Envlron. Sci. Technol. 1984, 18,365-369
Measurement of Nitrate Radical Concentrations in Continental Air Uirich F. Platt,t Arthur M. Winer, Heinz W. Biermann, Roger Atkinson, and James N. Pltts, Jr." Statewide Air Pollution Research Center, University of California, Riverside, California 9252 1
Nighttime profiles of the atmospheric concentrations of the nitrate radical (NO,), NOz, and 0, have been obtained by using a mobile differential ultraviolet/visible absorption spectrometer over path lengths from 3 to 17 km. Measurements were carried out at four semiarid/ desert sites in the southern California desert (Death Valley, Edwards Air Force Base, Phelan, and Whitewater), and nitrate radicals were observed at all four locations at typical concentrations of 10-100 parts per trillion. A decrease in the calculated NO3 lifetime with increasing relative humidity (RH) was observed with lifetimes up to -60 min for 550% RH, but 510 min for 250% RH. This suggests the involvement of water in the loss process for NO,, either in the gas phase or in the aqueous state at the ground or on aerosol surfaces. These experimental field data support our earlier hypothesis that this loss process of NO3 may constitute both a major sink for atmospheric NO, and a significant formation route for nitric acid, a key component of acid rain and fog.
-
~
~~~~
Introduction In order to understand the chemical cycles of NO and NOz and their roles as precursors to photochemical air pollution and acid deposition, the atmospheric chemistry of a number of other nitrogen-containing compounds which play a role in the production, storage, conversion, and sink mechanisms of NO, also need to be investigated. One such compound, the nitrate radical (NO,), has been included in models of the chemistry of the atmosphere for many years, but it is only recently that this species has been identified and measured in the troposphere (1-7). Measurements of the ambient concentrations of the NO3radical are of increasing importance in view of recent measurements of rate constants for the reactions of the NO, radical with a wide range of organics (8-13) which extend the earlier data of Niki and co-workers (14, 15). The magnitudes of these rate constants suggest that these reactions could be important loss processes for NO, and organics, as well as a formation route for nitric acid, a key constituent of acid deposition. We have previously employed a rapid-scanning differential optical absorption spectrometer (DOAS) over optical path lengths ranging from several hundred meters up to several kilometers to identify the NO, radical and measure its atmospheric concentrations at ground level as a function of time of day, geographical location, and season ( I , 3-6, 16,17). These concentrations have ranged from the detection limit of the DOAS system [-l part per trillon (ppt)] up to -350 ppt in a polluted air mass. Our previous data from locations such as California's South Coast Air Basin and Julich, West Germany, showed that while in polluted areas NO, appears to be present only for a few hours after sunset ( I ) , in rural air masses NO, could occasionally be detected throughout the night until its rapid photolysis at dawn (4). In many cases, however, no NO3was observed above our 1 ppt detection limit in either type of air mass (3). Even when NO, was observed, its concentrations always appeared to be far below the
-
'Present address: Institut fiir Chemie 3, Atmosphlirische Chemie, D-5170 Julich, West Germany. 0013-936X/84/09 18-0365$01.50/0
levels expected from the presently known NO, chemistry (2, 4 ) . To further investigate this phenomenon, we undertook an extensive new set of NO3 measurements in cleaner atmospheres and under a wide range of relative humidities with particular emphasis on the desert regions of California. We report here these observations of NO3at Death Valley, Edwards Air Force Base, Phelan, and Whitewater, CA (Figure l),and discuss the implications of our measurements for the atmospheric chemistry of the NO3 radical.
Experimental Methods The concentrations of NO, radicals and NO2 (and in some cases O,, HCHO, and HONO) were measured with a long-path DOAS system which has been described previously (6, 16, 18);hence, only a brief description will be given here. The concentrations of these species were derived from their light absorption in the visible (NO,) or near-UV (NO2,HCHO, and HONO) spectral regions. The light source employed for all measurements was a 450-W xenon high-pressure lamp whose radiation was collimated into a parallel beam by a spherical mirror (30-cm diameter, f = 25 cm) and transmitted horizontally in the open atmosphere over optical paths of -3-17 km. A Newtonian type telescope (30-cm aperture, f = 1.8 m) focused the received light on the entrance slit of a 0.5-m spectrograph (SPEX 1870). The spectrograph was equipped with a rotating metal disk carrying radial slits (100-pm width, 10-mm spacing) across the exit focal plane. A 35-nm segment of the spectrum was scanned repetitively in a chosen spectral region, The light intensity passing through the moving slits was monitored by a photomultiplier tube (EM1 9659$), and the analog signal was averaged for -25 ps, then digitized, and stored by a DEC MINC 11/23 computer. The computer averaged the scans and was also used to manipulate the data in spectral deconvolution and in the calculation of optical densities. Absorption bands with optical densities as low as (base 10) could be detected, and the detection limit for NO, ranged from 1ppt to several ppt, depending on the length of the light path used and the atmospheric particulate loadings during the experiments. As in most previous cases (I+), the NO, absorption cross section (udiff = 1.76 X 1O-l' cm2 molecule-l) given by Graham and Johnston (19) was used to convert the measured optical densities to concentration data. In addition to DOAS observations, supporting measurements of ozone (Dasibi, Model 1003AH), aerosol scatter (Meteorology Research Inc., Model 1550B integrating nephelometer), temperature, humidity (thermohydrograph and psychrometer), and solar radiation (Eppley Model PSP total radiation sensor) were also recorded continuously. A total of four measurement periods were conducted at four separate semiarid/desert sites in southern California (Figure 1)from Oct 1981 to May 1982. At all sites more than two-thirds of the optical paths (which ranged from 2.8 km at Whitewater to 17 km at Phelan) were at least 10 m above the ground. The four sites involved were selected to examine a range of atmospheric conditions.
0 1984 American Chemical Society
Environ. Sci. Technol., Vol. 18, No. 5, 1984 385
"-1
DEATH VALLEY
""I-
No3
20
MAY 4. 1982
MAY 3
Flgure 1.
'-1
F W 3. Nighttime mcsntratbn profilefor NO, at Deam Valley. CA. May 4.1982. (Une shown Is a Sroom cwe m u g h the data i g d n g possible local maxima and mlnima in the concenbatbn profiles.) NO, conc6nbationswere b&w the detection limn of 0.3 ppb and are hence
Measurement snes in southern California.
I
WATER
I
.
i20
f
not
shown.
EDWARDS A F B 100
I
SUNRISE
NO?
*
I
01
'
~
'
21 00
~
'
0 00
MAY 23
,
I
9 00
'' I '6~ b0 ' ' lo
MAY 24. 1982
Nb$tllmec c n m t b n proRes faNOs and NO9 at Edwards Air Force Base. CA. May 24. 1982. (Line shown Is a smooth curve through the data ignoring DOSSible local maxima and minima in the concentration profiles.) Fluwe 4.
0 00 O C T 30
3 00 OCT
I 00
31. 1981
F b r e 5. NiaMnme wncenbatbn proflle for NO, at Whnewater. CA. Oct 31,1981. Wneshown Is a smodh cwe thocgh thedata i g d n g possible local maxima and minima in concenbatbn profiles.) NO, CMICBnlraUnls were b b w the d e t m b n Rni of 0.9 ppb and are hence
not shown
5). The differences in the two NO, radical time-concentration profiles for Whitewater (Figures 2 and 5) can be attributed to differencea in rates of emission of NO, radical scavengers (e.g., NO) or to transport phenomena. How-
Table I. Concentrations of NO, and Associated Dataa NO,, PPt steady Bscat,e maxd NO,: ppb x ~ O m-I - ~ stateC Whitewater (2.8-km Optical Path)
date Oct 1 9 8 1 27/28 28/29 30131 31/Nov I Nov 1 9 8 1 213 516 10111 11/12 16/17 23/24 Dec 1 9 8 1 415 819 10111 11/12
RH,e %
temp,e "C
TNO,?
f
min
4
67 55
