Abstract
Abstract. Recent field studies have suggested that sea-salt particulate nitrate (NITs) photolysis may act as a significant local source of nitrogen oxides (NOx) over oceans. We present a study of the global impact of this process on oxidant concentrations in the marine boundary layer (MBL) using the GEOS-Chem model, after first updating the model to better simulate observed gas–particle phase partitioning of nitrate in the marine boundary layer. Model comparisons with long-term measurements of NOx from the Cape Verde Atmospheric Observatory (CVAO) in the eastern tropical North Atlantic provide support for an in situ source of NOx from NITs photolysis, with NITs photolysis coefficients about 25–50 times larger than corresponding HNO3 photolysis coefficients. Short-term measurements of nitrous acid (HONO) at this location show a clear daytime peak, with average peak mixing ratios ranging from 3 to 6 pptv. The model reproduces the general shape of the diurnal HONO profile only when NITs photolysis is included, but the magnitude of the daytime peak mixing ratio is under-predicted. This under-prediction is somewhat reduced if HONO yields from NITs photolysis are assumed to be close to unity. The combined NOx and HONO analysis suggests that the upper limit of the ratio of NITs : HNO3 photolysis coefficients is about 100. The largest simulated relative impact of NITs photolysis is in the tropical and subtropical marine boundary layer, with peak local enhancements ranging from factors of 5 to 20 for NOx, 1.2 to 1.6 for OH, and 1.1 to 1.3 for ozone. Since the spatial extent of the sea-salt aerosol (SSA) impact is limited, global impacts on NOx, ozone, and OH mass burdens are small ( ∼ 1–3 %). We also present preliminary analysis showing that particulate nitrate photolysis in accumulation-mode aerosols (predominantly over continental regions) could lead to ppbv-level increases in ozone in the continental boundary layer. Our results highlight the need for more comprehensive long-term measurements of NOx, and related species like HONO and sea-salt particulate nitrate, to better constrain the impact of particulate nitrate photolysis on marine boundary layer oxidant chemistry. Further field and laboratory studies on particulate nitrate photolysis in other aerosol types are also needed to better understand the impact of this process on continental boundary layer oxidant chemistry.
Highlights
Nitrogen oxides (NOx) are a key component of tropospheric chemistry due to their impact on oxidant chemical cycles
HNO3 uptake by coarse-mode sea-salt aerosols (SSA) continues after alkalinity is titrated, due in part to hydrochloric acid displacement, and the simulation of the marine boundary layer (MBL) nitrate partitioning between gas and particle phases is much improved
We find that the diurnal variations of both hypothesized that measured daytime nitrous acid (HONO) and NOx at Cape Verde Atmospheric Observatory (CVAO), as well as the absolute magnitude of HONO mixing ratios, are consistent with a NITs photolysis coefficient that is about 25–100
Summary
Nitrogen oxides (NOx) are a key component of tropospheric chemistry due to their impact on oxidant chemical cycles. Ye et al (2016) analyzed trace gas measurements from two aircraft flights over the western subtropical North Atlantic Ocean during the summer 2013 Nitrogen, Oxidants, Mercury and Aerosol Distributions, Sources and Sinks (NOMADSS) field campaign and hypothesized that measured daytime nitrous acid (HONO) concentrations in the MBL were indicative of a significant in situ source from p-NO3 photolysis (since HONO photolyzes rapidly to yield NOx, p-NO3 photolysis is effectively a NOx source) Their box model analysis suggested that the peak p-NO3 photolysis rate coefficient was on the order of 2 × 10−4 s−1, about 300 times larger than the corresponding photolysis coefficient for gas-phase nitric acid (HNO3). We perform an initial analysis of the impact of photolysis of nitrate on other aerosol types and consider its implications for continental boundary layer oxidant chemistry
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