The Chemistry of Nitrous Acid, HONO

Why are we interested in HONO?

Nitrous acid, HONO, is a source of the most important daytime radical: the hydroxyl radical. The OH radical is one of the key species in photochemical cycles responsible for ozone formation, which can lead to the so called "photochemical smog" in polluted regions. To understand the mechanisms that lead to this ozon-smog we need to know the sources of the OH radical.

One of the most poorly understood OH sources is the formation of nitrous acid, HONO, followed by its photolysis in sun-light [Cox, 1974: Cox ,1976; Stockwell and Calvert, 1979; Bongartz et al., 1991, 1994]

HONO + hn ® OH + NO (1)

Although this OH source mechanism has been known for nearly three decades [Johnston and Graham, 1974; Nash, 1974; Perner and Platt, 1979; Platt et al., 1980a, b], many questions about its importance remain open.

The following figure gives an overview of the chemistry of HONO which will be discussed in the next sections.

Sources of HONO

Sinks of HONO

The most important sink of HONO is photolysis (see reaction 1). The heterogeneous self-reaction of HONO has been investigated in the laboratory at high HONO concentrations [Febo et al, 1995], but it is unlikely that it plays a role in the atmosphere. HONO can also react with secondary and tertiary amines, forming carcinogenic nitrosamines [Pitts et al, 1978]. While this is not an important loss process, it has a direct impact on human health [Beckett et al., 1995; Rasmussen et al., 1995].

Observation of HONO and the Implication for Atmospheric Chemistry

Observations of HONO in the atmosphere show typical diurnal cycle. Since the major loss process is photolysis, HONO builds up during the night. At sunrise it is photolyzed into OH radicals and NO and its concentration drops. During the early morning hours HONO can be the most important source of OH radicals in the polluted atmosphere, as will be shown later. The concentration remains low during the day and rises again after sunset.

Observations of HONO in the polluted atmosphere often show nighttime mixing ratios of more than 10 ppb. Table 1 gives an overview of several observations of HONO in the polluted and remote atmosphere. No reliable daytime observations have been reported, but HONO mixing ratios up to 1 ppb at low sunlight intensities appear to be possible. A quantification of the daytime OH production by HONO photolysis has not been reported.

Table 1: Exaples of some maximum mixing ratios of HONO in the atmosphere and the quotient of HONO and NO2

 

Technique

Max. HONO

ppb

HONO/NO2

%

  

Juelich, Germany

DOAS

0.8

2.4

Perner & Platt 1979

Deuselbach, Ger.

DOAS

< 0.1

< 0.06 - < 2

Perner & Platt 1979

Los Angeles

DOAS

8

1 – 13

Harris et al. 1982

Göteborg, Swe.

Denuder

0.26 (avg.)

-

Ferm et al. 1983

Long Beach

DOAS, Den.

15

2.5

Appel et al. 1990

Ispra, Italy

OPSIS – DOAS

2

2

Notholt et al. 1992

Birmingham, UK

Den.,Cont. Anal.

10

1.5

Harrison et al. 1994

Milano, Italy

Denuder

17

up to 12

Febo et al. 1993, 1996

Zürich,Switz.

Wet Wall Den.

3.4

3.9 (avg.)

Zellweger et al. 1997

How important is HONO as OH source?

Simplified model calculations can be performed using varying HONO production rates explaining 0.2 ppb, 6 ppb, and 15 ppb, maximum HONO nighttime concentrations. These calculations show that, in the early morning, OH production from HONO photolysis exceeds OH production from the photolysis of ozone (figure 1). Later in the day, ozone photolysis dominates. In heavily polluted regions, HONO photolysis can be as much as 50% of the total OH production during the day (see following figure).

The numbers used for this calculation, as well as the OH production integrated over 24 hours, are displayed in table 2. Even integrated over a 24 hour period, the OH production by HONO photolysis can be comparable to the one by ozone photolysis.

Table 2: HONO formation rates used in the model calculations and the integrated OH production over 24 hours.

[O3]

ppb

HONO Formation

ppb/h

OH Production in 24h

ppb

O3 Photolysis

HONO Photolysis

Very Polluted

200

1.5

18.1

24.1

Medium P.

60

0.2

5.4

3.9

Cont. Backgr.

30

0.02

2.7

0.4

Why do we still not Understand HONO Chemistry?

The problem with HONO is that its concentration is strongly influenced by meteorological effects, i.e. the change from nighttime to daytime chemistry. During the night the boundary layer is at a much lower altitude, sometimes below 100m, than during the day, where it typically is located between 1000 m and 1500m. If HONO is formed close to the ground, the observed concentration at night will only reflect the lowest part of the troposphere. When the boundary layer moves upward during the morning the HONO is diluted. Assuming a nighttime boundary layer height of 150m and no HONO above the inversion, the dilution factor referring to a daytime inversion at 1500m is 10. Therefore, the measured source strength of HONO would be lower by a factor of 10. Even in this case HONO photolysis remains the most important OH source in the morning while it will only contribute ~10% of the OH production over a 24 hour period. At lower pollution levels the importance of HONO photolysis is smaller.

Why do we want to know more about the Chemistry of HONO?

A better understanding of the formation of HONO and its importance as a source of OH-radicals is highly desirable for the improvement of air pollution models. So far, none or only very oversimplified HONO production mechanisms, i.e. converting 10% of the NO2 into HONO, are used in these models. As shown above, a better understanding of HONO chemistry might have a severe impact on these models, our understanding of photochemical smog formation, and possible future political decisions regarding emission control strategies.

Open Questions

References