s−1 during dew evaporation was calculated using total dew volume (Vdew) and evaporation time (tevap) and represents a significant morning flux in a non-fertilized grassland. Assuming a boundary layer height of 150 m, the average mole ratio of NH4+ in dew to NH3 in the boundary layer at sunrise is roughly 1.6 ± 0.7. Furthermore, the observed loss of NH3 during nights with dew is approximately equal to the observed amount of NH4+ sequestered in dew at the onset of evaporation. Hence, there is strong evidence that dew is both a significant night-time reservoir and strong morning source of NH3. The possibility of rain evaporation as a source of NH3, as well as dew evaporation influencing species of similar water solubility (acetic acid, formic acid, and HONO), is also discussed. If release of NH3 from dew and rain evaporation is pervasive in many environments, then estimates of NH3 dry deposition and NHx ( ≡  NH3 + NH4+) wet deposition may be overestimated by models that assume that all NHx deposited in rain and dew remains at the surface.

Ammonia (NH3) emission reduction is key to limiting the deadly PM2.5 pollution globally. However, studies of long-term source apportionment of vertical NH3 are relatively limited. On the basis of the one-year measurements of weekly vertical profiles of δ 15N–NH3 at 5 heights (2, 15, 102, 180, and 320 m) on a 325-m meteorological tower in urban Beijing, we found that vertical profiles of NH3 concentrations generally remained stable with height. δ 15N–NH3 increased obviously as a function of height in cold seasons (with heating) and decreased in warm seasons (with fertilization), indicating a stronger human-induced seasonal variation via regional transport at higher altitudes. Relatively stable δ 15N–NH3 near the ground surface suggested the strong local emission. The results of isotopic mixing model (SIAR) indicate that source apportionment using measured δ 15N–NH3 only would overestimate the contribution of agricultural emissions to NH3. By using an estimation of initial δ 15N–NH3, we found that nonagricultural sources contributed ∼72% of NH3 on average. Our study suggests that (i) both persistent nonagricultural and periodic agricultural emissions drive atmospheric NH3 concentration and its vertical distribution in urban Beijing; and (ii) source apportionment based on measured δ 15N–NH3 only likely underestimates fossil fuel source contribution, if the combined NH x isotope effects are not considered.

Concentrated agricultural activities and animal feeding operations in the northeastern plains of Colorado represent an important source of atmospheric ammonia (NH 3 ). The NH 3 from these sources contributes to regional fine particle formation and to nitrogen deposition to sensitive ecosystems in Rocky Mountain National Park (RMNP), located ∼ 80 km to the west. In order to better understand temporal and spatial differences in NH 3 concentrations in this source region, weekly concentrations of NH 3 were measured at 14 locations during the summers of 2010 to 2015 using Radiello passive NH 3 samplers. Weekly (biweekly in 2015) average NH 3 concentrations ranged from 2.66 to 42.7 µg m −3 , with the highest concentrations near large concentrated animal feeding operations (CAFOs). The annual summertime mean NH 3 concentrations were stable in this region from 2010 to 2015, providing a baseline against which concentration changes associated with future changes in regional NH 3 emissions can be assessed. Vertical profiles of NH 3 were also measured on the 300 m Boulder Atmospheric Observatory (BAO) tower throughout 2012. The highest NH 3 concentration along the vertical profile was always observed at the 10 m height (annual average concentration of 4.63 µg m −3 ), decreasing toward the surface (4.35 µg m −3 ) and toward higher altitudes (1.93 µg m −3 ). The NH 3 spatial distributions measured using the passive samplers are compared with NH 3 columns retrieved by the Infrared Atmospheric Sounding Interferometer (IASI) satellite and concentrations simulated by the Comprehensive Air Quality Model with Extensions (CAMx). The satellite comparison adds to a growing body of evidence that IASI column retrievals of NH 3 provide very useful insight into regional variability in atmospheric NH 3 , in this case even in a region with strong local sources and sharp spatial gradients. The CAMx comparison indicates that the model does a reasonable job simulating NH 3 concentrations near sources but tends to underpredict concentrations at locations farther downwind. Excess NH 3 deposition by the model is hypothesized as a possible explanation for this trend.