The influence of the nocturnal boundary layer on secondary trace species in the atmosphere at Dorset, Ontario

Abstract

Abstract-The impact ofthe nocturnal boundary layer (NBL) on the concentrations of03' H 2 0 2 , PAN and CH20 has been studied for two 4-day periods, one in the summer of 1989 and the other in the spring of 1990. Where the presence of the nocturnal layer is clear, 03' PAN and CH 2 0 concentrations decrease rapidly and these species appear to be deposited to the surface. Break-up of the inversion in the morning returns concentrations to levels typical ofthe previous day. H 2 0 2 is removed much faster than the other species and it is replenished much more slowly. This appears to be due to the H 2 0 2 dissolving in water droplets resulting from the rapid cooling of the air. There is evidence for a morning maximum in PAN, likely due to the combination of high concentrations being brought downwards during the break-up of the NBL followed by thermal decomposition. Key word index:Ozone, hydrogen peroxide, formaldehyde, PAN, nocturnal boundary layer, deposition, nighttime chemistry. INTRODUCfIONField measurement studies are an important part of the atmospheric chemists' arsenal. They are performed to give data to assist in understanding specific questions in atmospheric chemistry, and for the development and evaluation of photochemical models.For convenience, most field measurements are made at the surface. However, the presence of the ground, and other local effects, can often significantly impact the observations, masking the larger scale chemical features one wishes to study. One such factor is the occurrence of a strong nocturnal boundary layer (NBL). By isolating the air being sampled from the bulk of the air aloft, for several hours each night, it often plays the principal role in determining the diurnal variation of the surface concentrations of a number of the more reactive species.In this paper we look in detail at the measurements of a number of secondary, reactive, trace gases, made at a rural site in Ontario, to see the effect of the NBL on their concentrations. The gases under study were 0 3 , HzO z , CHzO and PAN. These species have no direct emission sources, with the exception of a small amount of CHzO from combustion, and originate from atmospheric oxidation chemistry. By concentrating on these species, it is possible to focus on the chemistry that is occurring, rather than on the emission or transport processes. Furthermore, using the NBL to isolate a relatively small section of the atmosphere accentuates the observable consequences of any of these processes. Although the concentrations of this suite of species would be expected to be related, they have unique atmospheric lifetimes and decay mechanisms, and therefore they constitute a diverse set of molecules with which to probe our understanding of the system.The behaviour of ozone under the NBL is well documented (e.g. Cox et al., 1975;Harrison et al., 1978;Oltmans, 1981; Kelly et al., 1984; Broder and Gygax, 1985;Broder et al., 1981; Fehsenfeld et al., 1983;Lifshitz et al., 1988;Shepson et al., 1992b). However, the previously reported diurnal behavi...

Multi-year (2004–2008) record of nonmethane hydrocarbons and halocarbons in New England: seasonal variations and regional sources

“…Stable nocturnal boundary layer heights typically range from ∼50-200 m at midlatitude continental locations (e.g., Hastie et al, 1993;Gusten et al, 1998;McKendry and Lundgren, 2000;White et al, 2003); thus we chose 125 m as a representative value for the TF site (e.g., Talbot et al, 2005;Sive et al, 2007;Mao et al, 2008;White et al, 2008;. If we use H =50 m or 200 m, our emission rate estimates discussed below vary by ±60%.…”

“…We followed a simple box model approach which has been effectively used in previous studies to calculate emission and removal rates of trace gases in this region (i.e., Talbot et al, 2005;Zhou et al, 2005;Sive et al, 2007;White et al, 2008;Russo et al, 2010). This method uses measurements made on nights with low wind speeds and when a stable inversion layer has developed because under these conditions, the exchange of air between the nocturnal boundary layer (NBL) and the residual layer above is limited (e.g., Hastie et al, 1993;Gusten et al, 1998;Talbot et al, 2005). Therefore, advection and vertical mixing of air masses can be neglected.…”

Vertical nitrogen dioxide and ozone concentrations measured from a tethered balloon in the Lower Fraser Valley

“…It is generally felt that daytime measurements, after the breakup of the NBL, are truly representative because of the high turbulence, especially in the summer. The interpretation of nighttime data is more difficult because of the stratification of the atmosphere (Hastie et al, 1993;Shepson et al, 1992). The flights of 1 August allow us to examine this problem for both N0 2 and 0 3 , For these compounds, N0 2 has a surface source whereas 0 3 has a surface sink.…”

Air-chemistry "turbulence": power-law scaling and statistical regularity

“…3. All of them have clear spectral peaks at semi-diurnal and diurnal frequencies, corresponding to the local rush-hour emissions in the mornings and evenings and the photochemical reactions variability, respectively, in addition to the diurnal planetary boundary layer evolutions driven primarily by the daytime heating and nighttime cooling at the surface (e.g., Hastie et al, 1993;Güsten et al, 1998;Talbot, 2004, Talbot et al, 2005;Stephens et al, 2008). Among NO, NO 2 , NO x , CO, and SO 2 spectra, these two peaks have about the same pattern, i.e., the diurnal one is stronger than the semi-diurnal one.…”

“…However, there is still a lack of information on how chemical species are related to various dynamical scales in both time and space. Of course, the basic diurnal and semidiurnal oscillations found in the chemical observations (e.g., Hastie et al, 1993;Güsten et al, 1998;Talbot, 2004, Talbot et al, 2005;Stephens et al, 2008) are caused by variations in photochemical reactivity and traffic emissions, respectively, in addition to the diurnal boundary-layer variability. Stephens et al (2008) identified weekly signals in their study of the atmospheric chemical constituents in Mexico City, most likely due to the lower weekend emissions of ozone (O 3 ) precursors including nitrogen oxides (NO x , i.e., NO and NO 2 ) and reactive volatile organic compounds (VOCs), and reported references on this subject for other major cities and metropolitan regions.…”