4D-Var Inversion of European NH3 Emissions Using CrIS NH3 Measurements and GEOS-Chem Adjoint With Bi-Directional and Uni-Directional Flux Schemes.

We conduct the first 4D-Var inversion of NH3 accounting for NH3 bi-directional flux, using CrIS satellite NH3 observations over Europe in 2016. We find posterior NH3 emissions peak more in springtime than prior emissions at continental to national scales, and annually they are generally smaller than the prior emissions over central Europe, but larger over most of the rest of Europe. Annual posterior anthropogenic NH3 emissions for 25 European Union members (EU25) are 25% higher than the prior emissions and very close (<2% difference) to other inventories. Our posterior annual anthropogenic emissions for EU25, the UK, the Netherlands, and Switzerland are generally 10%-20% smaller than when treating NH3 fluxes as uni-directional emissions, while the monthly regional difference can be up to 34% (Switzerland in July). Compared to monthly mean in-situ observations, our posterior NH3 emissions from both schemes generally improve the magnitude and seasonality of simulated surface NH3 and bulk NH x wet deposition throughout most of Europe, whereas evaluation against hourly measurements at a background site shows the bi-directional scheme better captures observed diurnal variability of surface NH3. This contrast highlights the need for accurately simulating diurnal variability of NH3 in assimilation of sun-synchronous observations and also the potential value of future geostationary satellite observations. Overall, our top-down ammonia emissions can help to examine the effectiveness of air pollution control policies to facilitate future air pollution management, as well as helping us understand the uncertainty in top-down NH3 emissions estimates associated with treatment of NH3 surface exchange.

The development of loads of cations, anions, Cd and Pb in precipitation and of atmospheric concentrations of N-components, in Switzerland from 1988 to 2003.

This study presents the results of the analyses of Cd, Pb, cations and anions present in precipitation and dust at a pre-alpine and a suburban site in Switzerland in the period from 1988 to 2003. The aim of these measurements was to monitor the success of measures taken to diminish pollutant emissions. No change was found for Ca2+, K+, Na+ and Mg2+ loads--in line with expectations, as no reducing measures had been taken. Statistically significant and largely decreasing values (50-90%) were found for Cl- and Cd (linked to the fitting of filters in incineration plants), Pb (unleaded petrol), SO(2-)4 (diminishing the use of mineral oil with high S content), and the proton (lower HCl and SO2 emissions). A smaller decrease (up to 30%) or none was registered for oxidised nitrogen components (fitting cars with catalytic converters, but an increase in numbers of cars and trucks). No significant change was found for NH3 as farming techniques had undergone no major changes. The long-term measurements show that the measures taken to reduce emissions were successful. A shorter monitoring period would have been misleading owing to data variability and temporary incidents e.g. amount of precipitation.

Ammonia monitoring in Switzerland with passive samplers: patterns, determinants and comparison with modelled concentrations.

Gaseous ammonia (NH3) is an important form of N deposition to ecosystems, but it is not being routinely monitored in Switzerland. Therefore, a study was conducted to estimate annual means and seasonal patterns of NH3 concentrations for different site types in Switzerland, and to compare annual measured and modelled NH3 concentrations. NH3 concentrations were measured using the 'Zürcher' passive sampler, a Palmes type sampler with an acidic solution as absorbent. Twenty-four sampling sites were run for one year, and 17 for two years. The samplers were changed fortnightly or monthly. Spatial emission patterns were mapped by combining information on (1) the location of emission sources, (2) national statistics on NH3-emitting activities and (3) activity-specific emission factors. The spatial resolution was one hectare. The mean annual NH3 concentration in the ambient air of the 41 sites was 2.5+/-0.3 microg m(-3) (mean+/-standard error). It ranged from 0.4 to 7.5 microg m(-3). The site type and the season were the most important factors explaining the variation in the seasonal mean concentration. NH3 concentrations were highest in intensively used agricultural areas and in cities, and lowest in Alpine sites remote from emission sources. At 39 out of 41 sites, the NH3 concentrations were higher in summer (3.1+/-0.3 microg m(-3)) than in winter (2.0+/-0.3 microg m(-3)). Modelled NH3 concentrations did not systematically deviate from measured concentrations (r2 = 0.69). With the combined monitoring and modelling approach, it is now possible to obtain a reasonable and consolidated picture of the overall NH3 situation in Switzerland.

A passive sampling method to determine ammonia in ambient air.

Ambient ammonia concentrations, mainly originating from agricultural activities, have increased in the last few decades in Europe. As a consequence, critical loads on oligotrophic ecosystems such as forests and mires are greatly exceeded. Monitoring of ambient ammonia concentrations is necessary in order to investigate source-receptor relationships. Measuring ambient ammonia concentrations continuously with high time resolution is very expensive and cost-efficient systems are required. Where time resolution is of minor importance, several cost-effective systems, mainly dry denuder and passive samplers, can be applied. In this paper the Zürcher passive sampler, a diffusive sampling system, is presented. It is a Palmes type sampler with an acidic solution as absorbent and is easy to handle. It was tested at 46 sites in Switzerland over one year. The average concentration in ambient air was 2.5 microg m(-3) +/- 0.4 microg m(-3). The average of the blank values were 0.21 microg m(-3). The detection limit (double the standard deviation of the blank values) was 0.36 microg m(-3). Three passive samplers were exposed at each site and each period. The mean standard deviation of these triplicate measurements was 9.5%. Compared with a discontinuous tubular denuder system and a continuous annular denuder system, the deviation was less than 10%. The Zürcher passive sampler is a useful and cost-efficient tool to determine long-term average ammonia concentrations (one- to four-week periods) in ambient air for mean concentrations above 1 microg m(-3).