Air Pollution as a Climate Forcing: A Workshop

Day 2 Presentations

Aerosol Nitrate, a Dominant Atmospheric Trace Component in Europe?!

H.M. ten Brink and M. Schaap
Netherlands Energy Research Foundation (ECN), Petten, The Netherlands

Maps of nitrate and sulfate fields. See caption and text for more.

Figure 1: Concentration field of aerosol nitrate (A) in Europe in the winter season (October-March). The average dimension of a grid cell is 300 by 300 km. To put the nitrate field in perspective the analogous field for sulphate (B) is also given. Concentrations of the species are in the standard unit of µg/m3.

Introduction. Aerosols are comprised of a large number of chemical constituents, but models (1,2,3) indicate that sulphate is the dominant coolant. In the Netherlands another aerosol component, manmade nitrate, is at least as important as sulphate (4). In the IPCC-TAR (5) our finding is fully acknowledged, but it is stipulated that insufficient information exist for assessing the general magnitude of the forcing by this component. The reason is that there are insufficient reliable data (6). This in turn is due to the semi-volatile character of the compound, which gives rise to large underestimates of the actual concentrations (7). It was our aim to come up with new data, not only because of the importance of nitrate as a coolant but also because of other environmental issues in which the compound features. These are acidification and eutrofication (overfertilising of sensitive soils). Nitrate also contributes to those aerosols that are associated with respiratory diseases. (Legislators in the field of public health are more familiar with the term fine particulate matter, PM, as the collective term for the aerosols.)

Approach and Results. We started our study with a search for long-term data obtained with reliable measuring techniques (8,9), and found a consistent set in Europe. From this set and (other) corrected data (10) the average concentration of nitrate was calculated for the locations shown in table 1. We then constructed the concentration field by assuming that the point data are representative for the whole of the area depicted in Fig. 1A. The concentration field is obtained by interpolation. The point data are indeed representative for a larger region in the winter season: the concentrations of nitrate varies by less than 25% over distances corresponding to hundreds of kilometers, as illustrated in Fig. 2.

To put the concentration field for nitrate in perspective, the field for sulphate is also given (10,11) (Fig. 1B). Comparison shows that the current concentration of nitrate in winter in Europe north of the Alps is at least 60% of that of sulphate. Nitrate levels exceed those of sulphate in a region from southern England into Poland and Italy. Recently high concentrations also are reported from the Barcelona area in Spain, in line with the high concentrations of nitrate occurring in air flowing from southern Europe over the Atlantic (12).

The radiative cooling by nitrate, caused by reflection of solar radiation, was calculated with the well-known equation of Charlson et al. (13) developed for sulphate. We use the observation that the nitrate is present in particles with the same size as sulphate (4,10,12,14,15,16,17) and has a similar if not higher reflectivity (4,18). This is in sharp contrast to the a priori assumption that nitrate is present in large particles with low reflectivity and therefore of negligible importance for cooling (19,20).

Using values for the meteorological parameters in the Charlson-equation, taken from other sources (3), we arrive at a reduction in solar irradiance of 0.4 W/m2 for the winter season in the area depicted in Fig. 1. The, negative, radiative forcing effect of nitrate is largest in spring, with values as low as -1.5 W/m2. To put this into perspective: the regional warming forcing by the extra manmade greenhouse gases in the same season is +1.5 to +2 W/m2.

Scatter chart of winter nitrate concentrations. See caption and text for more.

Figure 2: Figure 2. Scatter diagram of the weekly averaged concentrations of aerosol nitrate in the winter season (Oct-Mar) in the period 1994 to 1997 with the central measuring location in the Netherlands, Bilthoven, as the reference site. The other data stem from a station at a distance of 140 km to the north in the Netherlands, a site 350 km to the north-east in Denmark and two locations to the east in Germany at a distance of 490 km and 570 km respectively.

It was found in the study that the nitrate is predominantly present in the form of ammonium nitrate, a well-known fertilising agent. However, such a fertiliser is not welcome in areas with sensitive, poor, soils and associated precious flora, where it causes eutrofication.

Aerosols (and nitrate) not only reflect the radiation from the sun but also reflect horizontal light. They are thereby causing haziness and obscuring the views in scenic areas in Europe and other regions. Moreover, nitrate contributes to the aerosols that man inhales, which are associated with respiratory diseases. While in itself nitrate may not be a harmful component, current legislation is concerned with the total mass of the aerosol to which nitrate contributes up to 30%. The highest concentrations of PM occur in the winter season and in such episodes the levels of nitrate seem to be especially elevated, e.g., in the Ruhr-area (Kuhlbusch, private communication).

Adams and Seinfeld (1) predict a quite large increase in the nitrate concentrations in the coming century. The reason is complicated but related to the availability of ammonia (from agriculture) that is necessary for forming aerosol nitrate. It is illustrative of the interplay of the major anthropogenic sources in the formation of nitrate.

Epilogue. We are continuously updating our data file and field with the latest data (for Europe). Data from East-Asia and Mexico are highly indicative of levels of nitrate comparable to those in Europe.


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Acknowledgement. The study was financed by the Ministry of Economic Affairs of the Netherlands and the Dutch National Research Program on Global Air Pollution and Climate Change. We gratefully acknowledge the data support by the authors of refs. 18,19 and 21 to 24.

Table 1: Average concentrations of aerosol nitrate, and winter values only (Oct-Mar) in the period 1994-1997, at the indicated stations.
De Zilk*NL4.24.6Idem
High MufflesGB2.6-Idem
Diabla GoraPL2.33.6Idem
# Country code
* Data obtained with adequate measuring methods; other values are corrected data or lower limits (10).
+ 1995-1998

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