# Summertime Aerosol Radiative Effects and Their Dependence on Temperature over the Southeastern USA
## 1 Introduction  
Aerosol particles are an important regulator of the Earth's climate. They scatter and absorb incoming solar radiation (e.g., [1]) and can act as initial formation sites for cloud droplets and thereby modify the properties and lifetime of clouds [2; 3; 4]. The magnitude of aerosol radiative effects remains the single largest uncertainty in the current estimates of the anthropogenic radiative forcing [5]. One of the key quantities needed for accurate estimates of anthropogenic radiative forcing is quantitative knowledge of the radiative effects of natural aerosol--after all, it is the change from the natural background that is important when quantifying human influence on climate. Recent studies have highlighted that the uncertainties in natural aerosol emissions may bias our current estimates of aerosol forcing more than the uncertainties associated with anthropogenic emissions [6]. Furthermore, our current understanding of future changes in natural aerosol radiative effects is very poor. The dominant source of natural aerosols over the Earth's vast forested regions is biogenic volatile organic compounds [7], which are oxidized in the atmosphere, forming semi-, low-, or non-volatile organic compounds [8]. Consequently, these compounds can condense onto existing aerosol particles or participate in new particle formation to form secondary organic aerosols (SOA) and significantly modify the chemical and optical properties of the particles [9]. In accordance with the expected positive temperature dependence of BVOC emissions [10; 11; 12; 13], several previous studies have shown that some aerosol properties, such as mass concentration and the number of cloud active aerosol particles, i.e., cloud condensation nuclei (CCN), indeed correlate positively with temperature at many forested sites (e.g., [14; 15]). Therefore, BVOC emissions could introduce a regionally relevant cooling feedback in a warming climate [9]. However, the evidence for a link between air temperature and the aerosol radiative effects due to increased BVOC emissions is not yet definite because in addition to BVOC emissions photochemistry and synoptic meteorology also depend on temperature and can affect SOA formation (e.g., [16]). Aerosol direct effects can be quantified via aerosol optical depth (AOD) from, e.g., satellites. [PERSON] et al. [17] observed a positive correlation between in situ observed SOA and regional satellite-based AOD in Ontario, Canada during periods of elevated temperature. Similarly, [PERSON] et al. [18] detected an exponential dependence of the AOD on the temperature in the southeastern USA, and hypothesized that it arose from enhanced natural BVOC emissions on warmer days. The links between temperature, BVOC emissions, and AOD are further complicated over regions with extensive SO\({}_{2}\) emissions, such as in the southeastern USA, due to the effect of isoprene oxidation on sulphate aerosol formation: the major oxidation products of isoprene are peroxides, which are generally attributed to the increased summertime formation of sulphate via aqueous phase reactions [18]. Therefore, changes in SO\({}_{2}\) emissions also affect the relationship between BVOC emissions and AOD. The southeastern USA is an interesting location for aerosol studies due to the strong interplay between biogenic and anthropogenic aerosol sources in that region. Consequently, extensive studies on aerosol over that region have been carried out (e.g., [19; 20; 21; 22; 23; 24; 25; 26; 27]). These studies concentrated on aerosol chemistry, trends in aerosols and the formation of SOA. For example, [PERSON] et al. [23] showed that anthropogenic pollution enhances isoprene-derived SOA concentrations significantly and a major fraction of organic aerosol is mediated by the pollution in the southeastern USA during summer. However, only three studies have estimated the direct radiative effect of aerosols in this region: [PERSON] et al.