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A proposed physical mechanism for ozone-meteorology correlations using land–atmosphere coupling regimes

A proposed physical mechanism for ozone-meteorology correlations using land–atmosphere coupling regimes

Atmospheric Environment 72 (2013) 50e59 Contents lists available at SciVerse ScienceDirect Atmospheric Environment journal homepage:

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Atmospheric Environment 72 (2013) 50e59

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Atmospheric Environment journal homepage:

A proposed physical mechanism for ozone-meteorology correlations using landeatmosphere coupling regimes Ahmed B. Tawfik a, b, *, Allison L. Steiner a a

Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Space Research Building, 2455 Hayward Street, Ann Arbor, MI 48109-2143, USA b Center for Ocean-Land-Atmosphere Studies, Institute of Global Environment and Society, 4041 Powder Mill Road, Calverton, MD 20705-3106, USA

h i g h l i g h t s < We propose a physical mechanism producing the ozone-meteorology gradient. < 17 years of hourly ozone and precursor observations and meteorological data are used. < Ozoneehumidity correlation is an artifact of landeatmosphere coupling regimes. < Evaporative fraction is a better predictor of ozone for soil water-limited regimes.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 July 2012 Received in revised form 25 February 2013 Accepted 2 March 2013

Correlations between surface ozone and meteorological variables exhibit a north-south gradient over the Eastern United States (US), with the ozoneetemperature correlation weakening and the ozoneehumidity correlation transitioning from positive to negative south of 37 N. Using 17 years of hourly August ozone, nitrogen oxide, and isoprene measurements from the Environmental Protection Agency’s Air Quality System and Photochemical Assessment Measurement Stations and hourly meteorological fields from the North American Land Data Assimilation System (Phase 2), we propose that the north-south transition and widely observed ozoneehumidity correlation results from a shift in the soil moisture-atmosphere coupling regime. Due to soil water limitations over the Southeast, evapotranspiration and specific humidity increase following precipitation events, and this coincides with reductions in temperature and ozone precursors. Therefore, the negative ozoneehumidity correlation in the Southeast is likely a manifestation of several meteorological factors directly influencing ozone production. Surface drying, as defined by the evaporative fraction, provides a better predictor of O3 than temperature, specific humidity, or radiation for the Southeast due to its ability to retain prior precipitation information and reflect sameday atmospheric conditions relevant to O3 production. Behavior of surface fluxes and coupling may be particularly relevant for prediction of seasonal and future O3 air quality, and further investigation into the links between landeatmosphere coupling and O3 is necessary. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Ozone-meteorology correlations Landeatmosphere coupling Humidity Evaporative fraction Eastern United States Surface ozone

1. Introduction The Eastern United States (US) is prone to high summer ozone concentrations (O3) caused by the combination of warm, stagnant meteorological conditions and abundant O3 precursor species, such as biogenic isoprene (Chameides et al., 1988; Fiore et al., 2005) and anthropogenic nitrogen oxides (Frost et al., 2006). Several modeling

* Corresponding author. Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Space Research Building, 2455 Hayward Street, Ann Arbor, MI 48109-2143, USA. E-mail addresses: abtawfi[email protected], abtawfi[email protected] (A.B. Tawfik). 1352-2310/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.

(Sillman and Samson, 1995; Dawson et al., 2007; Zeng et al., 2008) and observational (Cox and Chu, 1993; Camalier et al., 2007; Zheng et al., 2007; Bloomer et al., 2009; Blanchard et al., 2010; Davis et al., 2011) studies have found that changes in temperature and humidity are the two primary predictors for O3 for the Eastern US. This study presents a possible physical mechanism for describing the spatial pattern of temperature- O3 and humidity- O3 correlations not yet explained in the literature. 1.1. Surface ozone formation and behavior Photochemical production of tropospheric O3 occurs when hydrocarbon species are oxidized by the hydroxyl radical (OH) in the

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presence of reactive nitrogen oxides (NOx) and sunlight. This process is accelerated under warmer near-surface conditions; however, the system is highly nonlinear. For example, Steiner et al. (2010) showed that daily 1-h maximum O3 does not increase with temperatures over 312 K. Further, increased concentrations of atmospheric water vapor can provide a chemical sink for O3 reducing background concentrations in remote regions through an increased conversion of O(1D) to OH (Johnson et al., 1999). However, urban studies indicate that loss of O3 through this pathway is weak (Dawson et al., 2007), and in high NOx environments increased water vapor can lead to additional OH formation (Steiner et al., 2006) and increased ozone production (Kleinman et al., 2005). Using statistical methods, Camalier et al. (2007) demonstrated that primary O3-predictor variables have a clear north-south transition, where high latitude O3 (>38 N) is strongly positively correlated to daily maximum temperature and lower latitude O3 (<36 N) strongly negatively correlated to relative humidity. Davis et al. (2011) found that a widely used air quality model does not accurately capture the location of the north-south transition and underestimates the sensitivity of daily maximum 8-h O3 to temperature and relative humidity. Rasmussen et al. (2012) used a coupled chemistry-climate model to show that the temperatureozone relationship is overestimated north of 37 N and underestimated farther south. Both studies (Davis et al., 2011; Rasmussen et al., 2012) note that this north-south behavior suggests a latitudinal shift in the regime controlling O3 and may reflect inherent weaknesses in model meteorology or physics. 1.2. Landeatmosphereechemistry interactions Although there is agreement that temperature is a strong controller of O3 in the Northeast US, warmer temperatures often coincide with other meteorological conditions favorable to O3 production (Vukovich, 1995), such as stagnation events and reduced cloud cover. Cloud cover changes have been suggested as a possible cause of the negative O3-humidity correlation in the Southeastern US (Camalier et al., 2007; Davis et al., 2011), yet modeling studies have found that radiation perturbations from cloud cover produce minimal changes in surface O3 (Sillman and Samson, 1995; Dawson et al., 2007). Another modeling study found that drier soil moisture initialization increased sensible heat flux and warmed temperatures, promoting higher O3 throughout the boundary layer (Jacobson, 1999), pointing to a possible connection between soil moisture and O3. From the climate perspective, soil moisture is known to strongly influence sub-seasonal temperature and precipitation (Findell and Eltahir, 1997; Fennessy and Shukla, 1999; Pal and Eltahir, 2001; Fischer et al., 2007; Wu et al., 2007; Koster et al., 2010; Guo et al., 2011) because soil can retain information of prior precipitation events long after an event has passed. In some regions, this leads to changes in surface flux partitioning where drier (wetter) soils reduce (promote) subsequent precipitation and increase (decrease) temperatures. Near-surface water vapor also decreases over drier soils where soil water controls evapotranspiration (Betts, 2004, 2009; Santanello et al., 2011). This interaction between the soil and atmosphere is known as “landeatmosphere coupling” and can be categorized by two regimes: 1) evapotranspiration is limited by soil water availability, referred to as soil moisture-limited, and 2) evapotranspiration is limited by evaporative energy, referred to as energy-limited (Seneviratne et al., 2010). For soil moisture-limited regimes, the land surface impacts the atmosphere more readily leading to negative (positive) correlations between near-surface temperature (humidity) and evapotranspiration. When evapotranspiration is energy-limited, atmospheric conditions exert


control over evapotranspiration with weak feedback from soil moisture. Here we propose a mechanism that approaches the northsouth shift in ozone-meteorology correlations from the perspective of landeatmosphere interactions. By using the lande atmosphere coupling framework, the degree of coupling between surface and atmospheric variables can first be examined and then evaluated within the context of ozone production. This provides the necessary physical basis for describing ozonemeteorology relationships within the coupled climate system. We present evidence that the latitudinal change in ozoneespecific humidity correlations likely arise from evapotranspirationlimiting regimes. In this regard, near-surface specific humidity (hereinafter, “humidity”) acts as a passive surrogate in ozone formation rather than a direct causal variable. Evidence for this mechanism is explored using observational and reanalysis data over the Eastern US. 2. Data and methods 2.1. Observational chemical concentrations Hourly O3, NOy, NOx, and isoprene concentration data for August from 1994 to 2010 are obtained from the Environmental Protection Agency’s Air Quality System (EPA-AQS) Photochemical Assessment Monitoring Stations.1 Analysis is confined to stations within the Eastern US (25e50 N and 98e68 W). In this region, there are a total of 1221, 71, 376, and 223 stations measuring O3, NOy, NOx, and isoprene, respectively, with at least 650, 16, 126, and 30 active stations for any given year from 1994 through 2010. Ozone data are filtered to include stations and years with at least half a month of hourly measurements. Precursor species (NOy, NOx, and isoprene) data are not filtered due to limited data availability and to provide the maximum number of measurements for evaluation. The measurement method used for NOx in the EPA-AQS network has been shown to erroneously include peroxy acetyl nitrate (PAN), organic nitrates, and nitric acid (HNO3) (Fehsenfeld et al., 1987; Dunlea et al., 2007). Therefore, NOx measurements are treated as containing a subset of nitrogen oxide species that may be more effectively wet removed than NOx (¼NO2 þ NO). August is selected in this study due to peak summer temperatures and the potential for O3 exceedances. 2.2. North American land data assimilation system A suite of atmospheric and land surface data from Phase 2 of the North American Land Data Assimilation System (NLDAS2) is used for the climate data analysis. NLDAS2 provides hourly 2-m temperature, incident solar radiation (R), 2-m humidity, and precipitation at 1/8 by 1/8 degree grid spacing from 1994 to 2010 (Mitchell et al., 2004). With the exception of precipitation, NLDAS2 atmospheric forcing fields are elevation-adjusted quantities derived from the North American Regional Reanalysis (NARR; 32 km by 32 km) (Mesinger et al., 2006) and are temporally interpolated from 3-hourly to hourly, and spatially interpolated to the higher eighth-degree grid spacing required by NLDAS2 (Cosgrove et al., 2003). Incident radiation is bias-corrected using 5 years of hourly GOES-8 satellite data (Pinker et al., 2003). Precipitation is derived from PRISM topographically adjusted daily Climate Prediction Center (CPC) precipitation gauge data (Daly et al., 1994; Xie and Arkin, 1997) and is temporally decomposed into hourly

1 US EPA, 2011 Air Quality System Data Mart: US Environmental Protection Agency


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data using Stage II 4 km by 4 km hourly Doppler radar output or the CPC Hourly Precipitation Dataset (HPD) (Higgins et al., 1996). Surface variables (soil moisture, net surface radiation (Rn), and surface fluxes (sensible, H, and latent flux, lE)) are acquired from NLDAS2MOSAIC land surface model (Koster and Suarez, 1992, 1994). NLDAS2-MOSAIC is forced by NLDAS2 atmospheric fields, including surface pressure and 10-m wind components in addition to the variables described above. To avoid any single grid cell bias, NLDAS2 data are averaged over a 1/4 by 1/4 region around each EPA-AQS station.

3.2. Meteorological and climate controls on ozone

Several observed (Koster et al., 2009; Dirmeyer, 2011; Miralles et al., 2012) and model-based (Koster et al., 2006; Seneviratne et al., 2006) metrics have been developed to quantify lande atmosphere coupling, and these methods generally identify coupling in the same geographic areas (Seneviratne et al., 2010; Miralles et al., 2012). To identify factors limiting surface evapotranspiration, we employ a soil moisture-temperature coupling diagnostic (r(E,T)) defined as the Pearson correlation between evapotranspiration (E) and temperature (T) (Seneviratne et al., 2006). Negative values of r(E,T) are referred to as soil moisturelimited or “coupled” while positive values are energy-limited or “uncoupled”. We calculate a modified monthly version of r(E,T) using midday average (12pme4pm) E and T for each August (e.g. the correlation between 31 days of E and T for each year).

Zonally averaged anomalies can be used to understand environmental controls on O3 interannual variability (Fig. 2), where anomalous conditions are defined as deviations from the 1994e 2010 mean August conditions at EPA-AQS stations. In 1999, O3 anomalies reach 12 ppb from 30 to 36 N (Fig. 2a), and 4e7 ppb higher, during the historic 2007 drought in the Southeast (Luo and Wood, 2007). The lowest O3 anomalies occurred in 2009 from 32 to 38 N and were more than 10 ppb below the 17-year average during a year with anomalously wet soil moisture conditions (Fig. 2d). During the years with the greatest positive anomalies of O3 (1999, 2000, and 2007), R and temperature anomalies are also higher than average (Fig. 2b and c, respectively). For 1999 and 2000, R was about 40 W m2 greater than the 17-year mean and 20e 50 W m2 greater for 2007 over the Southeastern US, suggesting a reduction in cloud cover. Warm temperature anomalies are concurrent with R anomalies, with 2007 as the warmest year in the Southeast (>3 K), and 1999 and 2000 having weaker but nonnegligible positive anomalies (0.5e3 K). Temperature anomalies also correspond to drier than average soil moisture conditions (Fig. 2d), with 2000 and 2007 as the driest years. Soil moisture spatial anomalies follow the same pattern as temperature anomalies in the Southeast consistent with the landeatmosphere coupling framework presented in Section 1.2 highlighting the potential for landeatmosphere interactions to modulate O3.

3. Results

3.3. Landeatmosphere coupling and ozone

3.1. Spatial patterns of ozone concentrations and correlations

The soil moisture-temperature coupling parameter (r(E,T); Section 2.3) calculated from NLDAS2 data (Fig. 3) shows a similar spatial behavior as r(q,O3) (Fig. 1c). Specifically, stations where soil moisture perturbations exert greater control on near-surface temperatures (e.g. r(E,T) < 0) correspond to stations with a weaker r(T,O3) and negative r(q,O3) (Fig. 1b and c). This suggests that the ozone-meteorology correlations may be influenced by lande atmosphere interactions over the Southeastern US. To understand the relationships between predictor climate variables (T, q, R) and O3 we examine the zonally averaged interannual changes in r(E,T) (Fig. 4a). For stations south of 36 N, evapotranspiration is soil moisture limited (r(E,T) < 0) for all years except 2009 (Fig. 4a). North of 40 N, evapotranspiration tends to be energy-limited (r(E,T) > 0). In the Mid-Atlantic region (36e39 N)

2.3. Soil moisture-temperature coupling parameter

Average August midday (12e4pm) O3 over the Eastern US is between 45 and 65 ppb with the highest concentrations occurring along the Eastern Seaboard, the Ohio River Valley, and Tennessee River Valley (Fig. 1a). Fig. 1b shows the correlation between O3 and temperature (r(T,O3)) for August at each EPA-AQS station with at least 10 years of data. Consistent with prior studies (Camalier et al., 2007; Davis et al., 2011; Rasmussen et al., 2012), temperature is most strongly correlated to O3 north of 38 N (r(T,O3) > 0.7) and weakens south of 36 N (r(T,O3) < 0.5). The correlations between humidity and ozone (r(q,O3); Fig. 1c) show similar north-south decreases as r(T,O3), but with a transition from positive to negative correlation coefficients south of 37 N.




Fig. 1. a) August observed midday (12e4pm) average surface ozone concentrations (ppb), and Pearson’s correlation coefficient of ozone versus Phase 2 NLDAS reanalysis 2-m b) temperature and c) humidity from 1994 to 2010 at EPA-AQS stations.

A.B. Tawfik, A.L. Steiner / Atmospheric Environment 72 (2013) 50e59






Fig. 2. Zonally averaged anomalies for observed average midday a) surface ozone concentrations (ppb), b) NLDAS2 reanalysis incident solar radiation (W m2), c) NLDAS2 reanalysis 2-m temperatures (K), and d) NLDAS2-MOSAIC reanalysis soil moisture from 0 to 40 cm (mm) for August from 1994 to 2010 at EPA-AQS stations.

Fig. 3. Pearson’s correlation between NLDAS2 reanalysis evapotranspiration (E) and 2m temperature (T). Markers indicate particular EPA-AQS-PAMS stations for the Northeast (NE) and Atlanta Metropolitan Area (ATL) averaging regions: 46 (NE) and 16 (ATL) ozone stations, 20 (NE) and 5 (ATL) NOx stations, zero (NE) and 3 (ATL) NOy stations, and 11 (NE) and 4 (ATL) isoprene stations.

the limiting factor varies interannually, indicative of a regime shift as suggested by Davis et al. (2011) and Rasmussen et al. (2012). Zonally averaged correlations between midday O3 and environmental variables exhibit north-south features similar to the coupling parameter (Fig. 4b). Consistent with station data in Fig. 1, temperature is positively correlated with O3 throughout the Eastern US with stronger correlations at higher latitudes (Fig. 4b; r(O3,T) ¼ 0.1e0.6). In contrast, r(O3,q) exhibits a latitudinal gradient with a transition from negative values in the South (<36 N) to positive values in the North (>39 N). This transition occurs in the same Mid-Atlantic region (36e39 N) coinciding with the transition from soil water-limited to energy-limited regimes (Fig. 3). The transition zone corresponds to the region where mid-latitude synoptic systems become less prominent and convective events dominate (Li et al., 2005; Leibensperger et al., 2008; Fang et al., 2009). R is positively correlated with midday O3 at each zonal band with the strongest correlations from 33 to 40 N (r(O3,R) ¼ 0.3e0.4). The evaporative fraction (lE/RN) defines the portion of net surface radiation (RN) partitioned to evapotranspiration and can describe the degree of surface drying. lE/RN is negatively correlated with O3 over the Eastern US and the absolute value of r(O3, lE/RN) is greater than those of r(O3,T) or r(O3,q) (r(O3, lE/RN) < 0.6) south of 39 N (Fig. 4b). Given the strong correlation between lE/RN and O3, lE/RN may prove to be a better O3 predictor than other more commonly used variables for the Southeastern US. The correlation with midday O3 weakens to the north (>39 N) where evapotranspiration is no longer soil moisture limited. The physical mechanism underlying the strength of evaporative fraction as a


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Fig. 4. August zonally averaged (a) interannual variations in soil moistureetemperature coupling strength (r(E,T); Section 2.2) derived from NLDAS2 reanalysis and (b) zonally averaged correlation coefficient of observed midday average ozone against NLDAS2 reanalysis derived (red) 2-m temperature, (black) incident radiation, (blue) 2-m humidity, and (orange) evaporative fraction. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

predictor and the latitudinal behavior of correlations is described in Section 4. 3.4. Antecedent meteorological behavior To further evaluate our proposed physical mechanism, we examine the average antecedent temporal evolution of meteorology (Fig. 5) and chemical species (Fig. 6) prior to low (<40 ppb) and high (>70 ppb) midday O3 events. Two regions are examined, the Atlanta Metropolitan area (33e34.5 N and 83e85.4 W; ATL; soil-moisture limited) and a Northeast region (40e42 N and 73e 75 W; NE; energy-limited), to represent typical urban O3 nonattainment environments with long time series of chemical concentrations corresponding to different landeatmosphere coupling regimes. An average of all August high (142 for ATL; 78 for NE) and low (88 for ATL; 145 for NE) O3 days from 1994 to 2010 are presented in Figs. 5 and 6. Other geographic regions were examined and returned similar results (see Supplementary material). Average meteorological conditions (temperature, precipitation, humidity, wind speed) leading up to high and low O3 events are presented in Fig. 5. Comparing ATL and NE, there is a noticeable decrease (increase) in 2-m temperatures over the course of the 96 h prior to low (high) O3 events with a greater temperature change for NE (Fig. 5a). Both regions also record warmer temperatures prior to high O3 events. Accumulated precipitation totals for the 96 h prior demonstrate an even greater response between high and low O3 days for both regions (Fig. 5b). Precipitation totals are on average 19 mm prior to low O3 events and less than 6 mm for high O3 days. The difference in antecedent precipitation between high and low O3 events weakens beyond 60 h prior and converges at 4 mm, demonstrating that the influence of antecedent precipitation on midday O3 is approximately two days. In the NE, humidity gradually increases leading up to high O3 days (>70 ppb) from an average of 13e14.5 g kg1, with a sharper rise 36 h prior to the O3 maxima (Fig. 5c). This sharp increase coincides with warmer temperatures and less precipitation relative to low O3 days and may be indicative of a change in air mass following

frontal passage. For ATL, high O3 days correspond to lower midday humidity (w14.2 g kg1; Fig. 5c) whereas humidity increases slightly during the hours prior to low O3 conditions. Antecedent midday (12e4pm) average evaporative fraction (lE/RN) for ATL exhibits a 15% decrease in energy partitioning toward evapotranspiration under high O3 and w20% increase under low O3 (Fig. 5d). Overall, 90% of high (low) O3 days have midday average lE/RN less (greater) than 0.5 for ATL. For NE, only 60% of high O3 days have lE/ RN less than 0.5 and weaker evaporative fraction changes of 2% and 10% prior to high and low O3 conditions, respectively. This weaker response of evaporative fraction for the NE relative to ATL under comparable antecedent precipitation behavior further suggests a difference in landeatmosphere coupling regimes. Specifically, the evaporative fraction is less responsive to precipitation in the NE region as expected for energy-limited regimes. Stagnant air masses have been shown to promote pollutant accumulation (Vukovich, 1995) and are negatively correlated with the number of mid-latitude cyclone passages (Leibensperger et al., 2008). Using NLDAS2 10-m surface wind speed as an indicator of stagnation (Fig. 5e), winds are generally lighter approaching high O3 events indicating more stagnant conditions in both regions. For ATL, winds are w2 m s1 weaker for high O3 events with a weaker diurnal cycle (Fig. 5e), similar to results discussed in Blanchard et al. (2010). The NE exhibits weaker winds (w2 m s1) only during the nighttime hours prior to high O3 events, with little change during the day (0e0.6 m s1). An inland energy-limited region was also examined (see Supplementary material) and showed similar wind speed behavior as the NE region with little contrast in wind speed during the day between high and low O3 events, but calmer winds at night for high O3 events. Therefore, the diurnal wind behavior seen in the NE is likely not a result of its proximity to the Atlantic Ocean and a landesea breeze. 3.5. Antecedent precursor behavior The behavior of O3 and its precursors (NOx, NOy, and isoprene) are examined to identify the impacts of atmospheric and surface

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Fig. 5. Antecedent August a) 2-m temperature (K), b) accumulated precipitation (mm), c) 2-m humidity (g kg1), d) evaporative fraction, and e) 10-m wind speed (m s1) from NLDAS2 reanalysis for the hours approaching high (>70 ppb; dashed) and low (<40 ppb; solid) midday ozone concentrations averaged over EPA-AQS-PAMS sites for NE (black) and ATL (orange) from 1994 to 2010. NE and ATL regions defined in Fig. 3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

conditions on air quality (Fig. 6). For ATL, NOx is on average below 10 ppb prior to low O3 events and reaches 60 ppb approaching high O3 events (Fig. 6b). The difference in antecedent NOx concentrations between high and low O3 events is less pronounced for the NE with prior morning and nighttime (6e20 h prior) NOx concentrations of 20e32 ppb for low O3 events and 40e64 ppb for high O3events. Generally, the NE has higher midday NOx (w20 ppb) than ATL (w4 ppb; Fig. 6b). Elevated isoprene can increase photochemical O3 production under sufficiently high NOx (Chameides et al., 1988; Fiore et al., 2005). Isoprene concentrations range from <1 ppb on average at night to almost 4 ppb during the day for ATL (Fig. 6c), reflecting the strong biogenic isoprene source region in the Southeastern US (Guenther et al., 2006). The NE has lower isoprene with average midday values less than 1.5 ppb. Comparing the hours approaching high and low O3 for both regions, higher isoprene concentrations correspond to high O3 days and are likely driven by the warmer temperatures (1e4 K for ATL and 2e6 K for NE; Fig. 5a) resulting in higher biogenic isoprene emissions (Guenther et al., 1994; Petron et al., 2001). Although NOy measurements are only available for the ATL region, antecedent NOz (NOyeNOx) identifies the behavior of the primary chemical sink of NOx and the importance of wet removal. Antecedent NOz is 10 ppb higher the night before high O3 days, with little difference in high and low events prior to 20 h (Fig. 6d). The higher NOz concentrations observed prior to high O3 events correspond to dry (Fig. 5d) and stagnant conditions (Fig. 5e) with less wet removal (Fig. 5b).

4. Discussion Here we describe a mechanism controlling ozone-meteorology correlations under two soil moistureeatmosphere coupling regimes: 1) energy-limited evapotranspiration and 2) soil moisturelimited evapotranspiration. Environmental and chemical conditions leading up to high O3 events for energy-limited regimes (e.g., NE) are characterized by: little precipitation, weak changes in energy partitioning, warmer temperatures, higher humidity, more stagnant nighttime conditions, higher nighttime NOx concentrations, and slightly higher isoprene concentrations relative to low O3 events (Figs. 5 and 6). The hours prior to high O3 events for soil moisture-limited regimes (e.g., ATL) are characterized by: little precipitation, less energy partitioning toward evapotranspiration, warmer temperatures, lower humidity, more stagnant conditions, higher nighttime NOx and NOz concentrations, and slightly higher isoprene relative to low O3 events (Figs. 5 and 6). The two regimes differ only in the behavior of evaporative fraction and humidity suggesting that the commonly observed ozoneehumidity correlation over the Southeastern US may be a manifestation of soil moistureeatmosphere coupling regimes. This proposed physical mechanism is described below and summarized in Fig. 7. Precipitation events in both coupling regimes directly modify the chemical and meteorological environment. From the chemical perspective, greater precipitation effectively removes NOy species (Figs. 5b, 6b and d) through ventilation (Pickering et al., 1988; Vukovich, 1995), outflow (Merrill and Moody, 1996; Li et al., 2005; Leibensperger et al., 2008; Fang et al., 2009), and wet removal


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Fig. 6. As in Fig. 5, but for observed antecedent a) ozone, b) NOx, c) isoprene, and d) NOz (NOyeNOx) concentrations (ppb) for hours approaching high (>70 ppb; dashed) and low (<40 ppb; solid) midday ozone concentrations for a region in the Northeast (NE; black) and region around the Atlanta Metropolitan area (ATL; orange). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(Sickles and Shadwick, 2007) reducing ozone concentrations the following day (Figs. 6a and 7). This ozone precursor removal occurs in both coupling regimes but the nature of the precipitation event varies. In the case of energy-limited regimes (north of 37 N),

precipitation events are generally driven by synoptic events, such as frontal passages or mid-latitude cyclones (Whelpdale et al., 1984; Merrill and Moody, 1996; Leibensperger et al., 2008) supplying a cooler and less humid air mass following passage. This is

Fig. 7. The proposed chemical and meteorological pathways producing the ozoneehumidity correlations under energy-limited (north of 37 N) and soil moisture-limited (south of 37 N) evapotranspiration regimes.

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illustrated in the simultaneous sharp decrease in temperature and humidity prior to low ozone events under energy-limited regimes (Fig. 5a and c). Because evapotranspiration is energy-limited, additional soil water provided by the synoptic precipitation has a weak influence on evaporative fraction (Fig. 5d). Further, hours prior to high O3 days (>70 ppb) show slightly lower evaporative fraction (e.g. drier soil) and higher humidity (Fig. 5c and d) illustrating the weak control surface soil drying has on near-surface humidity. We speculate that the positive correlation between humidity and ozone found over the Northeastern US is likely driven by air mass changes following the passage of mid-latitude synoptic systems producing low ozone production conditions (cooler temperatures, ozone precursor removal, and outflow). This is in agreement with Li et al. (2005) and Leibensperger et al. (2008) who demonstrated the importance of mid-latitude systems for venting pollution episodes north of 35 N and 40 N. Under soil moisture-limited conditions (south of 37 N), precipitation events are characterized by convective activity. Similar to energy-limited regimes, convective precipitation events effectively remove surface NOy species (Fig. 6d) reducing subsequent day ozone (Fig. 6a). However, unlike energy-limited regimes, additional soil moisture provided by precipitation increases the evaporative fraction (Fig. 5d) and energy partitioned toward evapotranspiration instead of sensible heat flux. This reduction in sensible heat flux decreases near-surface temperatures (Fig. 5a) and increases evapotranspiration producing higher surface humidity (Fig. 5c). Therefore, the negative correlation between humidity and ozone is likely an artifact of coincident cooler temperatures, removal and ventilation of ozone and ozone precursors, and subsequent increases in evapotranspiration and humidity following precipitation over the Southeast. The transition from the energy-limited Northeast to the soil moisture-limited Southeast may provide an explanation for the north-south change in sign of ozoneehumidity correlations (Fig. 1c) and weakening of the ozoneetemperature correlation (Fig. 1b). Because the ozoneehumidity correlation is likely a manifestation of soil moistureeatmosphere coupling regimes, evaporative fraction could be a better predictor of ozone over the Southeastern US than temperature or humidity (Fig. 4b). The utility of evaporative fraction as a predictor arises from its ability to retain information of prior precipitation events (e.g. wet removal and ventilation) and instantaneous meteorological conditions (temperature and radiation) relevant to ozone production. Therefore, evaporative fraction could be a useful diagnostic encompassing both chemical and meteorological factors contributing to O3 formation rather than a direct control variable. The capability of evaporative fraction to be used as a predictor weakens in energy-limited regimes because, as defined by the landeatmosphere coupling framework, the land surface exerts weaker influence over atmospheric states. Past studies indicate that the NLDAS-MOSAIC model overestimates evapotranspiration and underestimates sensible heat flux relative to other land models and observations during the summer (Robock et al., 2003; Mo et al., 2011; Xia et al., 2012b). This discrepancy in energy partitioning places the transition from soil moisture-to-energy limited regimes farther north than other models (Xia et al., 2012a), however, all four land surface models used by NLDAS2 identified soil water-limited evapotranspiration south of 36 N during the summer over the Eastern US (Xia et al., 2012a). Therefore, analysis of the mechanism over the ATL and NE focus regions (Figs. 5 and 6) are not likely influenced by the choice of NLDAS2-MOSAIC. 5. Concluding remarks We demonstrate the potential importance of soil moisture and surface energy partitioning in describing a physical mechanism


underlying the north-south O3-meteorology correlations over the Eastern US using 17-years of observed EPA-AQS and NLDAS2 reanalysis data. We show that the distinct north-south gradients of temperature-ozone and humidity-ozone correlations (Camalier et al., 2007; Zheng et al., 2007; Davis et al., 2011; Rasmussen et al., 2012) likely arise from landeatmosphere coupling regimes. Specifically, it is proposed that near-surface humidity is a passive surrogate in ozone formation reflecting the simultaneous impact of surface drying on near-surface humidity and other ozone formation factors (temperature and radiation) in the Southeast. Because it is difficult to identify causal relationships of individual variables within the highly coupled climateechemistry system, analyzing ozone concentration behavior from the perspective of lande atmosphere coupling provides guidance for understanding the impact of climate on surface ozone. We showed that the evaporative fraction returned a higher correlation with O3 concentrations than temperature, humidity, or radiation for the Southeastern US due to its ability to capture prior day precipitation and instantaneous environmental variables relevant for O3 photochemical production. The ozoneeevaporative fraction correlation may therefore serve as a simple and useful diagnostic for assessing the degree of climatee chemistry interactions within fully interactive and offline chemistry models. Because the surface flux data needed to calculate the evaporative fraction are not widely available, this work highlights the need for co-located chemical, surface flux, and atmospheric measurements. The landeatmosphere coupling framework may also provide insight into the inability of chemistryeclimate models to reproduce observed O3-temperature relationships (Rasmussen et al., 2012) and the systematic overestimate of modeled ozone (greater than 10 ppb) over the Southeastern US (Fiore et al., 2009; Reidmiller et al., 2009). This work provides motivation for further investigation into the proposed mechanism and utility of the evaporative fraction using climateechemistry models and other long-term chemical observations. Acknowledgments We thank the two anonymous reviewers for their helpful and constructive comments. Portions of this work were supported by the NSF ATM 0809255 and the U.S.-Egypt Joint Science and Technology Fund. NLDAS2 data were acquired as part of the mission of NASA’s Earth Science Division and archived and distributed by the Goddard Earth Sciences (GES) Data and Information Services Center (DISC). Appendix A. Supplementary material Supplementary material related to this article can be found at References Betts, A.K., 2004. Understanding hydrometeorology using global models. Bulletin of the American Meteorological Society 85, 1673. Betts, A.K., 2009. Landesurfaceeatmosphere coupling in observations and models. Journal of Advances in Modeling Earth Systems 1. James.2009.1.4. Artn 4. Blanchard, C.L., Hidy, G.M., Tanenbaum, S., 2010. Nmoc, ozone, and organic aerosol in the Southeastern United States, 1999e2007 2 ozone trends and sensitivity to Nmoc emissions in Atlanta, Georgia. Atmospheric Environment 44, 4840e4849. Bloomer, B.J., Stehr, J.W., Piety, C.A., Salawitch, R.J., Dickerson, R.R., 2009. Observed relationships of ozone air pollution with temperature and emissions. Geophysical Research Letters 36. Artn L09803. Camalier, L., Cox, W., Dolwick, P., 2007. The effects of meteorology on ozone in urban areas and their use in assessing ozone trends. Atmospheric Environment 41, 7127e7137.


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