Atmospheric reactive nitrogen cycling over the ocean

Lead Research Organisation: University of York
Department Name: Chemistry


Atmospheric chemical processing drives the removal of emitted pollutants, and leads to the formation of ozone and secondary aerosol, which are harmful to human and environmental health, and contribute to climate forcing. Quantitative understanding of such chemistry is essential for the accurate prediction of current air quality and future atmospheric composition.

In the troposphere, gaseous chemical processing is critically dependent upon the abundance of nitrogen oxides (NOx, NO + NO2), which regulate atmospheric oxidising capacity, ozone formation and the major components of many aerosol particles. Globally, the dominant NOx sources are all continental (traffic, power generation, industry, soil emissions of NO); these are well understood in some locations, but are very uncertain and rapidly increasing in developing nations, particularly African megacities. Once in the atmosphere, NOx is converted to reservoir compounds such as PAN, which may release NOx after transport, and ultimately into nitric acid (HNO3) on timescales of days. Current understanding is that HNO3 is the final atmospheric sink for NOx, and is removed from the atmosphere by deposition. Consequently, at remote marine sites a number of days transit time from the coast, we would expect NOx levels to be very low, and the inorganic nitrogen budget to be dominated by unreactive transported HNO3.

Recent observations challenge this understanding: surprisingly high levels of NOx species, and HONO (a NOx precursor with a lifetime of a few minutes) have been observed over the tropical Atlantic ocean. This points to a missing source of HONO and NOx. It has been hypothesised that the photolytic conversion of particle-bound nitrate to gaseous HONO and NO2 may account for these observations and form the missing NOx source - a mechanism termed "renoxification" (Ye et al., Nature 2016). We have performed proof-of-concept measurements and modelling of HONO and NOx levels at the Cape Verde observatory in the tropical Atlantic, which we have found to be consistent with this mechanism (Reed et al., ACP, 2017) - however, order of magnitude uncertainties over the rate and products of particle nitrate photolysis remain, and observational evidence for its occurrence on dominant aerosol species (dust, sulfate aerosol) is missing, meaning that impacts on the global-scale are unknown.

This project aims to address these uncertainties, through integrating existing ground-based, aircraft and satellite observations with targeted new field and laboratory studies. We will focus upon a natural laboratory, the tropical Atlantic region where we will probe the emissions and evolution of nitrogen species in the outflow of polluted air from the developing regions of West Africa to the clean marine environment of the mid Atlantic (Cape Verde Observatory, CVO). Specifically, we will (1) Use the tropical Atlantic as a natural laboratory to study renoxification during different seasons and aerosol regimes, alongside laboratory studies to parameterise the particulate nitrate photolysis process; (2) integrate this new understanding into a global chemistry-transport model to evaluate the recycling and transformations of NOx during transport, and hence the impacts of these process in the tropical Atlantic ocean, and upon our understanding of atmospheric chemical processing globally.

Planned Impact

We have identified 5 distinct 'impact groups': Scientists, Operational air-quality community, Policy makers, Education Groups and the General Public.

We will engage with researchers through traditional academic practice including online, open-access publication of results and presentation at conferences; open-access to datasets; improvements in open-source models; direct liaison with user community. Improvements in the GEOS-Chem model made during this project will be included into the standard version of the model and disseminated through its user community via the GEOS-Chem web and git sites. We will accelerate update of our findings through exploitation of direct links with beneficiaries from the project, including in groups such as the Master Chemical Mechanism (MCM) development committee, the GEOS-Chem steering committee, and with DEFRA Air Quality Expert Group members.

Operational Air-quality community:
The operational weather prediction community is now moving towards providing global atmospheric composition forecasts at approaching air quality scales. NASA Goddard is about to launch global 5-day air-quality forecasts at 25 km resolution in its new release of its GEOS system, using GEOS-Chem as the atmospheric composition module. As this project will make improvements in our understanding of NOx chemistry within GEOS-Chem, this information will be relatively seamlessly transferred into the NASA GEOS system and into their forecasts. In addition, the ECMWF have a long-running operational atmospheric composition forecasting activity, and are currently evaluating inclusion of GEOS-Chem into their Copernicus Atmosphere Monitoring Service (CAMS). If taken forward this will provide a further mechanism for the project results to influence operational air quality forecasting.

Policy makers:
The WHO global burden of disease study estimates that Cape Verde has the 10th (out of 182), highest exposure to PM2.5 globally. Thus, Cape Verde has a pressing need to understand current and future air quality problems so that policies and strategies can be developed. Although much of the high levels of ambient PM is due to dust, Cape Verde also has a growing vehicle fleet, an electricity supply almost entirely dependent on diesel generators, 57% urbanization, and heavy use of biomass in the rural areas. We are already informally advising the National Meteorological and Geophysics Institute (INMG) on a new mobile air-quality monitoring laboratory which they have been running in the capital of Praia. They are however in urgent need of low cost, low power and low maintenance solutions to effectively monitor air quality across a range of sites. Within this project we propose to trial in Cape Verde a novel clustered air quality sensor package now available in WACL, a device recently evaluated in Beijing. We will train our local science manager at Cape Verde in the running and calibration of the package of sensors, the latter using calibration standards available at the CVAO. This will form trial data that we intend to use in future GCRF-type funding applications to work with Cape Verde to identify where and how policy needs to intervene to deliver measurable improvements in air quality.

Educational material and Presentations to the General Public:
The team of PIs has a significant track record of public outreach. We identify several mechanisms to continue to engage schools and the general public in our research at no extra cost to the project.
Description Nitrogen oxides (NO and NO2 = NOx) in the atmosphere are central to mediating the abundance of ozone, OH radicals, atmospheric processing and the formation of many secondary pollutant species. Recently, the view of HNO3 as a permanent sink for NOx has been challenged by (limited) observational evidence that particulate phase nitrate (pNO_3^-) can be efficiently photolyzed to HONO and NOx via a so-called "renoxification" process, a process previously suggested by laboratory studies. There is however a very high uncertainty in the renoxification "enhancement factor" (f, the ratio of the photolysis rate constants of pNO_3^- and gas phase HNO3), with laboratory and field studies reporting values spanning values between ~1 and 1700. In particular, while field evidence for this process has suggested f of around 300 on marine sea salt aerosol, recent laboratory experiments7 and field observations of NOx/HNO3 ratios have derived enhancement factors of 1-30 and have suggested therefore that renoxification plays only a limited role in atmospheric chemistry.
Our extensive aircraft and ground-based observations made in this project of HONO and NOx in the remote marine troposphere (the best place to identify such chemistry due to the complex nature of the nitrogen oxide budget and the dominance of fresh emission sources in urban / continental regions) confirm that renoxification is occurring at levels which are significant for atmospheric chemistry, on mixed sea-salt, dust and biomass burning aerosol (i.e. not just on sea-salt aerosol), and suggest a mechanism which is consistent with surface-enhancement of NO3- ions, which has been demonstrated by previous laboratory studies. This surface-enhanced mechanism means that the efficiency of renoxification reduces strongly with the concentration of bulk nitrate, which reconciles the very large discrepancies in renoxification photolysis rate constants found across multiple laboratory and field studies.
This work has important implications for atmospheric oxidants and their trends, especially as nitrate aerosols are becoming increasing more important in the atmosphere due to an increase in precursor ammonia emissions and a decline of ammonium-sulfate aerosol.
Exploitation Route too early to say
Sectors Chemicals,Environment

Title CEDA: "ARNA-2 FAAM Aircraft Project" (data not yet complete). 
Description Airborne atmospheric measurements from core and non-core instrument suites data on board the FAAM BAE-146 aircraft collected for ARNA-1 and ARNA-2 FAAM Aircraft Projects. 
Type Of Material Database/Collection of data 
Year Produced 2021 
Provided To Others? No  
Impact Not yet any impact 
Description WMO GAW Training Webinar on Reactive gases 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Study participants or study members
Results and Impact Delivered an Introductory talk for the WMO GAW Training Webinar on Reactive gases
Year(s) Of Engagement Activity 2021